EPA­
450/
4­
92­
001
A
TIERED
MODELING
APPROACH
FOR
ASSESSING
THE
RISKS
DUE
TO
SOURCES
OF
HAZARDOUS
AIR
POLLUTANTS
by
David
E.
Guinnup
U.
S.
ENVIRONMENTAL
PROTECTION
AGENCY
Office
of
Air
Quality
Planning
and
Standards
Technical
Support
Division
Research
Triangle
Park,
NC
27711
March
1992
ii
DISCLAIMER
This
report
has
been
reviewed
by
the
Office
of
Air
Quality
Planning
and
Standards,

U.
S.
Environmental
Protection
Agency,
and
has
been
approved
for
publication.
Any
mention
of
trade
names
or
commercial
products
is
not
intended
to
constitute
endorsement
or
recommendation
for
use.
iii
TABLE
OF
CONTENTS
DISCLAIMER
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ii
FIGURES
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TABLES
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1.0
INTRODUCTION
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1
1.1
Background
and
Purpose
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1
1.2
Risk
Assessment
in
Title
III
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2
1.3
Overview
of
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5
1.4
General
Modeling
Requirements,
Definitions,
and
Limitations
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6
2.0
TIER
1
ANALYSES
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2.1
Introduction
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2.2
Long­
term
Modeling
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9
2.2.1
Maximum
Annual
Concentration
Estimation
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2.2.2
Cancer
risk
assessment
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12
2.2.3
Chronic
Noncancer
Risk
Assessment
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13
2.3
Short­
term
Modeling
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13
2.3.1
Maximum
Hourly
Concentration
Estimation
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14
2.3.2
Acute
Hazard
Index
Assessment
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15
3.0
TIER
2
ANALYSES
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19
3.1
Introduction
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19
3.2
Long­
term
Modeling
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19
3.2.1
Maximum
Annual
Concentration
Estimation
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3.2.2
Cancer
Risk
Assessment
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21
3.2.3
Chronic
Noncancer
Risk
Assessment
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22
3.3
Short­
term
Modeling
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22
iv
3.3.1
Maximum
Hourly
Concentration
Estimation
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22
3.3.2
Acute
Hazard
Index
Assessment
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24
4.0
TIER
3
ANALYSES
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25
4.1
Introduction
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25
4.2
Long­
term
Modeling
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25
4.2.1
Maximum
Annual
Concentration
Estimation
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25
4.2.2
Cancer
Risk
Assessment
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28
4.2.3
Chronic
Noncancer
Risk
Assessment
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4.3
Short­
term
Modeling
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4.3.1
Maximum
Hourly
Concentration
Estimation
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30
4.3.2
Acute
Hazard
Index
Exceedance
Assessment
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32
5.0
ADDITIONAL
DETAILED
ANALYSES
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35
6.0
SUMMARY
OF
DIFFERENCES
BETWEEN
MODELING
TIERS
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37
REFERENCES
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39
APPENDIX
A
­
ELECTRONIC
BULLETIN
BOARD
ACCESS
INFORMATION
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41
APPENDIX
B
­
REGIONAL
METEOROLOGISTS/
MODELING
CONTACTS
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43
APPENDIX
C
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EMISSION
RATE
ESTIMATION
GUIDANCE
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45
v
FIGURES
Number
1
Schematic
of
Example
Facility
with
Long­
Term
Impact
Locations
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27
2
Schematic
of
Example
Facility
with
Short­
Term
Impact
Locations
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33
vi
TABLES
Number
1
Normalized
Maximum
Annual
Concentrations,
(
µ
g/
m3)/(
T/
yr)
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11
2
Normalized
Maximum
1­
Hour
Average
Concentrations,
(
µ
g/
m3)/(
g/
s)
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16
3
Differences
between
Modeling
Tiers
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37
1
1.0
INTRODUCTION
1.1
Background
and
Purpose
Title
III
of
the
Clean
Air
Act
Amendments
of
1990
(
CAAA)
sets
forth
a
framework
for
regulating
major
sources
of
hazardous
(
or
toxic)
air
pollutants
which
is
based
on
the
implementation
of
MACT,
the
maximum
achievable
control
technology,
for
those
sources.
Under
this
framework,
prescribed
pollution
control
technologies
are
to
be
installed
without
the
a
priori
estimation
of
the
health
or
environmental
risk
associated
with
each
individual
source.
The
regulatory
process
is
to
proceed
on
a
source
category­
by­
source
category
basis,
with
a
list
of
source
categories
to
be
published
by
the
end
of
1991,
and
a
schedule
for
their
regulation
to
be
published
a
year
later.
After
the
implementation
of
MACT,
it
will
be
incumbent
on
the
United
States
Environmental
Protection
Agency
(
EPA)
to
assess
the
residual
health
risks
to
the
population
near
each
source
within
a
regulated
source
category.
The
results
of
this
residual
risk
assessment
will
then
be
used
to
decide
if
further
reduction
in
toxic
emissions
is
necessary
for
each
source
category
(
refer
to
§
112(
f)
of
the
CAAA).
These
decisions
will
hinge
primarily
on
a
determination
of
the
lifetime
cancer
risk
for
the
"
maximum
exposed
individual"
for
each
source
as
well
as
the
determination
of
whether
the
exposed
population
near
each
source
is
protected
from
noncancer
health
effects
with
an
"
ample
margin
of
safety".
The
determination
of
lifetime
cancer
risk
involves
the
estimation
of
long­
term
ambient
concentrations
of
toxic
pollutants
whereas
the
determination
of
noncancer
health
effects
can
involve
the
estimation
of
long­
term
and
short­
term
ambient
concentrations.

Since
the
measurement
of
long­
term
and
short­
term
ambient
concentrations
for
each
toxic
air
pollutant
(
189
pollutants
as
listed
in
§
112(
b))
in
the
vicinity
of
each
source
is
a
prohibitively
expensive
task,
it
is
envisioned
that
the
process
of
residual
risk
determination
would
involve
performing
analytical
simulations
of
toxic
air
pollutant
dispersion
for
all
sources
(
or
a
subset
of
sources)
within
each
source
category.
Such
simulations
will
subsequently
be
coupled
with
health
effects
information
and
compared
to
available
population
data
to
quantify
human
exposure,
cancer
risk,
noncancer
health
risks,
and
ecological
risks.

In
addition
to
mandating
the
residual
risk
assessment
process,
the
CAAA
provide
for
the
exemption
of
source
categories
and
pollutants
from
the
MACT­
based
regulatory
process
if
it
can
be
demonstrated
that
the
risks
associated
with
that
source
category
or
pollutant
are
below
specified
levels
of
concern.
EPA­
approved
risk
assessments
would
need
to
be
performed
to
justify
such
an
exemption,
and
the
CAAA
provide
for
petition
processes
to
approve
or
deny
claims
that
a
source
category
or
a
specific
pollutant
should
not
be
subject
to
regulation.

The
purpose
of
this
document
is
to
provide
guidance
on
the
use
of
EPA­
approved
procedures
which
may
be
used
to
assess
risks
due
to
the
atmospheric
dispersion
of
emissions
of
hazardous
air
pollutants.
It
is
likely
that
the
techniques
described
herein
will
be
useful
with
respect
to
several
decision­
making
processes
associated
with
the
implementation
of
CAAA
Title
III
(
e.
g.,
petition
to
add
or
delete
a
pollutant
from
the
list
of
hazardous
air
pollutants,
petition
to
delete
a
source
category
from
the
list
of
source
categories,
demonstration
of
source
modification
offsets,
etc.).
In
addition,
these
procedures
may
serve
as
the
basis
for
the
residual
risk
determination
process
described
above.
The
guidance
addresses
the
estimation
of
long­
term
and
short­
term
ambient
concentrations
resulting
from
the
atmospheric
dispersion
of
known
emissions
of
hazardous
air
pollutants,
and
subsequently
addresses
the
techniques
currently
used
to
quantify
the
cancer
risks
and
noncancer
risks
associated
with
the
predicted
ambient
concentrations.
It
describes
a
tiered
approach
which
progresses
from
simple
conservative
screening
estimates
(
provided
in
the
form
of
lookup
tables)
to
more
complex
modeling
methodologies
using
computer
models
and
site­
specific
data.
In
addition
to
providing
guidance
to
assist
in
the
CAAA
Title
III
implementation
process,
it
is
being
2
provided
to
the
general
public
to
assist
State
and
local
air
pollution
control
agencies
as
well
as
sources
of
hazardous
air
pollutants
in
their
own
assessments
of
the
impacts
of
these
sources.

While
the
methods
described
herein
comprise
the
most
up­
to­
date
means
for
assessing
the
impacts
of
sources
of
toxic
air
pollution,
they
are
subject
to
future
revision
as
new
scientific
information
becomes
available,
possibly
as
a
result
of
the
risk
assessment
methodology
study
being
conducted
by
the
National
Academy
of
Sciences
(
NAS)
under
mandate
of
section
112(
o)
of
the
CAAA
(
report
due
to
Congress
from
NAS
in
May,
1993).

1.2
Risk
Assessment
in
Title
III
As
mentioned
above,
several
provisions
of
CAAA
Title
III
describe
the
need
to
consider
ambient
concentration
impacts
and
their
associated
health
risks
in
establishing
the
regulatory
process
for
sources
of
toxic
air
pollutants.
Specifically,
these
are:

1.
A
pollutant
may
be
deleted
via
a
petition
process
from
the
list
of
hazardous
or
toxic
pollutants
subject
to
regulation
if
the
petition
demonstrates
(
among
other
things)
that
"
ambient
concentrations
...
of
the
substance
may
not
reasonably
be
anticipated
to
cause
any
adverse
effects
to
the
human
health."
(
§
112(
b)(
3)(
C))

2.
A
pollutant
may
be
added
to
the
list
if
a
petition
demonstrates
that
"
ambient
concentrations
...
of
the
substance
are
known
to
cause
or
may
reasonably
be
anticipated
to
cause
adverse
effects
to
human
health."(
§
112(
b)(
3)(
B))

3.
An
entire
source
category
may
be
deleted
from
the
list
of
source
categories
subject
to
regulation
if
a
petition
demonstrates,
for
the
case
of
carcinogenic
pollutants,
that
"
no
source
in
the
category
...
emits
(
carcinogenic)
air
pollutants
in
quantities
which
may
cause
a
lifetime
risk
of
cancer
greater
than
one
in
one
million
to
the
individual
in
the
population
who
is
most
exposed
to
emissions
of
such
pollutants
from
the
source,"
(
§
112(
c)(
9)(
B)(
i))
and,
for
the
case
of
noncarcinogenic
yet
toxic
pollutants,
that
"
emissions
from
no
source
in
the
category
...
exceed
a
level
which
is
adequate
to
protect
public
health
with
an
ample
margin
of
safety
and
no
adverse
environmental
effect
will
result
from
emissions
from
any
source."
(
§
112(
c)(
9)(
B)(
ii))

4.
Within
eight
years
after
a
source
category
has
been
subject
to
a
MACT
regulation,
EPA
must
determine
whether
additional
regulation
of
that
source
category
is
necessary
based
on
an
assessment
of
the
residual
risks
associated
with
the
sources
in
that
category.
Based
on
such
an
assessment,
additional
regulation
of
the
source
category
is
deemed
necessary
if
"
promulgation
of
such
standards
is
required
in
order
to
provide
an
ample
margin
of
safety
to
protect
the
public
health"
with
respect
to
noncancer
health
effects,
or
if
the
MACT
standards
"
do
not
reduce
lifetime
excess
cancer
risks
to
the
individual
most
exposed
to
emissions
from
a
source
in
the
category
or
subcategory
to
less
than
one
in
one
million"
with
respect
to
carcinogens,
or
if
a
determination
is
made
"
that
a
more
stringent
standard
is
necessary
to
prevent
...
an
adverse
environmental
effect."
(
§
112(
f)(
2)(
A))

In
the
context
of
these
provisions,
decisions
are
to
be
made
based
on
whether
or
not
the
predicted
impact
of
a
source
exceeds
some
level
of
concern.
For
comparison
to
specified
levels
of
concern,
source
impacts
are
quantified
in
four
ways:

1.
lifetime
cancer
risk;

2.
chronic
noncancer
hazard
index;
3
3.
acute
noncancer
hazard
index,
and;

4.
frequency
of
acute
hazard
index
exceedances.

These
impact
measures
are
discussed
in
more
detail
in
the
next
few
paragraphs.
It
is
worth
noting
at
this
point
that
insofar
as
knowledge
is
available
regarding
the
effects
of
specific
hazardous
pollutants
on
the
environment,
it
may
be
possible
to
use
ecological
hazard
index
values
to
quantify
such
impacts.
Such
calculations
may
proceed
on
a
track
which
is
parallel
to
the
calculation
of
health
hazard
index
values.
However,
until
specific
methodologies
for
ecological
risk
assessment
are
adopted,
the
techniques
identified
in
this
document
will
remain
limited
to
the
assessment
of
human
health
risks
due
to
inhalation
of
hazardous
air
pollutants.

For
carcinogenic
pollutants,
the
level
of
concern
is
the
risk
of
an
individual
contracting
cancer
by
being
exposed
to
ambient
concentrations
of
that
pollutant
over
the
course
of
a
lifetime,
or
lifetime
cancer
risk.
For
the
purposes
of
§
112(
c),
the
criterion
specified
in
the
CAAA
is
1
in
1,000,000
lifetime
cancer
risk
for
the
most
exposed
individual,
or
the
individual
exposed
to
the
highest
predicted
concentrations
of
a
pollutant.
(
For
other
purposes,
the
lifetime
cancer
risk
specifying
the
level
of
concern
may
be
higher
or
lower.)
Lifetime
cancer
risks
are
calculated
by
multiplying
the
predicted
annual
ambient
concentrations
(
in

g/
m3)
of
a
specific
pollutant
by
the
unit
risk
factor
or
unit
risk
estimate
(
URE)
1
for
that
pollutant,
where
the
unit
risk
factor
is
equal
to
the
upper
bound
lifetime
cancer
risk
associated
with
inhaling
a
unit
concentration
(
1

g/
m3)
of
that
pollutant.
Since
predicted
annual
pollutant
concentrations
around
a
source
vary
as
a
function
of
position,
so
do
lifetime
cancer
risk
estimates.
Thus,
decisions
involving
whether
the
impact
of
a
source
or
group
of
sources
is
above
some
level
of
concern
typically
focus
on
the
highest
predicted
concentration
(
and
hence
the
highest
predicted
lifetime
cancer
risk)
outside
the
facility
fenceline.
The
EPA
has
developed
unit
risk
factors
for
a
number
of
possible,
probable,
or
known
human
carcinogens,
and
will
be
developing
additional
cancer
unit
risk
factors
as
more
information
becomes
available.
For
the
purposes
of
this
document,
cancer
risks
resulting
from
exposure
to
mixtures
of
multiple
carcinogenic
pollutants
will
be
assessed
by
summing
the
cancer
risks
due
to
each
individual
pollutant,
regardless
of
the
type
of
cancer
which
may
be
associated
with
any
particular
carcinogen.
2
For
pollutants
causing
noncancer
health
effects
from
chronic
or
acute
exposure,
the
levels
of
concern
are
chronic
and
acute
concentration
thresholds,
respectively,
which
would
be
derived
from
health
effects
data,
taking
into
account
scientific
uncertainties.
For
purposes
of
estimating
potential
long­
term
impacts
of
hazardous
air
pollutants,
EPA
has
derived
for
some
pollutants
(
and
will
derive
for
others)
chronic
inhalation
reference
concentration
(
RfC)
1
values,
which
are
defined
as
estimates
of
the
lowest
concentrations
of
a
single
pollutant
to
which
the
human
population
can
be
exposed
over
a
lifetime
without
appreciable
risk
of
deleterious
effects.
For
purposes
of
specific
chronic
noncancer
risk
assessment,
EPA
may
designate
the
RfC
value,
or
some
fraction
or
multiple
thereof,
as
the
appropriate
long­
term
noncancer
level
of
concern.
For
purposes
of
specific
acute
noncancer
risk
assessment,
the
EPA
may
designate
acute
reference
thresholds
as
the
appropriate
short­
term
noncancer
level
of
concern.
For
the
purposes
of
this
document,
long­
term
noncancer
levels
of
concern
will
be
referred
to
as
chronic
concentration
thresholds,
and
short­
term
noncancer
levels
of
concern
will
be
referred
to
as
acute
concentration
thresholds.
For
ease
of
implementation,
acute
concentration
thresholds
will
be
designated
for
1­
hour
averaging
times.
This
does
not
necessarily
mean
that
exposure
data
indicate
deleterious
health
effects
from
exposure
times
of
1
hour,
but
rather
that
the
1­
hour
acute
concentration
threshold
has
been
derived
such
that
it
is
protective
of
the
exposure
duration
of
concern.

The
risk
with
respect
to
long­
or
short­
term
deleterious
noncancer
health
effects
associated
with
exposure
to
a
pollutant
or
group
of
pollutants
is
quantified
by
the
hazard
index.
The
chronic
noncancer
hazard
index
is
calculated
by
dividing
the
modeled
annual
concentration
of
a
pollutant
by
4
its
chronic
concentration
threshold
value.
The
acute
noncancer
hazard
index
is
calculated
by
dividing
the
modeled
1­
hour
concentration
of
a
pollutant
by
its
acute
concentration
threshold
value.
If
multiple
pollutants
are
being
evaluated,
the
(
chronic
or
acute)
hazard
index
at
any
location
is
calculated
by
dividing
each
predicted
(
annual
or
1­
hour)
concentration
at
that
location
by
its
(
chronic
or
acute)
concentration
threshold
value
and
summing
the
results.
2
If
the
hazard
index
is
greater
than
1.0,
this
represents
an
exceedance
of
the
level
of
concern
at
that
location.
For
pollutants
which
can
cause
deleterious
health
effects
from
acute
exposures,
exceedances
of
a
level
of
concern
may
occur
at
any
location
and
at
any
time
throughout
the
modeling
period.
Thus,
the
frequency
with
which
any
location
experiences
an
exceedance
also
becomes
a
measure
of
the
risk
associated
with
a
modeled
source.
Frequency
of
acute
hazard
index
exceedances
is
only
addressed
by
the
most
refined
analysis
methods
referred
to
in
this
document.

Information
on
UREs
and
RfCs
is
accessible
through
the
Integrated
Risk
Information
System
(
IRIS),
EPA
Environmental
Criteria
and
Assessment
Office
(
ECAO)
in
Cincinnati,
Ohio,
(
513)
569­
7254.

1.3
Overview
of
Document
This
document
is
divided
into
three
major
sections,
each
section
addressing
a
different
level
of
sophistication
in
terms
of
modeling,
referred
to
as
"
tiers".
The
first
tier
is
a
simplified
screening
procedure
in
which
the
user
can
estimate
maximum
off­
site
ground­
level
concentrations
without
extensive
knowledge
regarding
the
source
and
without
the
need
of
a
computer.
The
second
tier
is
a
more
sophisticated
screening
technique
which
requires
a
bit
more
detailed
knowledge
concerning
the
source
being
modeled
and,
in
addition,
requires
the
execution
of
a
computer
program.
The
third
tier
involves
site­
specific
computer
simulations
with
the
aid
of
computer
programs
and
detailed
source
parameters.
Since
the
effects
of
toxic
air
pollutants
may
be
of
concern
from
both
a
long­
term
and
a
short­
term
perspective,
each
tier
is
divided
into
two
parts.
The
first
part
addresses
dispersion
modeling
to
assess
long­
term
ambient
concentrations
(
important
from
a
cancer­
causing
or
chronic
noncancer
effects
standpoint)
and
the
second
addresses
dispersion
modeling
for
the
estimation
of
short­
term
concentrations
(
important
from
an
acute
toxicity
perspective).

It
should
be
noted
that
this
document
is
intended
to
be
used
in
conjunction
with
the
User's
Guides
for
the
models
described:
SCREEN3,
TOXST4,
and
TOXLT5.
It
is
not
intended
to
replace
or
reproduce
the
contents
of
these
documents.
In
addition,
the
reader
may
wish
to
consult
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6
for
more
detailed
information
on
the
consistent
application
of
air
quality
models.
Modelers
may
also
wish
to
use
the
EPA's
TSCREEN7
modeling
system
to
assist
in
the
Tier
2
computer
simulation
of
certain
toxic
release
scenarios.
It
should
be
noted,
however,
that
toxic
pollutant
releases
which
TSCREEN
treats
as
heavier­
than­
air
are
not
to
be
modeled
using
techniques
described
herein.
Atmospheric
dispersion
of
such
pollutants
requires
a
more
refined
analysis,
such
as
those
described
in
Reference
8.
Model
codes,
user's
guides,
and
associated
documentation
referred
to
in
this
document
can
be
obtained
through
the
Technology
Transfer
Network
(
TTN)
of
the
EPA's
Office
of
Air
Quality
Planning
and
Standards
(
OAQPS),
and
access
information
is
provided
in
Appendix
A.

The
modeling
tiers
are
designed
such
that
the
concentration
estimates
from
each
tier
should
be
less
conservative
than
the
previous
one.
This
means
that,
for
a
given
situation,
a
Tier
1
modeled
impact
should
be
greater
than,
or
more
conservative
than,
the
Tier
2
modeled
impact,
and
the
Tier
2
modeled
impact
should
be
more
conservative
than
the
Tier
3
modeled
impact.
Progression
from
5
one
tier
of
modeling
to
the
next
thus
involves
the
use
of
levels
of
concern,
as
defined
above.
For
example,
if
the
results
of
a
Tier
1
analysis
indicate
an
exceedance
of
a
level
of
concern
with
respect
to
either
(
1)
the
maximum
predicted
cancer
risk,
(
2)
the
maximum
predicted
chronic
noncancer
hazard
index,
or
(
3)
the
maximum
predicted
acute
hazard
index,
the
analyst
may
wish
to
perform
a
Tier
2
analysis.
If
all
three
of
these
impact
measures
are
below
their
specified
levels
of
concern,
there
should
be
no
need
to
perform
a
more
refined
simulation,
and
thus,
there
should
be
no
need
to
progress
to
the
next
tier
of
modeling.
Since
the
establishment
of
levels
of
concern
for
each
specific
hazardous
air
pollutant
is
not
a
part
of
this
effort,
this
document
will
refer
to
generic
levels
of
concern,
and
users
will
need
to
consult
subsequent
EPA
documents
to
determine
the
specific
levels
of
concern
for
their
particular
pollutant
or
pollutant
mixture
and
for
the
particular
purpose
of
their
modeling
efforts.

1.4
General
Modeling
Requirements,
Definitions,
and
Limitations
This
document
describes
modeling
methodologies
for
point,
area,
and
volume
sources
of
atmospheric
pollution.
A
point
source
is
an
emission
which
emanates
from
a
specific
point,
such
as
a
smokestack
or
vent.
An
area
source
is
an
emission
which
emanates
from
a
specific,
well­
defined
surface,
such
as
a
lagoon,
landfarm,
or
open­
top
tank.
Sources
referred
to
as
having
"
fugitive"
emissions
(
e.
g.,
multiple
leaks
within
a
specific
processing
area)
are
typically
modeled
as
area
sources.
The
methods
used
in
this
document
are
generally
considered
to
be
applicable
for
assessing
impacts
of
a
source
from
the
facility
fenceline
out
to
a
50
km
radius
of
the
source
or
sources
to
be
modeled.
There
is
no
particular
upper
or
lower
limit
on
emission
rate
values
for
which
these
techniques
apply.

For
the
purposes
of
this
document,
"
source"
means
the
same
thing
as
"
release",
and
"
air
toxic"
means
the
same
thing
as
"
hazardous
air
pollutant".
It
should
be
noted
that
"
area
source"
as
defined
in
the
previous
paragraph
is
not
the
same
as
the
"
area
source"
defined
by
the
CAAA.
Modeling
techniques
described
in
this
document
are
specifically
intended
for
use
in
the
simulation
of
a
finite
number
of
well­
defined
sources,
not
for
simulation
of
a
large
number
of
ill­
defined
small
sources
distributed
across
a
large
region,
as
might
well
be
the
case
for
some
"
area
sources"
specified
in
the
CAAA.
Simulation
of
the
acute
and
chronic
impacts
of
such
area
sources
may
utilize
the
RAM
model9
and
the
CDM
2.0
model10,
respectively.
Consult
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6
for
additional
information.
The
reader
should
note
that
relatively
small,
well­
defined
groups
of
sources,
however,
may
be
modeled
using
the
techniques
described
herein.

This
document
does
not
address
the
simulation
of
facilities
located
in
complex
terrain.
Those
interested
in
modeling
facilities
with
possible
complex
terrain
effects
are
directed
to
consult
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6
or
their
EPA
Regional
Office
modeling
contact
for
assistance
in
this
area
(
see
listing
in
Appendix
B).

In
order
to
conduct
an
impact
assessment,
it
is
necessary
to
have
estimates
of
emission
rates
of
each
pollutant
from
each
source
or
release
point
being
included
in
the
assessment.
Emission
rates
may
be
best
estimated
from
experimental
measurements
or
sampling,
where
such
test
methods
are
available.
Alternatively,
mass
balance
calculations
or
use
of
emission
factors
developed
for
specific
types
of
processes
may
be
used
to
quantify
emission
rates.
The
procedures
discussed
in
this
document
do
not
address
the
emission
estimation
process.
Guidance
for
source­
specific
emission
rate
estimation
and
emission
test
methods
is
available
in
other
EPA
documentation
(
e.
g.,
see
References
11
through
15
and
Appendix
C).
Additional
information
on
emission
measurement
techniques
is
available
through
the
OAQPS
TTN
(
see
Appendix
A).

Since
many
sources
of
hazardous
air
pollutants
are
intermittent
in
nature
(
e.
g.,
batch
process
emissions),
the
techniques
in
this
document
have
been
developed
to
allow
the
treatment
of
intermittent
sources
as
well
as
continuous
types
of
sources.
It
is
important
to
understand
the
different
6
treatment
of
emission
rates
for
both
types
of
sources
when
carrying
out
either
the
analysis
of
a
longterm
impact
or
a
short­
term
impact.
In
a
long­
term
impact
analysis,
the
emission
rate
used
for
modeling
is
based
on
the
amount
of
pollutant
emitted
over
a
1
year
period,
regardless
of
whether
the
emission
process
is
a
continuous
or
intermittent
one.
In
addition,
to
assess
the
worst­
case
impact
of
a
source
or
group
of
sources,
long­
term
emission
rates
used
in
model
simulations
should
reflect
the
emission
rates
for
a
plant
or
process
which
is
operating
at
full
design
capacity.
In
a
short­
term
impact
analysis,
the
emission
rate
used
for
modeling
is
based
on
the
maximum
amount
of
pollutant
emitted
over
a
1
hour
period,
during
which
the
source
is
emitting.
The
Tier
1
and
Tier
2
procedures
evaluate
the
combined
worst­
case
impacts
of
intermittent
sources
as
if
they
are
all
emitting
at
the
same
time,
whereas
the
Tier
3
procedures
incorporate
a
more
realistic
treatment
of
intermittent
sources
by
turning
them
on
and
off
throughout
the
simulation
period
according
to
user­
specified
frequency
of
occurrence
of
each
release.
This
frequency
of
occurrence
should
reflect
the
normal
operating
schedule
of
the
source
when
operating
at
maximum
design
capacity.

In
addition
to
emission
rate
estimates,
it
is
necessary
to
have
quantitative
information
about
the
sources
to
conduct
a
detailed
impact
assessment.
Tier
1
analyses
require
information
about
the
height
of
the
release
above
ground
level
and
the
shortest
distance
from
the
release
point
to
the
facility
fenceline.
Higher
tiers
of
analysis
require
additional
information
including,
but
not
limited
to:

Stack
height
Inside
stack
diameter
Exhaust
gas
exit
velocity
Exhaust
gas
exit
temperature
Dimensions
of
structures
near
each
source
Dimensions
of
ground­
level
area
sources
Exact
release
and
fenceline
location
Exact
location
of
receptors
for
determining
worst­
case
impacts
Land
use
near
the
modeled
facility
Terrain
features
near
the
facility
Duration
of
short­
term
release
Frequency
of
short­
term
release
Where
appropriate,
this
document
will
address
the
best
means
of
obtaining
these
input
data.
In
some
more
complex
cases,
the
modeling
contact
at
the
nearest
EPA
Regional
Office
may
need
to
be
consulted
for
specific
modeling
guidance
(
see
listing
in
Appendix
B).

Depending
on
the
specific
purpose
of
the
impact
assessment,
it
may
be
difficult
for
the
modeler
to
decide
which
sources
(
or
release
points)
and
which
pollutants
should
be
included
in
a
particular
analysis
or
simulation.
Since
these
questions
pertain
to
the
particular
purposes
for
which
the
impact
assessment
is
being
performed,
they
are
not
addressed
by
this
document.
Instead,
this
document
refers
to
and
provides
guidance
for
modeling
various
scenarios
including
single­
source,
multiple­
source,
single­
pollutant,
and
multiple­
pollutant
scenarios.
Subsequent
EPA
documents
will
address
the
questions
of
which
sources
and
which
pollutants
should
be
included
in
an
impact
analysis
for
a
specific
regulatory
purpose.
7
2.0
TIER
1
ANALYSES
2.1
Introduction
Tier
1
analysis
of
a
stationary
source
(
or
group
of
sources)
of
toxic
pollutant(
s)
is
performed
to
address
the
question
of
whether
or
not
the
source
has
the
potential
to
cause
a
significant
impact.
This
"
screening"
analysis
is
performed
by
using
tables
of
lookup
values
to
obtain
the
"
worst­
case"
impact
of
the
source
being
modeled.
The
analysis
is
performed
to
assess
both
the
potential
long­
and
short­
term
impacts
of
the
source.
If
the
predicted
screening
impacts
are
less
than
the
appropriate
levels
of
concern,
no
further
modeling
is
indicated.
If
the
predicted
screening
impacts
are
above
any
levels
of
concern,
further
analysis
of
those
impacts
at
a
higher
Tier
may
be
desirable
to
obtain
more
accurate
results.

The
Tier
1
"
lookup
tables"
have
been
created
as
tools
which
may
be
easily
used
to
estimate
conservative
impacts
of
sources
of
toxic
pollutants
with
a
minimal
amount
of
information
concerning
those
sources.
The
normalized
annual
and
1­
hour
concentration
tables
were
created
based
on
conservative
simulations
of
toxic
pollutant
sources
with
Gaussian
plume
dispersion
models.
In
this
context,
"
conservative"
simulations
use
conservative
assumptions
regarding
meteorology,
building
downwash,
plume
rise,
etc.

2.2
Long­
term
Modeling
Long­
term
modeling
of
toxic
or
hazardous
air
pollutants
is
aimed
at
the
estimation
of
annual
average
pollutant
concentrations
to
which
the
public
might
be
exposed
as
the
result
of
emissions
from
a
specific
source
or
group
of
sources.
From
the
EPA
regulatory
viewpoint,
this
"
public"
does
not
include
employees
of
the
facility
responsible
for
the
emissions
(
this
is
the
jurisdiction
of
the
Occupational
Safety
and
Health
Agency,
OSHA).
Thus,
the
impact
assessment
focuses
on
estimating
concentrations
"
off­
site",
or
outside
the
facility
boundary.
For
carcinogens,
the
calculation
of
cancer
risk
proceeds
by
multiplying
annual
concentrations
by
pollutant­
specific
cancer
potency
factors
derived
from
health
effects
data.
The
impacts
of
pollutants
with
chronic
noncancer
effects
are
generally
assessed
by
comparing
predicted
annual
concentrations
with
chronic
threshold
concentrations
which
are
again
derived
from
experimental
health
data.
For
the
purposes
of
protecting
the
general
public
against
"
worst­
case"
pollutant
concentrations,
the
analysis
is
focused
on
predicting
the
worst­
case,
or
maximum
annual
average
concentrations.

2.2.1
Maximum
Annual
Concentration
Estimation
A
long­
term
Tier
1
analysis
requires
the
following
information:

1.
annual
average
emission
rate
of
each
pollutant
from
each
source
included
in
the
simulation
(
T/
yr).
These
emissions
do
not
have
to
be
continuously
emitted,
but
rather
should
represent
the
total
amount
of
pollutant
which
is
generated
by
this
source
in
a
year.
Note
that
the
tons
used
in
this
regard
are
English
tons
(
1
T.
=
2000
lb.).
Also
note
that,
for
Tier
1
analyses,
the
emission
rate
from
an
area
source
represents
the
total
emissions
from
the
area,
not
the
emissions
per
square
unit
area.

2.
height
of
the
release
point
above
ground
(
m),
for
each
point
source.

3.
source
types
(
point
or
area).
Point
sources
typically
include
exhaust
vents
(
pipes
or
stacks),
or
any
other
type
of
release
that
causes
toxic
materials
to
enter
the
atmosphere
from
a
well­
defined
location,
at
a
well­
defined
rate.
Area
sources
may
also
be
well­
defined,
but
8
differ
from
point
sources
in
that
the
extent
over
which
the
release
occurs
is
substantial.

4.
maximum
horizontal
distance
across
each
area
source
(
m).

5.
nearest
distance
to
property­
line
(
m).
Concentration
estimates
are
needed
at
locations
that
are
accessible
to
the
general
public.
This
is
typically
taken
to
be
any
point
at
or
beyond
the
property­
line
of
a
facility.
Estimate
the
distance
from
the
point
of
each
release
to
the
nearest
point
on
the
fenceline.
(
This
need
not
be
the
same
fenceline
point
for
each
release.)
If
the
source
is
characterized
as
an
area
source,
this
distance
should
be
measured
from
the
nearest
edge
of
the
area
source,
not
from
the
center.

Once
these
five
items
are
determined
for
each
release
(
or
source),
screening
estimates
of
normalized
maximum
annual
concentrations
resulting
from
each
release
are
obtained
from
Table
1
using
the
following
procedure.

1.
For
an
area
source,
select
the
"
side
length"
in
the
table
(
10m,
20m,
30m)
which
is
less
than
or
equal
to
the
maximum
horizontal
distance
across
the
source.

2.
For
a
point
source,
select
the
largest
"
emission
height"
in
the
table
(
0m,
2m,
5m,
10m,
35m,
or
50m)
that
is
less
than
or
equal
to
the
estimated
height
of
release.

3.
Select
the
largest
distance
in
the
table
(
10m,
30m,
50m,
100m,
or
200m)
that
is
less
than
or
equal
to
the
nearest
distance
to
the
property­
line.

4.
Take
the
appropriate
normalized
maximum
annual
concentration
for
this
release
height
and
distance
from
the
table,
and
multiply
by
the
emission
rate
of
each
toxic
substance
(
T/
yr)
in
the
release
to
obtain
the
concentration
estimate
(

g/
m3).
DO
NOT
INTERPOLATE
TABLE
VALUES.

For
example,
consider
the
situation
in
which
a
toxic
pollutant
A
is
released
at
a
rate
of
14.6
T/
yr
from
a
vent­
pipe
that
is
40m
tall,
and
which
is
attached
to
a
building
that
is
4m
tall,
10m
long,
and
5m
wide.
The
nearest
boundary
of
the
facility
is
located
65m
from
the
pipe.
A
value
of
35m
should
be
selected
for
the
emission
height,
because
all
larger
entries
in
the
table
exceed
the
TABLE
1.
Normalized
Maximum
Annual
Concentrations,
(
µ
g/
m3)/(
T/
yr)

Emission
Side
Source
height,
length,
b
typea
m
m
Normalized
maximum
concentration
at
or
beyond:
c
10
m
30
m
50
m
100
m
200
m
500
m
A
A
A
P
P
P
P
P
P
P
0
0
0
0
2
5
10
20
35
50
10
20
30
­­
­­
­­
­­
­­
­­
­­
9.56E+
2
5.15E+
2
3.51E+
2
5.41E+
3
1.87E+
2
9.62E+
1
2.77E+
1
6.91E+
0
2.26E+
0
1.11E+
0
3.02E+
2
1.83E+
2
1.31E+
2
7.92E+
2
1.42E+
2
7.46E+
1
2.44E+
1
4.52E+
0
2.26E+
0
1.10E+
0
1.64E+
2
1.07E+
2
7.92E+
1
3.25E+
2
1.35E+
2
5.18E+
1
2.11E+
1
4.52E+
0
1.13E+
0
1.11E+
0
6.48E+
1
4.78E+
1
3.74E+
1
9.67E+
1
7.28E+
1
2.72E+
1
1.36E+
1
3.80E+
0
1.11E+
0
4.69E­
1
2.32E+
1
1.91E+
1
1.61E+
1
2.91E+
1
2.64E+
1
1.48E+
1
7.17E+
0
2.44E+
0
8.98E­
1
4.23E­
1
5.53E+
0
5.04E+
0
4.58E+
0
6.08E+
0
5.96E+
0
5.18E+
0
2.88E+
0
1.06E+
0
4.41E­
1
2.53E­
1
aSource
type
P=
Point
source,
type
A
=
Area
source.

bSide
length
of
square
area
source.

cDistance
downwind
of
an
area
source
indicates
distance
from
downwind
edge
of
the
area
source.
10
actual
height
of
release
of
40m.
Concentrations
should
be
estimated
for
a
distance
of
50m,
because
once
again,
all
greater
entries
in
the
table
exceed
the
actual
distance
of
65m.
The
appropriate
normalized
maximum
annual
concentration
is
1.13
(

g/
m3)/(
T/
yr).
Multiplying
by
the
emission
rate
of
14.6
T/
yr
results
in
a
maximum
annual
concentration
estimate
for
screening
purposes
equal
to
16.5

g/
m3.

2.2.2
Cancer
risk
assessment
Once
the
maximum
annual
concentration
has
been
estimated
for
each
release
being
modeled,
upper
bound
lifetime
maximum
individual
cancer
risk
may
be
estimated
by
multiplying
the
maximum
annual
concentration
estimates
of
each
carcinogenic
pollutant
by
the
unit
cancer
risk
factor
for
that
pollutant
and
then
summing
results.
This
approach
assumes
that
all
cancer
risks
are
additive,
regardless
of
the
organ
system
which
may
be
affected.
It
should
be
noted
that
this
approach
assumes
that
all
worst­
case
impacts
occur
at
the
same
location.
While
this
assumption
may
not
be
very
realistic,
it
does
help
to
insure
that
Tier
1
results
are
conservative,
and,
therefore
protective
of
the
public.

As
an
example
of
this
approach,
suppose
one
is
simulating
a
plant
which
emits
2
pollutants,
A
and
B,
through
4
different
stacks
such
that
pollutant
A
is
released
from
stacks
1
and
2,
and
pollutant
B
is
released
from
stacks
2,
3,
and
4.
In
this
example,
stack
1
is
the
same
as
that
described
in
the
example
above.
After
going
through
the
above
procedure
to
estimate
the
maximum
annual
concentrations
of
each
pollutant
from
each
stack,
the
results
are:

Source
Compound
Max
impact
Stack
1
Pollutant
A
16.5

g/
m3
Stack
2
Pollutant
A
5.49

g/
m3
Stack
2
Pollutant
B
2.35

g/
m3
Stack
3
Pollutant
B
4.13

g/
m3
Stack
4
Pollutant
B
24.9

g/
m3
Suppose
that
the
unit
cancer
risk
factors
for
pollutants
A
and
B
are
known
to
be
1.0
X
10­
7
and
2.0
X
10­
7
(

g/
m3)­
1,
respectively.
The
Tier
1
maximum
cancer
risk
is
calculated
for
the
individual
releases
and
pollutants
and
summed
as
follows:

Source
Compound
Max
impact
Max
risk
Stack
1
Pollutant
A
16.5

g/
m3
1.65
X
10­
6
Stack
2
Pollutant
A
5.49

g/
m3
5.49
X
10­
7
Stack
2
Pollutant
B
2.35

g/
m3
4.70
X
10­
7
Stack
3
Pollutant
B
4.13

g/
m3
8.26
X
10­
7
Stack
4
Pollutant
B
24.9

g/
m3
4.98
X
10­
6
­­­­­­­­­­­­
Total
risk
8.48
X
10­
6
If
we
are
assessing
the
impact
of
this
group
of
sources
in
relation
to
the
CAAA
specified
level
of
concern
of
1
X
10­
6
lifetime
cancer
risk,
and
since
the
maximum
Tier
1
risk
is
greater
than
the
CAAA
specified
concern
level
of
1
X
10­
6,
this
source
warrants
further
modeling
on
the
basis
of
cancer
risk
(
note
that
this
does
not
rule
out
the
need
to
investigate
acute
or
chronic
noncancer
risks).

2.2.3
Chronic
Noncancer
Risk
Assessment
11
For
all
pollutants
which
pose
a
chronic
noncancer
threat
to
health,
an
assessment
of
the
magnitude
of
this
threat
is
made
using
the
hazard
index
approach.
The
chronic
noncancer
hazard
index
is
calculated
by
summing
the
maximum
annual
concentrations
for
each
pollutant
divided
by
the
chronic
threshold
concentration
value
for
that
pollutant.
If
the
calculated
hazard
index
is
greater
than
1.0,
the
release
or
releases
being
simulated
may
pose
a
threat
to
the
public,
and
further
modeling
may
be
indicated.
It
should
again
be
noted
that,
for
the
sake
of
erring
conservatively,
this
approach
assumes
that
the
worst­
case
impacts
of
all
releases
occur
at
the
same
location.

As
an
example
of
the
above
procedure,
suppose
that
pollutants
A
and
B
in
the
example
above
pose
a
chronic
noncancer
health
risk,
and
their
respective
chronic
concentration
threshold
values
are
20.0
and
5.0

g/
m3,
respectively.
The
chronic
noncancer
hazard
index
would
be
formulated
as
follows:

Source
Compound
Max
impact
Hazard
index
Stack
1
Pollutant
A
16.5

g/
m3
0.825
Stack
2
Pollutant
A
5.49

g/
m3
0.275
Stack
2
Pollutant
B
2.35

g/
m3
0.470
Stack
3
Pollutant
B
4.13

g/
m3
0.826
Stack
4
Pollutant
B
24.9

g/
m3
4.980
­­­­­­­­­­­­
Total
hazard
index
7.376
In
this
case,
one
of
the
individual
hazard
index
values
exceeds
1.0,
the
total
hazard
index
for
the
modeled
facility
exceeds
1.0,
and
further
modeling
at
a
higher
Tier
may
be
desired.

2.3
Short­
term
Modeling
Short­
term
modeling
of
toxic
or
hazardous
air
pollutants
is
aimed
at
the
estimation
of
1­
hour
average
pollutant
concentrations
to
which
the
public
might
be
exposed
as
the
result
of
emissions
from
a
specific
source
or
group
of
sources.
Again,
from
the
EPA
regulatory
viewpoint,
this
"
public"
does
not
include
employees
of
the
facility
responsible
for
the
emissions
(
this
is
the
jurisdiction
of
OSHA).
Thus,
the
impact
assessment
focuses
on
estimating
concentrations
"
off­
site",
or
outside
the
facility
boundary.
From
the
short­
term
perspective,
the
health
effects
of
most
concern
vary,
but
they
are
those
which
create
detrimental
health
effects
as
the
result
of
short­
term
exposure
to
toxic
pollutants.
The
risks
associated
with
such
exposures
are
generally
assessed
by
comparing
1­
hour
predicted
concentrations
with
acute
threshold
concentrations
which
are
derived
from
experimental
health
data.
For
the
purposes
of
protecting
the
general
public
against
"
worst­
case"
pollutant
concentrations,
the
analysis
is
focused
on
predicting
the
worst­
case,
or
maximum
1­
hour
average
concentrations.

2.3.1
Maximum
Hourly
Concentration
Estimation
A
short­
term
Tier
1
analysis
requires
the
following
information:

1.
maximum
1­
hour
average
emission
rate
of
each
pollutant
from
each
source
included
in
the
simulation
(
g/
s).
If
the
release
is
a
continuous,
constant­
rate
emission,
then
this
value
is
equivalent
to
the
release
rate
for
long­
term
modeling,
except
that
it
is
expressed
in
g/
s
instead
of
T/
yr.
(
To
convert
from
T/
yr
to
g/
s,
divide
by
34.73;
to
convert
from
g/
s
to
T/
yr,
multiply
by
34.73.)
If
the
release
is
intermittent,
such
as
a
batch
process,
this
value
is
equivalent
to
the
maximum
number
of
grams
emitted
during
any
hour
when
the
release
is
occurring
divided
by
12
3600.
Again
note
that,
for
Tier
1
analyses,
the
emissions
from
an
area
source
represent
the
total
emissions
from
that
source,
not
just
the
emissions
per
unit
surface
area.

2.
height
of
each
release
above
ground
(
m),
for
point
sources.

3.
source
types
(
point
or
area).
Point
sources
typically
include
exhaust
vents
(
pipes
or
stacks),
or
any
other
type
of
release
that
causes
toxic
materials
to
enter
the
atmosphere
from
a
well­
defined
location,
at
a
well­
defined
rate.
Area
sources
may
also
be
well­
defined,
but
differ
from
point
sources
in
that
the
extent
over
which
the
release
occurs
is
substantial.

4.
maximum
horizontal
distance
across
each
area
source
(
m).

5.
nearest
distance
to
property­
line
(
m).
Concentration
estimates
are
needed
at
locations
that
are
accessible
to
the
general
public.
This
is
typically
taken
to
be
any
point
at
or
beyond
the
property­
line
of
a
facility.
Estimate
the
distance
from
the
point
of
each
release
to
the
nearest
point
on
the
fenceline.
(
This
need
not
be
the
same
fenceline
point
for
each
release.)
If
the
source
is
characterized
as
an
area
source,
this
distance
should
be
measured
from
the
nearest
edge
of
the
area
source,
rather
from
than
the
center
of
the
area
source.

Once
these
five
items
are
determined
for
each
release,
screening
estimates
of
maximum
1­
hour
average
concentrations
resulting
from
each
release
are
obtained
from
Table
2
using
the
following
procedure.

1.
For
area
sources,
select
the
"
side
length"
in
the
table
(
10m,
20m,
30m)
which
is
less
than
or
equal
to
the
maximum
horizontal
distance
across
the
source.

2.
For
point
sources,
select
the
largest
"
emission
height"
in
the
table
(
0m,
2m,
5m,
10m,
35m,
or
50m)
that
is
less
than
or
equal
to
the
estimated
height
of
release.

3.
For
each
source,
select
the
largest
distance
in
the
table
(
10m,
20m,
50m,
100m,
or
200m)
that
is
less
than
or
equal
to
its
nearest
distance
to
the
property­
line.

4.
Take
the
normalized
maximum
1­
hour
average
concentration
for
this
release
and
fenceline
distance,
and
multiply
by
the
emission
rate
of
each
toxic
pollutant
(
g/
s)
in
the
release
to
obtain
the
maximum
off­
site
1­
hour
average
concentration
estimates
(

g/
m3).
DO
NOT
INTERPOLATE
TABLE
VALUES.

For
example,
again
consider
the
situation
in
which
a
toxic
material
A
is
released
from
a
vent­
pipe
that
is
40m
tall,
and
which
is
attached
to
a
building
that
is
4m
tall,
10m
long,
and
5m
wide.
The
nearest
boundary
of
the
facility
is
located
65m
from
the
pipe.
For
the
short­
term
assessment,
it
has
been
determined
that
the
maximum
emissions
of
A
that
can
occur
during
any
hour
of
the
year
is
1800g,
therefore
the
emission
rate
for
short­
term
assessment
is
1800g/
3600s
=
0.50
g/
s.
A
value
of
35m
is
again
selected
for
the
emission
height,
because
all
larger
entries
in
the
table
exceed
the
actual
height
of
release.
Concentrations
are
estimated
for
a
distance
of
50m,
because
once
again,
all
greater
entries
in
the
table
exceed
the
actual
distance
of
65m.
The
appropriate
normalized
maximum
1­
hour
average
concentration
is
3.94E+
2
(

g/
m3)/(
g/
s).
Multiplying
by
the
emission
rate
of
0.50
g/
s
results
in
a
maximum
hourly
concentration
estimate
for
screening
purposes
equal
to
197

g/
m3.

2.3.2
Acute
Hazard
Index
Assessment
For
all
pollutants
which
pose
a
threat
to
health
based
on
acute
exposure,
an
assessment
of
the
magnitude
of
this
threat
is
made
using
the
acute
hazard
index
approach,
similar
to
that
used
in
chronic
noncancer
risk
assessment.
In
this
case,
however,
the
acute
hazard
index
is
calculated
by
summing
the
maximum
1­
hour
concentrations
for
each
pollutant
divided
by
the
acute
concentration
threshold
value
for
that
pollutant.
It
should
again
be
noted
that,
for
the
sake
of
erring
conservatively,
this
approach
assumes
that
the
worst­
case
impacts
of
all
releases
can
occur
simultaneously
at
the
same
location.
Similar
to
the
chronic
risk
assessment,
if
the
calculated
hazard
index
is
greater
than
1.0,
the
release
or
releases
being
simulated
may
pose
a
significant
threat
to
the
public,
and
further
modeling
at
a
higher
Tier
may
be
indicated.
14
TABLE
2.
Normalized
Maximum
1­
hour
Average
Concentrations,
(
µ
g/
m3)/(
g/
s)

Emission
Side
Source
height,
length,
b
typea
m
m
Normalized
maximum
concentration
at
or
beyond:
c
10
m
30
m
50
m
100
m
200
m
500
m
A
A
A
P
P
P
P
P
P
P
0
0
0
0
2
5
10
20
35
50
10
20
30
­­
­­
­­
­­
­­
­­
­­
3.32E+
5
1.79E+
5
1.22E+
5
1.88E+
6
6.51E+
4
3.34E+
4
9.61E+
3
2.45E+
3
7.84E+
2
3.84E+
2
1.05E+
5
6.36E+
4
4.54E+
4
2.75E+
5
4.92E+
4
2.59E+
4
8.49E+
3
1.57E+
3
7.84E+
2
3.84E+
2
5.70E+
4
3.72E+
4
2.75E+
4
1.13E+
5
4.69E+
4
1.80E+
4
7.36E+
3
1.57E+
3
3.94E+
2
3.84E+
2
2.25E+
4
1.66E+
4
1.30E+
4
3.36E+
4
2.53E+
4
9.44E+
3
4.71E+
3
1.32E+
3
3.85E+
2
1.63E+
2
8.07E+
3
6.62E+
3
5.59E+
3
1.01E+
4
9.18E+
3
5.13E+
3
2.49E+
3
8.46E+
2
3.12E+
2
1.47E+
2
1.92E+
3
1.75E+
3
1.59E+
3
2.11E+
3
2.07E+
3
1.80E+
3
1.00E+
3
3.67E+
2
1.53E+
2
8.77E+
1
aSource
type
P=
Point
source,
type
A
=
Area
source.

bSide
length
of
square
area
source.

cDistance
downwind
of
an
area
source
indicates
distance
from
downwind
edge
of
the
area
source.
15
As
an
example
of
the
acute
hazard
index
approach,
consider
the
same
plant
being
simulated
in
Section
2.2.2,
but
this
time
the
maximum
1­
hour
concentrations
are
determined
using
the
procedure
in
Section
2.3.2
to
be
the
following:

Source
Compound
Max
1­
hr
impact
Stack
1
Pollutant
A
197

g/
m3
Stack
2
Pollutant
A
257

g/
m3
Stack
2
Pollutant
B
110

g/
m3
Stack
3
Pollutant
B
301

g/
m3
Stack
4
Pollutant
B
367

g/
m3
Further
suppose
that
pollutants
A
and
B
pose
health
problems
from
acute
exposures
with
acute
threshold
concentration
values
of
200
and
100

g/
m3,
respectively.
The
acute
hazard
index
is
calculated
as
follows:

Source
Compound
Max
1­
hr
impact
Hazard
index
Stack
1
Pollutant
A
197

g/
m3
0.985
Stack
2
Pollutant
A
257

g/
m3
1.285
Stack
2
Pollutant
B
110

g/
m3
1.100
Stack
3
Pollutant
B
301

g/
m3
3.010
Stack
4
Pollutant
B
367

g/
m3
3.670
­­­­­­­­­­­­­
Total
hazard
index
10.050
In
this
case,
4
of
the
individual
hazard
index
values
exceeds
1.0,
the
total
hazard
index
for
the
modeled
plant
exceeds
1.0,
and
further
modeling
at
a
higher
Tier
may
be
desired.
16
17
3.0
TIER
2
ANALYSES
3.1
Introduction
Tier
2
analysis
of
a
stationary
source
(
or
group
of
sources)
of
toxic
pollutant(
s)
may
be
desired
if
the
results
of
a
Tier
1
analysis
indicate
an
exceedance
of
a
level
of
concern
with
respect
to
one
or
more
of
the
following:
(
1)
the
maximum
predicted
cancer
risk;
(
2)
the
maximum
predicted
chronic
noncancer
hazard
index,
or;
(
3)
the
maximum
predicted
acute
hazard
index.
Note
that
in
situations
where
only
one
or
two
of
the
Tier
1
criteria
are
exceeded,
only
those
analyses
which
exceed
the
Tier
1
criteria
may
need
to
be
performed
at
the
higher
Tier.
For
example,
if
the
Tier
1
analysis
showed
cancer
risk
and
chronic
noncancer
risks
to
be
of
concern
while
the
acute
risk
analysis
showed
no
cause
for
concern,
only
long­
term
modeling
for
cancer
risk
and
chronic
noncancer
risk
may
need
to
be
performed
at
Tier
2.
Tier
2
analyses
are
slightly
more
sophisticated
than
Tier
1
analyses,
and
therefore
require
additional
input
information
as
well
as
a
computer
for
their
execution.
Tier
2
analyses
are
structured
around
the
EPA's
SCREEN
model
and
its
corresponding
documentation3.
The
SCREEN
model
source
code
and
documentation
is
available
through
the
OAQPS
TTN
(
see
Appendix
A).

Again,
similar
to
the
Tier
1
analysis,
if
any
of
the
predicted
impacts
from
Tier
2
are
above
the
appropriate
levels
of
concern,
further
modeling
is
indicated
at
a
higher
Tier.

3.2
Long­
term
Modeling
Long­
term
Tier
2
modeling
utilizes
the
SCREEN3
model
to
estimate
1­
hour
maximum
concentrations,
and
then
utilizes
a
conservative
conversion
factor
to
derive
maximum
annual
concentration
values
from
the
SCREEN
predictions16,17.
These
maximum
annual
concentration
estimates
are
used
to
assess
cancer
risk
and
chronic
noncancer
risk
exactly
as
in
Sections
2.2.2
and
2.2.3
of
this
document.

3.2.1
Maximum
Annual
Concentration
Estimation
In
addition
to
the
information
required
to
perform
a
Tier
1
long­
term
analysis,
a
Tier
2
analysis
requires
the
following
information:

1.
the
inside
diameter
of
the
stack
at
the
exit
point
(
m).

2.
the
stack
gas
exit
velocity
(
m/
s)

3.
the
stack
gas
exit
temperature
(
K)

4.
a
determination
of
whether
the
area
surrounding
the
modeled
facility
is
urban
or
rural.
This
is
usually
assessed
on
the
basis
of
land
use
in
the
vicinity
of
the
facility.
Refer
to
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6
for
additional
guidance
on
this
determination.

5.
downwash
potential.
Downwash
effects
must
be
included
in
dispersion
estimates
for
point
(
stack)
sources
whenever
the
point
of
release
is
located
on
the
roof
of
a
building
or
structure,
or
within
the
lee
of
a
nearby
structure.
The
potential
for
downwash
is
determined
in
the
*
Note:
The
maximum
horizontal
dimension
is
defined
as
the
largest
possible
alongwind
distance
the
structure
could
occupy.

18
following
way.
First,
estimate
the
heights
and
maximum
horizontal
dimensions*
of
the
structures
nearest
the
point
of
release.
For
each
structure,
determine
which
of
these
two
dimensions
is
less,
and
call
this
length
L.
If
the
structure
is
less
than
5L
away
from
the
source,
then
this
structure
may
cause
downwash.
For
every
structure
satisfying
this
criterion,
calculate
a
height
by
multiplying
L
by
1.5,
and
adding
this
to
the
actual
height
of
the
structure.
If
any
calculated
height
exceeds
the
height
of
the
release,
then
downwash
calculations
must
be
made
for
that
release.

Once
these
items
are
determined
for
each
release
being
modeled,
estimates
of
maximum
concentrations
from
each
release
are
obtained
through
individual
SCREEN
runs
for
each
release.
Recommendations
for
each
SCREEN
run
are
as
follows:

1.
The
emission
rates
used
for
Tier
1
long­
term
modeling
should
be
converted
from
T/
yr
to
g/
s
(
divide
T/
yr
by
34.73).
Area
source
emission
rates
should
be
converted
to
g/
s/
m2
by
dividing
by
the
total
area
of
the
source.

2.
Choose
the
default
atmospheric
temperature
of
293K.

3.
For
each
release,
exercise
the
automated
distance
array
choosing
as
the
minimum
receptor
distance
the
appropriate
nearest
fenceline
distance
for
that
release,
and
choosing
50
km
as
the
maximum
receptor
distance.
The
maximum
concentration
for
that
release
will
then
be
chosen
as
the
maximum
at
or
beyond
the
nearest
fenceline
distance.

4.
The
option
for
flagpole
receptors
should
not
be
used.

5.
For
each
release,
the
maximum
1­
hour
concentration
should
be
noted.

6.
Maximum
annual
concentrations
should
be
calculated
for
each
release
by
multiplying
predicted
maximum
1­
hour
concentrations
by
0.08.

As
an
example
of
the
Tier
2
long­
term
analysis,
consider
Stack
1
from
the
Tier
1
example.
To
consider
downwash
possibilities,
the
maximum
horizontal
dimension
is
first
estimated
as
{(
10m)
2
+
(
5m)
2}
1/
2
=
11.2m.
The
dimension
L
is
then
4m,
and
the
maximum
stack
height
for
which
downwash
is
possible
would
be
4m
+
1.5
X
4m
=
10m.
Since
the
actual
stack
height
is
40m,
downwash
need
not
be
considered
in
the
SCREEN
simulation.
The
emission
rate
specified
in
the
example
of
14.6
T/
yr
is
converted
to
g/
s
to
be
used
in
the
SCREEN
simulation,
resulting
in
an
emission
rate
of
14.6/
34.73
=
0.42
g/
s.
In
addition
to
the
actual
stack
height
(
40m)
and
minimum
fenceline
distance
(
65m),
input
parameters
for
the
SCREEN
simulation
are:

Inside
stack
diameter
0.5m
Stack
gas
exit
velocity
5.6
m/
s
Stack
gas
exit
temperature
303
K
Plant
location
urban
The
results
from
the
SCREEN
simulation
indicate
that
the
maximum
1­
hour
concentration
at
or
beyond
65m
is
32.5

g/
m3,
occurring
165m
downwind.
Using
the
recommended
conversion
factor
19
of
0.08,
the
maximum
annual
concentration
is
estimated
as
2.6

g/
m3
(
this
value
can
be
contrasted
with
the
Tier
1
estimation
of
16.5

g/
m3
).

3.2.2
Cancer
Risk
Assessment
Maximum
annual
concentrations
for
all
releases
of
carcinogens
should
be
multiplied
by
the
appropriate
unit
cancer
risk
factor
and
summed
to
estimate
the
maximum
cancer
risk.
It
should
be
noted
that
this
approach,
as
in
Tier
1,
presumes
that
all
worst­
case
impacts
occur
at
the
same
location.
While
this
assumption
may
not
be
very
realistic,
it
does
help
to
insure
that
the
results
of
a
Tier
2
analysis
are
conservative
and
therefore
protective
of
the
public.
More
receptor­
specific
risk
calculations
are
addressed
in
the
Tier
3
analyses.

Borrowing
again
from
the
Tier
1
example,
maximum
annual
impacts
for
each
source
and
pollutant
combination
are
estimated
using
the
SCREEN
model.
Risk
estimates
are
then
made
by
summing
the
risk
due
to
each
release,
regardless
of
downwind
distance
to
maximum
impact.
The
results
are:

Source
Compound
Max
impact
Max
risk
Stack
1
Pollutant
A
2.60

g/
m3
2.60
X
10­
7
Stack
2
Pollutant
A
1.34

g/
m3
1.34
X
10­
7
Stack
2
Pollutant
B
0.58

g/
m3
1.16
X
10­
7
Stack
3
Pollutant
B
0.62

g/
m3
1.24
X
10­
7
Stack
4
Pollutant
B
3.70

g/
m3
7.40
X
10­
7
­­­­­­­­­­­­
Total
risk
1.38
X
10­
6
For
this
example,
the
maximum
lifetime
cancer
risk
estimated
using
the
Tier
2
methods
is
a
factor
of
6
lower
than
that
estimated
in
the
Tier
1
analysis.
However,
the
cancer
risk
level
still
exceeds
1
X
10­
6,
indicating
that
modeling
at
a
higher
Tier
may
be
desirable.

3.2.3
Chronic
Noncancer
Risk
Assessment
As
in
Tier
1,
maximum
annual
concentrations
are
divided
by
their
chronic
concentration
threshold
values
and
summed
to
calculate
the
hazard
index
values.
Again,
this
approach
conservatively
assumes
that
all
worst­
case
impacts
occur
at
the
same
location.

Continuing
with
the
example,
the
chronic
noncancer
hazard
index
is
recalculated
using
the
Tier
2
estimated
long­
term
impacts.
Threshold
concentration
values
for
chronic
noncancer
effects
again
are
taken
as
20.0
and
5.0

g/
m3
for
pollutants
A
and
B,
respectively.
The
following
results:

Source
Compound
Max
impact
Hazard
index
Stack
1
Pollutant
A
2.60

g/
m3
0.130
Stack
2
Pollutant
A
1.34

g/
m3
0.067
Stack
2
Pollutant
B
0.58

g/
m3
0.116
Stack
3
Pollutant
B
0.62

g/
m3
0.124
Stack
4
Pollutant
B
3.70

g/
m3
0.740
­­­­­­­­­­­­
Total
hazard
index
1.177
20
The
chronic
noncancer
hazard
index
estimated
in
Tier
2
is
a
good
deal
less
than
that
estimated
for
the
same
sources
in
Tier
1.
Even
though
none
of
the
individual
source/
pollutant
combinations
exceeds
a
chronic
threshold
concentration
value,
the
total
hazard
index
exceeds
1.0,
and
further
analysis
at
Tier
3
is
indicated
for
chronic
noncancer
effects.

3.3
Short­
term
Modeling
Short­
term
Tier
2
modeling
utilizes
the
SCREEN3
model
to
estimate
1­
hour
maximum
concentrations
directly.
These
maximum
1­
hour
concentration
estimates
are
used
to
assess
acute
hazard
index
values
exactly
as
in
Section
2.3.2
of
this
document.

3.3.1
Maximum
Hourly
Concentration
Estimation
In
addition
to
the
information
required
to
perform
a
Tier
1
short­
term
analysis,
a
Tier
2
analysis
requires
the
following
information
for
stack
sources:

1.
the
inside
diameter
of
the
stack
at
the
exit
point
(
m).

2.
the
stack
gas
exit
velocity
(
m/
s)

3.
the
stack
gas
exit
temperature
(
K)

4.
a
determination
of
whether
the
area
surrounding
the
modeled
facility
is
urban
or
rural.
This
is
usually
assessed
on
the
basis
of
land
use
in
the
vicinity
of
the
facility.
Refer
to
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6
for
additional
guidance
on
this
determination.

5.
downwash
potential.
Downwash
effects
must
be
included
in
dispersion
estimates
for
point
sources
whenever
the
point
of
release
is
located
on
the
roof
of
a
building
or
structure,
or
within
the
lee
of
a
nearby
structure.
The
potential
for
downwash
is
determined
in
the
following
way.
First,
estimate
the
heights
and
maximum
horizontal
dimensions
of
the
structures
nearest
the
point
of
release.
For
each
structure,
determine
which
dimension
is
less,
and
call
this
length
L.
If
the
structure
is
less
than
5L
away
from
the
source,
then
this
structure
may
cause
downwash.
For
every
structure
satisfying
this
criterion,
calculate
a
height
by
multiplying
L
by
1.5,
and
adding
this
to
the
actual
height
of
the
structure.
If
any
calculated
height
exceeds
the
height
of
the
release,
then
downwash
calculations
must
be
used
for
that
release.

Once
these
items
are
determined
for
each
release
being
modeled,
estimates
of
maximum
concentrations
from
each
release
are
obtained
through
individual
SCREEN
runs
for
each
release.
Recommendations
for
each
SCREEN
run
are
as
follows:

1.
Choose
the
default
atmospheric
temperature
of
293K.

2.
Area
source
emission
rates
reflect
the
total
emission
rate
from
divided
by
the
area
of
the
source.

3.
For
each
release,
exercise
the
automated
distance
array
choosing
as
the
minimum
receptor
distance
the
appropriate
nearest
fenceline
distance
for
that
release,
and
choosing
50
km
as
the
maximum
receptor
distance.
The
maximum
concentration
for
that
release
will
then
be
chosen
as
the
maximum
at
or
beyond
the
nearest
fenceline
distance.

4.
The
option
for
flagpole
receptors
should
not
be
used.
21
5.
For
each
release,
the
maximum
1­
hour
concentration
should
be
noted.

Using
this
approach
with
the
Stack
1
example,
the
SCREEN
model
is
exercised
with
the
stack
parameters
specified
in
Section
3.2.1.
The
maximum
short­
term
emission
rate
of
0.50
g/
s
(
see
Section
2.3.1),
however,
is
used
to
estimate
the
maximum
1­
hour
source
impact.
The
results
of
the
SCREEN
model
indicate
that
the
maximum
1­
hour
concentration
is
38.8

g/
m3,
again
occurring
165m
downwind.

3.3.2
Acute
Hazard
Index
Assessment
As
in
Tier
1,
maximum
1­
hour
concentrations
are
divided
by
their
acute
threshold
concentration
values
and
summed
to
calculate
the
acute
hazard
index
values.
Again,
this
approach
conservatively
assumes
that
all
worst­
case
impacts
can
occur
simultaneously
at
the
same
location.

To
illustrate
this
procedure,
short­
term
impacts
from
the
example
plant
are
assessed
using
the
hazard
index
approach.
Again
the
acute
threshold
concentration
values
are
taken
as
200
and
100

g/
m3,
respectively.
The
results
are:

Source
Compound
Max
1­
hr
impact
Hazard
index
Stack
1
Pollutant
A
34.8

g/
m3
0.174
Stack
2
Pollutant
A
70.5

g/
m3
0.352
Stack
2
Pollutant
B
29.9

g/
m3
0.299
Stack
3
Pollutant
B
50.0

g/
m3
0.500
Stack
4
Pollutant
B
60.4

g/
m3
0.604
­­­­­­­­­­­­­
Total
hazard
index
1.925
For
this
example,
the
acute
hazard
index
estimated
in
Tier
2
is
roughly
20%
of
that
estimated
for
the
same
sources
in
Tier
1.
However,
since
the
total
hazard
index
exceeds
1.0,
further
analysis
at
Tier
3
is
indicated
for
health
effects
resulting
from
acute
exposures.
22
4.0
TIER
3
ANALYSES
4.1
Introduction
Tier
3
analysis
of
a
stationary
source
(
or
group
of
sources)
of
toxic
pollutant(
s)
may
be
desired
if
the
results
of
a
Tier
2
analysis
indicate
an
exceedance
of
a
level
of
concern
with
respect
to
one
or
more
of
the
following:
(
1)
the
maximum
predicted
cancer
risk;
(
2)
the
maximum
predicted
chronic
noncancer
hazard
index,
or;
(
3)
the
maximum
predicted
acute
hazard
index.
Tier
3
analysis
of
a
stationary
source
(
or
group
of
sources)
of
toxic
pollutant(
s)
is
performed
to
provide
the
most
scientifically­
refined
indication
of
the
impact
of
that
source.
This
Tier
involves
the
utilization
of
sitespecific
source
and
plant
layouts
as
well
as
meteorological
information.
In
contrast
to
the
previous
Tiers,
Tier
3
allows
for
a
more
realistic
simulation
of
intermittent
sources
and
combined
source
impacts.
In
addition,
results
from
short­
term
analyses
indicate
not
only
if
a
risk
level
of
concern
can
be
exceeded,
but
how
often
that
level
of
concern
might
be
exceeded
during
an
average
year.
Dispersion
modeling
for
the
Tier
3
analysis
procedure
is
based
on
use
of
the
EPA's
Industrial
Source
Complex
(
ISC2)
model18,
and
as
such
utilizes
many
of
the
same
techniques
recommended
in
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6
approach
to
the
dispersion
modeling
of
criteria
pollutants.

To
facilitate
the
dispersion
modeling
of
toxic
air
pollutants,
the
EPA
has
developed
TOXLT
(
TOXic
modeling
system
Long­
Term)
5
for
refined
long­
term
analyses,
and
TOXST
(
TOXic
modeling
system
Short­
Term)
4
for
refined
short­
term
analyses.
The
TOXLT
system
incorporates
the
ISCLT2
(
long­
term)
directly
to
calculate
annual
concentrations
and
the
TOXST
system
incorporates
the
ISCST2
(
short­
term)
model
directly
to
calculate
hourly
concentrations.
Codes
and
user's
guides
for
both
TOXLT
and
TOXST
are
available
via
electronic
bulletin
board
(
see
Appendix
A).

4.2
Long­
term
Modeling
Long­
term
Tier
3
modeling
uses
the
TOXLT5
modeling
system
to
estimate
maximum
annual
concentrations
and
maximum
cancer
risks.
The
TOXLT
modeling
system
uses
the
ISCLT2
model
to
calculate
these
annual
concentrations
at
receptor
sites
which
are
specified
by
the
user.
A
postprocessor
called
RISK
subsequently
calculates
lifetime
cancer
risks
and
chronic
noncancer
hazard
index
values
at
each
receptor.

4.2.1
Maximum
Annual
Concentration
Estimation
In
addition
to
the
information
required
to
perform
a
Tier
2
long­
term
analysis,
the
Tier
3
long­
term
analysis
requires
the
following
information:

1.
five
years
of
meteorological
data
from
the
nearest
National
Weather
Service
(
NWS)
station.
These
data
are
for
the
most
recent,
readily­
available
consecutive
five
year
period.
NWS
data
are
available
through
the
electronic
bulletin
board
(
see
Appendix
A).
Alternatively,
one
or
more
years
of
meteorological
data
from
on­
site
measurements
may
be
substituted.
These
data
should
be
obtained
and
quality­
assured
using
procedures
consistent
with
the
"
Guideline
on
Air
Quality
Modeling
(
Revised)"
6.

2.
plant
layout
information,
including
all
emission
point
and
fenceline
locations.
This
information
should
be
sufficiently
detailed
to
allow
the
modeler
to
specify
emission
point
and
fenceline
receptor
locations
within
2
meters.

3.
pollutant­
specific
data
concerning
deposition
or
decay
half­
life,
if
applicable.
23
Once
these
data
have
been
obtained,
an
input
file
should
be
prepared
for
execution
of
the
ISCLT2
model
using
the
guidance
available
in
the
ISC2
User's
Guide18.
The
ISCLT2
model
should
then
be
executed
using
the
TOXLT
system.
Procedures
utilized
should
also
be
consistent
with
the
TOXLT
User's
Guide5
(
available
via
electronic
bulletin
board,
see
Appendix
A).
Specific
recommendations
concerning
the
development
of
these
inputs
include:

1.
Annual
emission
rates
should
be
converted
to
g/
s
for
input.
The
TOXLT
modeling
system
uses
"
base
emission
rates"
and
"
emission
rate
multipliers"
to
specify
the
emission
rate
for
each
pollutant/
source
combination.
Thus,
for
a
given
pollutant
and
source
the
emission
rate
equals
the
base
emission
rate
(
specified
in
the
ISCLT2
input
file)
times
the
emission
rate
multiplier
for
that
pollutant/
source
combination
(
specified
in
the
RISK
input
file).
In
general,
the
input
file
to
the
ISCLT2
program
should
specify
the
same
emission
rates
used
in
previous
modeling
tiers
for
each
source,
and
emission
rate
multipliers
of
1.0
should
then
be
provided
as
inputs
to
the
RISK
post­
processor.
(
This
doesn't
necessarily
have
to
be
the
case,
as
long
as
the
product
of
the
emission
rate
provided
as
input
to
ISCLT2
and
the
emission
rate
multiplier
provided
as
input
to
RISK
equals
the
actual
emission
rate
being
modeled
for
each
source.)
In
the
case
where
more
than
one
pollutant
is
being
emitted
from
the
same
source,
that
source
should
only
be
included
once
in
the
ISCLT2
input
file,
and
emission
rate
multipliers
should
be
provided
to
the
RISK
post­
processor
for
each
pollutant
being
emitted
from
that
source.

2.
In
general,
each
source
should
be
modeled
as
a
single
ISCLT2
source
group.
However,
all
sources
of
a
single
pollutant
may
be
grouped
into
a
single
ISCLT2
source
group.
Each
source
of
more
than
one
pollutant
should
be
modeled
as
a
single
ISCLT2
source
group
by
itself.

3.
Input
switches
to
the
ISCLT2
model
should
be
set
to
allow
the
creation
of
the
master
file
inventory
for
post­
processing.
The
regulatory
default
mode
should
be
used.
Choose
the
printed
output
option
for
tabulating
the
greatest
impacts
of
each
source.

4.
STability
ARray
(
STAR)
summaries
of
the
NWS
meteorological
data
should
be
created
using
the
STAR
program
(
this
program
and
a
description
of
its
use
are
available
on
the
electronic
bulletin
board,
see
Appendix
A).
These
should
be
included
in
the
input
file
according
to
the
ISCLT2
User's
Guide.

5.
A
polar
or
rectangular
receptor
grid
may
be
used,
but
with
sufficient
detail
to
accurately
estimate
the
highest
concentrations.
The
design
of
the
receptor
network
should
consider
the
long­
term
results
of
the
earlier
modeling
tiers
such
that
the
highest
resolution
of
receptors
is
in
the
vicinity
of
the
highest
predicted
impacts.
Additional
receptors
may
need
to
be
added
in
sufficient
detail
to
accurately
resolve
the
highest
concentrations.

6.
Where
appropriate,
direction­
specific
building
downwash
dimensions
should
be
included
for
each
radial
direction.

The
printed
ISCLT2
output
will
indicate
the
top
10
impacts
for
each
source
group,
while
the
master
file
inventory
will
contain
all
of
the
annual
concentration
predictions
from
each
source
group
at
each
receptor.

Continuing
with
the
example
from
Tiers
1
and
2,
TOXLT
was
utilized
to
perform
site­
specific
ISCLT2
dispersion
modeling
for
the
4
stacks
in
the
example.
Each
of
the
stacks
was
modeled
as
an
individual
source
group.
A
STAR
summary
of
five
years
of
meteorological
data
from
the
nearest
NWS
site
was
utilized
along
with
specific
source
and
plant
boundary
locations
according
to
Figure
1
below.
Stacks
are
represented
in
the
Figure
as
open
circles,
with
stacks
3
and
4
located
at
the
same
24
Stacks
Plant
Boundary
N
1
2
3,4
W
Y
Z
X
50
meters
Figure
1.
Schematic
of
Example
Facility
with
Long­
Term
Impact
Locations
place.
A
rectangular
receptor
grid
(
indicated
by
the
filled
circles)
with
50m
spacing
outside
the
plant
boundary
was
used
to
obtain
concentration
predictions.
Neither
pollutant
was
presumed
to
decompose
in
the
atmosphere.

The
results
of
the
dispersion
modeling
indicated
the
following
maximum
annual
off­
site
concentrations
for
each
of
the
source/
pollutant
combinations:

Source
Compound
Max
impact
Location
Stack
1
Pollutant
A
.788

g/
m3
X
Stack
2
Pollutant
A
.305

g/
m3
Y
Stack
2
Pollutant
B
.131

g/
m3
Y
Stack
3
Pollutant
B
.172

g/
m3
Z
Stack
4
Pollutant
B
.976

g/
m3
Z
It
should
be
noted
that
the
maximum
concentrations
from
each
source/
receptor
combination
were
not
co­
located.
The
positions
of
the
maximum
concentration
from
each
source
are
indicated
on
Figure
1
corresponding
to
the
letters
X,
Y,
and
Z
in
the
table
above.
In
general,
the
Tier
3
maximum
concentration
values
are
25
to
30%
as
high
as
the
Tier
2
values.

4.2.2
Cancer
Risk
Assessment
Concentrations
from
the
ISCLT2
master
file
inventory
are
used
by
the
RISK
post­
processor
to
calculate
cancer
risks
at
each
receptor
site
in
the
ISCLT2
receptor
array.
RISK
can
then
provide
summaries
of
the
calculated
risks
according
to
user
specifications.
Use
of
the
RISK
post­
processor
requires
the
following
considerations:
25
1.
As
stated
above,
emission
rate
multipliers
for
each
pollutant
from
each
source
should
be
provided
as
inputs
to
the
RISK
post­
processor
such
that
the
product
of
the
base
emission
rate
input
to
ISCLT2
and
the
emission
rate
multiplier
input
to
RISK
equals
the
emission
rate
being
modeled.

2.
Unit
cancer
risk
factors
are
provided
to
RISK
either
in
the
RISK
post­
processor
input
file
or
through
an
interactive
process
in
TOXLT.

3.
The
RISK
post­
processor
output
options
should
be
exercised
to
provide
the
total
cancer
risk
at
each
receptor
due
to
all
pollutants,
as
well
as
individual
pollutant
or
source
contributions
to
these
receptor­
specific
risks.

If
the
maximum
predicted
lifetime
cancer
risk
in
the
receptor
grid
is
less
than
the
designated
level
of
concern
(
e.
g.,
1
X
10­
6),
placement
of
additional
receptors
in
the
ISCLT2
receptor
array
should
be
considered
as
a
means
of
ensuring
that
the
simulation
is
not
underestimating
maximum
risk.
If
the
maximum
cancer
risk
in
the
receptor
array
is
greater
than
the
designated
level
of
concern,
additional
runs
of
the
RISK
post­
processor
may
be
performed
using
reduced
emission
rate
multipliers
to
assess
the
impacts
of
possible
emission
control
scenarios.
If
the
analysis
shows
no
cancer
risk
greater
than
the
designated
level
of
concern
and
the
receptor
array
is
deemed
adequate,
the
modeled
source
is
considered
to
be
in
compliance
with
the
specified
criterion.
In
the
case
of
non­
compliance,
it
may
be
desirable
on
the
part
of
the
modeler
to
conduct
a
more
refined
analysis.
Section
5.0
if
this
document
discusses
some
of
the
possibilities
for
further
modeling
refinements.

The
output
of
the
RISK
post­
processor
for
the
example
plant
indicates
that
the
maximum
lifetime
cancer
risk
outside
the
plant
boundary
is
2.9
X
10­
7,
located
at
point
W
on
Figure
1.
Such
a
result
would
indicate
that
the
facility
would
not
cause
a
significant
cancer
risk
to
the
public,
according
to
the
cancer
risk
level
specified
by
the
CAAA
of
1990.

4.2.3
Chronic
Noncancer
Risk
Assessment
In
this
assessment,
concentrations
from
the
ISCLT2
master
file
inventory
are
used
by
the
RISK
post­
processor
to
calculate
chronic
noncancer
hazard
index
values
for
a
specific
noncancer
effect
at
each
receptor
site
in
the
ISCLT2
receptor
array.
RISK
can
then
provide
summaries
of
the
calculated
index
values
according
to
user
specifications.
A
separate
RISK
simulation
should
be
performed
for
each
chronic
noncancer
effect
being
considered.
Use
of
the
RISK
post­
processor
requires
the
following
considerations:

1.
As
stated
above,
emission
rate
multipliers
for
each
pollutant
from
each
source
should
be
provided
as
inputs
to
the
RISK
post­
processor
such
that
the
product
of
the
emission
rate
input
to
ISCLT2
and
the
emission
rate
multiplier
input
to
RISK
equals
the
actual
emission
rate
being
modeled.

2.
Chronic
threshold
concentration
values
for
the
specific
noncancer
effect
are
provided
to
RISK
either
in
the
RISK
post­
processor
input
file
or
through
an
interactive
process
in
TOXLT.

3.
The
RISK
post­
processor
output
options
should
be
exercised
to
provide
the
total
noncancer
hazard
index
at
each
receptor
due
to
all
pollutants,
as
well
as
individual
pollutant
or
source
contributions
to
these
receptor­
specific
hazard
indices.

If
the
maximum
hazard
index
value
in
the
receptor
grid
exceeds
1.0,
emission
reduction
scenarios
can
be
performed
(
again,
using
reduced
emission
rate
multipliers)
to
determine
how
this
26
hazard
index
value
can
be
reduced
below
1.0.
If
the
maximum
hazard
index
value
in
the
receptor
grid
does
not
exceed
1.0,
the
source(
s)
being
modeled
is
considered
to
be
in
compliance
with
the
specified
criteria.
In
the
case
of
non­
compliance,
it
may
be
desirable
on
the
part
of
the
modeler
to
conduct
a
more
refined
analysis.
Section
5.0
if
this
document
discusses
such
possibilities.

Using
the
chronic
noncancer
threshold
concentration
values
for
pollutants
A
and
B
of
20.0
and
5.0

g/
m3,
respectively,
the
RISK
post­
processor
was
exercised
for
the
example
facility
to
obtain
a
maximum
hazard
index
value
of
0.27
located
at
point
Z
on
Figure
1.
This
result,
which
is
approximately
30%
of
the
Tier
2
result,
would
indicate
that
the
facility
does
not
present
a
significant
chronic
noncancer
risk
in
its
current
configuration.

4.3
Short­
term
Modeling
Short­
term
Tier
3
modeling
uses
the
TOXST
modeling
system4
to
estimate
maximum
hourly
concentrations
and
the
receptor­
specific
expected
annual
number
of
exceedances
of
short­
term
concentration
thresholds.
For
multiple
pollutant
scenarios,
this
amounts
to
the
number
of
times
the
acute
hazard
index
value
exceeds
1.0.
The
model
uses
the
ISCST2
model
to
calculate
these
hourly
concentrations
at
receptor
sites
which
are
specified
by
the
user.
Acute
hazard
index
values
are
subsequently
calculated
at
each
receptor
by
the
TOXX
post­
processor,
in
which
a
Monte
Carlo
simulation
is
performed
for
intermittent
sources
to
assess
the
average
number
of
times
per
year
the
acute
hazard
index
value
exceeds
1.0
at
each
receptor.

4.3.1
Maximum
Hourly
Concentration
Estimation
In
addition
to
the
information
required
to
perform
a
Tier
2
analysis,
the
Tier
3
short­
term
analysis
requires
the
following
information:

1.
five
years
of
meteorological
data
from
the
nearest
National
Weather
Service
(
NWS)
station.
These
data
are
for
the
most
recent,
readily­
available
consecutive
five
year
period.
NWS
data
are
available
through
the
electronic
bulletin
board
(
see
Appendix
A).
Alternatively,
one
or
more
years
of
meteorological
data
from
on­
site
measurements
may
be
substituted.
These
data
should
be
obtained
and
quality­
assured
using
procedures
consistent
with
the
"
Guideline
on
Air
Quality
Modeling
(
Revised)"
6.

2.
plant
layout
information,
including
all
emission
point
and
fenceline
locations.
This
information
should
be
sufficiently
detailed
to
allow
the
modeler
to
specify
emission
point
and
fenceline
receptor
locations
within
2
meters
of
their
actual
locations.

3.
pollutant­
specific
data
concerning
deposition
or
reactivity,
if
applicable.

4.
source­
specific
data
concerning
the
annual
average
number
of
releases
and
their
duration
for
all
randomly­
scheduled
intermittent
releases.

Once
these
data
have
been
obtained,
an
input
file
should
be
prepared
for
execution
of
the
ISCST2
model
using
the
guidance
available
in
the
ISC2
User's
Guide18.
The
ISCST2
model
should
then
be
executed
using
the
TOXST
system.
Procedures
utilized
should
also
be
consistent
with
the
TOXST
User's
Guide5
(
available
through
the
electronic
bulletin
board,
see
Appendix
A).
Specific
recommendations
concerning
the
development
of
these
inputs
include:

1.
Maximum
hourly
emission
rates
are
used
for
the
analysis.
The
TOXST
modeling
system
uses
"
base
emission
rates"
and
"
emission
rate
multipliers"
to
specify
the
emission
rate
for
each
pollutant/
source
combination.
Thus,
for
a
given
pollutant
and
source
the
emission
rate
equals
27
pcutoff

LACT

n
i

1
(
Npol)
i
the
base
emission
rate
(
specified
in
the
ISCST2
input
file)
times
the
emission
rate
multiplier
for
that
pollutant/
source
combination
(
specified
in
the
TOXX
input
file).
The
input
file
to
the
ISCST2
program
should
contain
the
same
emission
rates
used
in
previous
modeling
tiers
for
each
source,
and
the
input
file
to
the
TOXX
post­
processor
should
be
provided
unit
emission
rate
multipliers
(
1.0).
If
more
than
one
pollutant
is
being
emitted
from
the
same
source,
that
source
may
be
included
once
in
the
ISCST2
input
file
with
a
unit
emission
rate
(
1.0)
and
the
individual
pollutant
emission
rates
may
be
provided
to
the
TOXX
post­
processor.
(
It
should
be
noted
that
this
may
complicate
the
interpretation
of
the
printed
ISCST2
output.
Alternatively,
multiple
pollutants
from
the
same
source
may
be
modeled
as
individual
sources
with
actual
emission
rates
in
ISCST2
and
unit
emission
rates
in
TOXX.
This
may
require
more
computing
time,
but
may
allow
direct
interpretation
of
concentration
predictions
in
the
ISCST2
printed
output.
Regardless
of
which
method
is
used,
the
modeler
should
take
care
that
the
product
of
the
emission
rate
used
in
ISCST2
and
the
emission
rate
used
in
TOXX
equals
the
emission
rate
of
the
pollutant
and
source
being
modeled.)

2.
All
continuous
sources
of
the
same
pollutant
should
be
modeled
as
one
ISCST2
source
group.
Each
intermittent
source
operating
independently
from
one
another
should
be
modeled
as
a
separate
ISCST2
source
group.
All
intermittent
sources
of
the
same
pollutant
emitting
at
the
same
time
may
be
modeled
in
the
same
ISCST2
source
group.
However,
each
source
of
more
than
one
pollutant
should
be
modeled
as
a
source
group
by
itself.

3.
Input
parameters
in
the
ISCST2
input
file
should
be
set
in
accordance
with
the
TOXST
User's
Guide.
The
regulatory
default
mode
should
be
used.
The
ISCST2
output
options
should
be
chosen
to
provide
summary
results
of
the
top
50
impacted
receptors
for
each
source
group.
(
As
noted
earlier,
if
unit
emission
rates
are
being
used
in
ISCST2,
interpretation
of
the
concentration
impacts
as
absolute
may
be
inappropriate.)

4.
Meteorological
input
files
for
ISCST2
may
be
created
from
NWS
meteorological
data
using
the
RAMMET
program
(
this
program
and
a
description
of
its
use
are
available
on
the
electronic
bulletin
board,
see
Appendix
A).

5.
A
polar
or
rectangular
receptor
grid
may
be
used,
but
with
sufficient
detail
to
accurately
estimate
the
highest
concentrations
from
each
source.
The
design
of
the
receptor
network
should
consider
the
short­
term
results
of
the
earlier
modeling
tiers
such
that
the
highest
resolution
of
receptors
is
in
the
vicinity
of
the
highest
predicted
impacts.
Additional
receptors
may
need
to
be
added
in
sufficient
detail
to
accurately
resolve
the
highest
concentrations.

6.
Where
appropriate,
direction­
specific
building
downwash
dimensions
should
be
included
for
each
radial
direction.

7.
The
ISCST2
model
option
to
create
a
TOXFILE
output
for
post­
processing
should
be
chosen.
The
concentration
threshold
value
(
called
"
pcutoff")
used
to
reduce
the
size
of
this
binary
concentration
output
file
should
be
chosen
appropriately
to
eliminate
predicted
concentration
values
below
possible
concern.
Although
it
may
be
set
higher,
a
good
rule
of
thumb
for
setting
this
value
is:
28
Stacks
Plant
Boundary
N
1
2
3,4
R
Q
S
Figure
2.
Schematic
of
Example
Facility
with
Short­
Term
Impact
Locations
where
LACT
is
the
lowest
acute
concentration
threshold
value
in
the
group
of
pollutants
being
modeled,
and
Npol
i
is
the
number
of
pollutants
emitted
from
ISCST2
source
group
i.

The
printed
ISCST2
output
will
indicate
the
top
50
impacts
for
each
ISCST2
source
group,
and
the
TOXFILE
will
contain
all
of
the
concentrations
above
the
cutoff
value
from
each
ISCST2
source
group
at
each
receptor.

The
ISCST2
model
was
exercised
for
the
example
facility.
The
maximum
1­
hour
concentrations
for
each
source/
pollutant
combination
were
determined
to
be
as
follows:

Source
Compound
Max
impact
Location
Stack
1
Pollutant
A
34.5

g/
m3
Q
Stack
2
Pollutant
A
67.9

g/
m3
R
Stack
2
Pollutant
B
29.1

g/
m3
R
Stack
3
Pollutant
B
39.2

g/
m3
S
Stack
4
Pollutant
B
47.5

g/
m3
S
The
locations
of
the
predicted
maximum
1­
hour
concentrations
are
shown
in
Figure
2.
The
maximum
impacts
from
each
source
were
only
slightly
lower
than
those
from
the
Tier
2
analysis.

4.3.2
Acute
Hazard
Index
Exceedance
Assessment
Concentrations
from
the
ISCST2
master
file
inventory
are
used
by
the
TOXX
post­
processor
to
calculate
acute
hazard
index
values
for
each
hour
of
a
multiple­
year
simulation
period
at
each
receptor
site
in
the
ISCST2
receptor
array.
The
program
then
counts
the
number
of
times
a
hazard
index
value
exceeds
1.0
(
an
exceedance)
and
prints
out
a
summary
report
which
indicates
the
average
29
number
of
times
per
year
an
exceedance
occurs
at
each
receptor.
The
use
of
the
TOXX
postprocessor
requires
the
following
considerations:

1.
As
stated
above,
in
most
cases
unit
emission
rate
multipliers
for
each
pollutant
from
each
source
are
used
as
inputs
to
the
TOXX
post­
processor.

2.
Acute
threshold
concentration
values
are
provided
to
TOXX
as
the
health
effects
thresholds
in
the
TOXX
post­
processor
input
file.

3.
The
TOXX
output
option
should
be
chosen
to
output
the
exceedances
in
polar
grid
format.
Exceedance
counts
at
discrete
fenceline
receptors
will
appear
at
the
end
of
this
table
in
the
order
in
which
discrete
receptor
locations
were
input
to
ISCST2.

4.
If
only
one
pollutant
is
being
modeled,
the
additive
exceedance
calculation
option
should
not
be
chosen.
If
multiple
pollutants
are
being
modeled,
the
additive
exceedance
calculation
option
should
be
chosen.
The
TOXX
post­
processor
should
be
set
to
perform
400
or
more
simulation
years
(
maximum
1000).
Unless
otherwise
specified
by
EPA
guidance,
background
concentrations
for
toxic
air
pollutants
should
be
set
equal
to
0.

5.
The
frequency
of
operation
for
each
emission
source
is
specified
by
providing
values
for
the
probability
of
the
source
switching
on
and
the
duration
of
the
release.
For
each
continuous
emission,
the
probability
of
the
source
switching
on
is
1.0,
and
for
each
intermittent
emission
source,
the
probability
of
the
source
switching
on
is
equal
to
the
average
number
of
releases
per
year
divided
by
8760
(
the
number
of
hours
in
a
non­
leap
year).
The
duration
of
release
for
each
continuous
source
should
be
set
equal
to
1.0,
and
the
duration
of
release
for
each
intermittent
release
should
be
specified
as
the
nearest
integer
hour
which
is
not
less
than
the
release
duration.
(
For
example,
if
the
average
release
duration
is
less
than
1
hour,
the
duration
of
release
should
be
set
equal
to
1;
if
the
average
release
duration
is
3.2
hours,
the
duration
of
release
should
be
set
equal
to
4.0)

If
the
maximum
number
of
acute
hazard
index
exceedances
in
the
receptor
grid
is
less
than
some
specified
value
(
e.
g.,
0.1,
equivalent
to
an
average
of
1
hourly
exceedance
every
10
years,
or
0.5,
equivalent
to
an
average
of
1
hourly
exceedance
every
2
years),
the
modeled
source
is
considered
to
be
in
compliance
with
the
acute
threshold
concentration
criteria.
However,
resimulation
with
placement
of
additional
receptors
in
the
ISCST2
receptor
array
should
be
considered
as
a
means
of
assuring
that
the
simulation
is
not
underestimating
the
maximum
acute
hazard
index.
If
the
maximum
number
of
hazard
index
exceedances
in
the
receptor
array
is
greater
than
the
specified
value,
additional
runs
of
the
TOXX
post­
processor
with
reduced
emission
rate
multipliers
may
be
performed
to
assess
the
impacts
of
possible
emission
control
scenarios.
In
the
case
of
non­
compliance,
it
may
be
desirable
on
the
part
of
the
modeler
to
conduct
a
more
refined
analysis.
Section
5.0
of
this
document
discusses
such
possibilities.

The
TOXX
post­
processor
was
exercised
for
the
example
facility
using
the
results
from
the
ISCST2
simulation.
The
frequency
of
operation
for
each
source
ranged
from
0.14
to
0.84,
reflecting
the
actual
yearly
frequency
of
"
on"
time
for
each
source.
The
output
showed
that
a
few
of
the
receptors
at
or
very
near
the
fenceline
experienced
impacts
resulting
in
a
hazard
index
value
of
1.0
or
greater,
and
that
the
maximum
frequency
of
these
hazard
index
exceedances
was
0.20
along
the
northern
plant
boundary,
or
an
average
of
1
hourly
exceedance
every
five
years.
Comparing
this
result
with
the
Tier
2
result
indicates
that
the
hazard
index
rarely
exceeds
1.0
because
in
a
Tier
3
analysis
the
maximum
impacts
from
each
release
rarely
occur
at
the
same
place
and
time.
Depending
on
the
specified
compliance
criteria,
this
result
may
or
may
not
indicate
that
the
facility
causes
a
significant
health
risk
from
acute
exposures
in
its
current
configuration.
30
5.0
ADDITIONAL
DETAILED
ANALYSES
If
any
Tier
3
analyses
indicate
non­
compliance
with
any
of
the
user­
specified
criteria,
it
may
be
desirable
to
conduct
an
additional,
more
refined
analysis.
This
may
mean
the
use
of
on­
site
meteorological
data
or
it
may
mean
that
a
more
appropriate
modeling
procedure
is
deemed
applicable
for
the
specific
case.
The
determination
of
an
appropriate
alternative
modeling
procedure
can
only
be
made
in
a
manner
consistent
with
the
approach
outlined
in
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6.

In
some
cases,
the
EPA
may
allow
exposure
assessments
to
incorporate
available
information
on
actual
locations
of
residences,
potential
residences,
businesses,
or
population
centers
for
the
purpose
of
establishing
the
probability
of
human
exposure
to
the
predicted
levels
of
toxic
pollution
near
the
source
being
modeled.
In
such
cases,
use
of
the
Human
Exposure
Model
(
HEM
II)
19
with
the
ISCLT
dispersion
model
is
preferred.
Again,
if
the
use
of
other
modeling
procedures
is
desired,
the
approval
of
a
more
appropriate
alternative
modeling
procedure
can
only
be
made
in
a
manner
consistent
with
the
approach
outlined
in
Section
3.2
of
the
"
Guideline
on
Air
Quality
Models
(
Revised)"
6.
31
37
6.0
SUMMARY
OF
DIFFERENCES
BETWEEN
MODELING
TIERS
To
summarize
the
major
differences
between
the
3
modeling
tiers
described
in
this
document,
Table
3
below
briefly
lists
the
input
requirements,
output
parameters,
and
assumptions
associated
with
each
tier.
This
Table
may
be
used
to
quickly
determine
whether
a
given
scenario
may
be
modeled
at
any
particular
tier.
Within
each
tier,
cancer
unit
risk
estimates,
chronic
noncancer
concentration
thresholds,
and
acute
concentration
thresholds
are
required
to
convert
concentration
predictions
into
cancer
risks,
chronic
noncancer
risks,
and
acute
noncancer
risks,
respectively.

Modeling
Tier
Input
Requirements
Output
Parameters
Major
Assumptions
Tier
1
emission
rate,
stack
height,
minimum
distance
to
fenceline
maximum
off­
site
concentrations,
worst­
case
cancer
risk
or
worst­
case
noncancer
hazard
index
(
short­
and
long­
term)
Worst­
case
meteorology,
worst­
case
downwash,
worst­
case
stack
parameters,
short­
term
releases
occur
simultaneously,
maximum
impacts
co­
located,
cancer
risks
additive,
noncancer
risks
additive
Tier
2
emission
rate,
stack
height,
minimum
distance
to
fenceline,
stack
velocity,
stack
temperature,
stack
diameter,
rural/
urban
site
classification,
building
dimensions
for
downwash
calculation
maximum
offsite
concentrations,
worst­
case
cancer
risk
and/
or
worstcase
noncancer
hazard
index
(
short­
and
longterm
Worst­
case
meteorology,
short­
term
releases
occur
simultaneously,
maximum
impacts
co­
located,
cancer
risks
additive,
noncancer
risks
additive
Tier
3
emission
rate,
stack
height,
actual
fenceline
and
release
point
locations,
stack
velocity,
stack
temperature,
stack
diameter,
rural/
urban
site
classification,
local
meteorological
data,
receptor
locations
for
concentration
predictions,
frequency
and
duration
of
short­
term
(
intermittent)
releases
concentrations
at
each
receptor
point,
long­
term
cancer
risk
estimates,
chronic
noncancer
hazard
index
estimates
at
each
receptor
point,
annual
hazard
index
exceedance
rate
at
each
receptor
cancer
risks
additive,
noncancer
risks
additive
38
39
REFERENCES
1.
Environmental
Protection
Agency,
1988.
Glossary
of
Terms
Related
to
Health,
Exposure,
and
Risk
Assessment.
EPA­
450/
3­
88­
016.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

2.
Environmental
Protection
Agency,
1987.
The
Risk
Assessment
Guidelines
of
1986.
EPA­
600/
8­
87­
045.
United
States
Environmental
Protection
Agency,
Washington,
DC
20460.

3.
Environmental
Protection
Agency,
1992.
The
SCREEN
Model
User's
Guide.
EPA­
450/
4­
92­
006.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711
in
preparation).

4.
Environmental
Protection
Agency,
1992.
Toxic
Modeling
System
Short­
Term
(
TOXST)
User's
Guide.
EPA­
450/
4­
92­
002.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711
(
in
preparation).

5.
Environmental
Protection
Agency,
1992.
Toxic
Modeling
System
Long­
Term
(
TOXLT)
User's
Guide.
EPA­
450/
4­
92­
003.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711
(
in
preparation).

6.
Environmental
Protection
Agency,
1988.
Guideline
on
Air
Quality
Models
(
Revised).
EPA­
450/
2­
78­
027R.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

7.
Environmental
Protection
Agency,
1990.
User's
Guide
to
TSCREEN:
A
Model
for
Screening
Toxic
Air
Pollutant
Concentrations.
EPA­
450/
4­
90­
013.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

8.
Environmental
Protection
Agency,
1991.
Guidance
on
the
Application
of
Refined
Dispersion
Models
for
Air
Toxic
Releases.
EPA­
450/
4­
91­
007.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

9.
Catalano,
J.
A.,
D.
B.
Turner,
and
J.
H.
Novak,
1987.
User's
Guide
for
RAM
­­
Second
Edition.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

10.
Irwin,
J.
S.,
T.
Chico,
and
J.
A.
Catalano.
CDM
2.0
­­
Climatological
Dispersion
Model
­­
User's
Guide.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

11.
Environmental
Protection
Agency,
1991.
Procedures
for
Establishing
Emissions
for
Early
Reduction
Compliance
Extensions.
Draft.
EPA­
450/
3­
91­
012a.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

12.
Environmental
Protection
Agency,
1978.
Control
of
Volatile
Organic
Emissions
from
Manufacturers
of
Synthesized
Pharmaceutical
Products.
EPA­
450/
2­
78­
029.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.
40
13.
Environmental
Protection
Agency,
1980.
Organic
Chemical
Manufacturing
Volumes
1­
10.
EPA­
450/
3­
80­
023
through
028e.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

14.
Environmental
Protection
Agency,
1980.
VOC
Fugitive
Emissions
in
Synthetic
Organic
Chemicals
Manufacturing
Industry
­
Background
Information
for
Proposed
Standards.
EPA­
450/
3­
80­
033a.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

15.
Environmental
Protection
Agency,
1990.
Protocol
for
the
Field
Validation
of
Emission
Concentrations
from
Stationary
Sources.
EPA­
450/
4­
90­
015.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

16.
Pierce,
T.
E.,
Turner,
D.
B.,
Catalano,
J.
A.,
Hale,
F.
V.,
1982.
"
PTPLU:
A
Single
Source
Gaussian
Dispersion
Algorithm."
EPA­
600/
8­
82­
014.
United
States
Environmental
Protection
Agency,
Washington,
DC
20460.

17.
California
Air
Pollution
Control
Officers
Association
(
CAPCOA),
1987.
Toxic
Air
Pollutant
Source
Assessment
Manual
for
California
Air
Pollution
Control
District
and
Applications
for
Air
pollution
Control
District
Permits,
Volumes
1
and
2.
CAPCOA,
Sacramento,
CA.

18.
Environmental
Protection
Agency,
1987.
User's
Guide
for
the
Industrial
Source
Complex
(
ISC2)
Dispersion
Models,
Volumes
1,
2
and
3.
EPA­
450/
4­
92­
008a,
b,
and
c.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.

19.
Environmental
Protection
Agency,
1991.
Human
Exposure
Model
(
HEM­
II)
User's
Guide.
EPA­
450/
4­
91­
010.
United
States
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711.
41
APPENDIX
A
ELECTRONIC
BULLETIN
BOARD
ACCESS
INFORMATION
The
Office
of
Air
Quality
Planning
and
Standards
(
OAQPS)
of
the
EPA
has
developed
an
electronic
bulletin
board
network
to
facilitate
the
exchange
of
information
and
technology
associated
with
air
pollution
control.
This
network,
entitled
the
OAQPS
Technology
Transfer
Network
(
TTN),
is
comprised
of
individual
bulletin
boards
that
provide
information
on
OAQPS
organization,
emission
measurement
methods,
regulatory
air
quality
models,
emission
estimation
methods,
Clean
Air
Act
Amendments,
training
courses,
and
control
technology
methods.
Additional
bulletin
boards
will
be
implemented
in
the
future.

The
TTN
service
is
free,
except
for
the
cost
of
the
phone
call,
and
may
be
accessed
from
any
computer
through
the
use
of
a
modem
and
communications
software.
Anyone
in
the
world
wanting
to
exchange
information
about
air
pollution
control
can
access
the
system,
register
as
a
system
user,
and
obtain
full
access
to
all
information
areas
on
the
network
after
a
1
day
approval
process.
The
system
allows
all
users
to
peruse
through
information
documents,
download
computer
codes
and
user's
guides,
leave
questions
for
others
to
answer,
communicate
with
other
users,
leave
requests
for
technical
support
from
the
OAQPS,
or
upload
files
for
other
users
to
access.
The
system
is
available
24
hours
a
day,
7
days
a
week,
except
for
Monday,
8­
12
a.
m.
EST,
when
the
system
is
down
for
maintenance
and
backup.

The
model
codes
and
user's
guides
referred
to
in
this
document,
in
addition
to
the
document
itself,
are
all
available
on
the
TTN
in
the
bulletin
board
entitled
SCRAM,
short
for
Support
Center
for
Regulatory
Air
Models.
Procedures
for
downloading
these
codes
and
documents
are
also
detailed
in
the
SCRAM
bulletin
board.

Documentation
on
EPA­
approved
emission
test
methods
is
available
on
the
TTN
in
the
bulletin
board
entitled
EMTIC,
short
for
the
Emission
Measurement
Testing
Information
Center.
Procedures
for
reading
or
downloading
these
documents
are
also
detailed
in
the
EMTIC
bulletin
board.

The
TTN
may
be
accessed
at
the
phone
number
(
919)­
541­
5742,
for
users
with
1200
or
2400
bps
modems,
or
at
the
phone
number
(
919)­
541­
1447,
for
users
with
a
9600
bps
modem.
The
communications
software
should
be
configured
with
the
following
parameter
settings:
8
data
bits;
1
stop
bit;
and
no
(
N)
parity.
Users
will
be
asked
to
create
their
own
case
sensitive
password,
which
they
must
remember
to
be
able
to
access
the
network
on
future
occasions.
The
entire
network
is
menu­
driven
and
extremely
user­
friendly,
but
any
users
requiring
assistance
may
call
the
systems
operator
at
(
919)­
541­
5384
during
normal
business
hours
EST.
42
43
APPENDIX
B
REGIONAL
METEOROLOGISTS/
MODELING
CONTACTS
Ian
Cohen
James
W.
Yarbrough
Rebecca
Calby
EPA
Region
I
(
ATS­
2311)
EPA
Region
VI
(
6T­
AP)
EPA
Region
V(
5AR­
18J)
J.
F.
K.
Federal
Building
1445
Ross
Avenue
77
W.
Jackson
Boston,
MA
02203­
2211
Dallas,
TX
75202­
2733
Chicago,
IL
60604
FTS:
835­
3229
FTS:
255­
7214
FTS:
886­
6061
Com:
(
617)
565­
3225
Com:
(
214)
255­
7214
Com:
(
312)
886­
6061
E­
mail:
EPA9136
E­
mail:
EPA9663
E­
mail:
EPA9553
FAX:
FTS
835­
4939
Fax:
FTS
255­
2164
Fax:
FTS
886­
5824
Robert
Kelly
Richard
L.
Daye
Robert
Wilson
EPA
Region
II
EPA
Region
VII
EPA
Region
X
(
ES­
098)
26
Federal
Plaza
726
Minnesota
Avenue
1200
Sixth
Avenue
New
York,
NY
10278
Kansas
City,
KS
66101
Seattle,
WA
98101
FTS:
264­
2517
FTS:
276­
7619
FTS:
399­
1530
Com:
(
212)
264­
2517
Com:
(
913)
551­
7619
Com:
(
206)
442­
1530
E­
mail:
EPA9261
E­
mail:
EPA9762
E­
mail:
EPA9051
Fax:
FTS
264­
7613
Fax:
FTS
276­
7065
Fax:
399­
0119
Alan
J.
Cimorelli
Kevin
Golden
EPA
Region
III
(
3AM12)
EPA
Region
VIII
(
8ART­
TO)
841
Chestnut
Building
999
18th
Street
Philadelphia,
PA
19107
Denver
Place
­­
Suite
500
Denver,
CO
80202­
2405
FTS:
597­
6563
FTS:
776­
0952
Com:
(
215)
597­
6563
Com:
(
303)
293­
0952
E­
mail:
EPA9358
E­
mail:
EPA9853
Fax:
FTS
597­
7906
Fax:
FTS
330­
7559
Lewis
Nagler
Carol
Bohnenkamp
EPA
Region
IV
EPA
Region
IX
(
A­
2­
1)
345
Courtland
Street,
N.
E.
75
Hawthorne
Street
Atlanta,
GA
30365
San
Francisco,
CA
94105
FTS:
257­
3864
FTS:
484­
1238
Com:
(
404)
347­
2864
Com:
(
415)
744­
1238
E­
mail:
EPA9470
E­
mail:
EPA9930
Fax:
FTS
257­
5207
Fax:
FTS
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1076
44
45
APPENDIX
C
EMISSION
RATE
ESTIMATION
GUIDANCE
In
order
to
conduct
an
impact
assessment,
it
is
necessary
to
determine
emission
rates
of
each
pollutant
from
each
source
or
release
point
included
in
the
assessment.
Use
of
data
collected
from
properly
conducted
source
tests
is
normally
preferred.
However,
care
should
be
taken
to
ensure
that
emissions
data
are
collected
at
operating
rates
which
are
representative
of
the
time
period
for
health
effects
assessment.
Maximum
operating
rates
and
other
worst
case
conditions
best
reflect
peak
shortterm
emissions
for
assessment
of
acute
health
effects.
Conversely,
emissions
testing
conducted
at
average
operating
rates
and
conditions
are
a
better
representation
of
longer­
term
emissions
and
would
be
appropriate
for
evaluating
chronic
exposures.
It
is
also
important
to
verify
that
quality
assurance
procedures
were
established
and
followed
throughout
the
period
of
testing.
Where
this
is
difficult
to
establish,
data
should
be
used
cautiously.

Data
collected
from
material
balances
may
be
acceptable
for
certain
sources
but
may
be
of
poor
quality
where
differences
of
large
numbers
are
necessary
to
determine
emissions
(
e.
g.,
refinery
fugitive
emissions).
Emission
factors
from
AP­
42,1
based
upon
the
results
of
properly
conducted
source
tests
from
a
representative
sample
of
the
source
population
may
be
acceptable
for
estimating
emissions
from
large
groupings
of
sources
(
e.
g.,
area
sources),
but
generally
are
not
the
method
of
choice
for
estimating
emissions
from
individual
sources.
Exceptions
to
this
include
sources
such
as
storage
tanks
which
are
difficult
to
test
under
a
full
range
of
conditions
affecting
emissions
and
are
best
addressed
through
the
emissions
estimation
methodologies
in
AP­
42.

When
developing
testing
programs,
it
should
be
noted
that
EPA
Reference
Methods
are
available
for
only
a
limited
number
of
toxic
compounds.
2
However,
EPA
has
published
a
compilation
of
available
test
methods
for
toxic
compounds
which
should
be
consulted.
3
EPA
has
also
developed
a
validation
protocol
that
may
be
utilized
in
advance
to
validate
data
that
may
be
developed
by
industry
or
others.
4
Emission
factors
are
available
for
a
limited
number
of
compounds.
Literature
citations
for
air
toxic
emissions
are
found
in
EPA's
"
Crosswalk
for
Air
Toxic
Emission
Factors
(
XATEF).
EPA's
"
SPECIATE"
database
contains
speciation
profiles
which
may
also
be
applied
to
VOC
emissions
or
emission
factors
to
crudely
estimate
contributions
of
toxic
components.
In
most
cases,
data
in
XATEF
and
SPECIATE
are
based
upon
testing
of
limited
source
populations.
Generally,
these
data
should
not
be
used
for
estimating
individual
source
emissions.

Recent
guidance
has
been
issued
by
EPA
on
procedures
to
estimate
source
baseline
emissions
for
toxic
air
contaminants
from
selected
source
categories.
5,6
In
addition,
EPA
has
developed
guidance
on
estimating
emissions
from
area
sources
of
toxics
emissions.
7
Finally,
EPA
has
published
a
series
of
documents
for
point,
area
and
mobile
sources
to
aid
in
the
preparation
of
emission
inventories.
8,9,10,11
However,
considerable
latitude
exists
regarding
the
proper
use
of
this
guidance
when
developing
source
emission
estimates.
To
ensure
that
technically
credible
estimation
techniques
are
employed,
personnel
from
the
Emission
Factors
and
Methodologies
Section
in
the
Emissions
Inventory
Branch
of
EPA's
Office
of
Air
Quality
Planning
and
Standards
should
be
consulted
during
the
emission
estimation
process.

Appendix
C
References
46
1.
Compilation
of
Air
Pollutant
Emission
Factors,
Volume
I:
Stationary
Point
and
Area
Sources,
and
Volume
II:
Mobile
Sources,
Fourth
Edition,
and
Supplements,
AP­
42,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
1985
and
ff.

2.
Code
of
Federal
Regulations,
Title
40,
Part
60,
Appendix
A.
Part
61,
Appendix
B.

3.
Screening
Methods
for
the
Development
of
Air
Toxics
Emission
Factors,
EPA­
450/
4­
91­
021.
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711,
September
1991.

4.
Protocol
for
the
Field
Validation
of
Emission
Concentrations
from
Stationary
Sources,
EPA­
450/
4­
90­
015.
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC
27711,
April
1991.

5.
Procedures
for
Establishing
Emissions
for
Early
Reduction
Compliance
Extensions,
Volume
1
­­
Synthetic
Organic
Chemical
Manufacturing,
Ethylene
Oxide
Sterilization,
And
Chromium
Electroplating,
EPA­
450/
3­
91­
012a,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
July
1991.

6.
Procedures
for
Estimating
and
Allocating
Area
Source
Emissions
of
Air
Toxics,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
March
1989.

7.
Procedures
for
Emission
Inventory
Preparation,
Volume
I:
Emission
Inventory
Fundamentals,
EPA­
450/
4­
81­
026a,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
September
1981.

8.
Procedures
for
Emission
Inventory
Preparation,
Volume
II:
Point
Sources,
EPA­
450/
4­
81­
026b,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
September
1981.

9.
Procedures
for
Emission
Inventory
Preparation,
Volume
III:
Area
Sources,
EPA­
450/
4­
81­
026c,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
September
1981.

10.
Procedures
for
Emission
Inventory
Preparation,
Volume
IV:
Mobile
Sources,
EPA­
450/
4­
81­
026d,
U.
S.
Environmental
Protection
Agency,
Research
Triangle
Park,
NC,
July
1989.
47
TECHNICAL
REPORT
DATA
(
Please
read
Instructions
on
reverse
before
completing)

1.
REPORT
NO.

EPA­
450/
4­
92­
001
2.
3.
RECIPIENT'S
ACCESSION
NO.

4.
TITLE
AND
SUBTITLE
A
Tiered
Modeling
Approach
for
Assessing
the
Risks
Due
to
Sources
of
Hazardous
Air
Pollutants
5.
REPORT
DATE
6.
PERFORMING
ORGANIZATION
CODE
7.
AUTHOR(
S)

David
E.
Guinnup
8.
PERFORMING
ORGANIZATION
REPORT
NO.

9.
PERFORMING
ORGANIZATION
NAME
AND
ADDRESS
U.
S.
Environmental
Protection
Agency
Office
of
Air
Quality
Planning
and
Standards
Technical
Support
Division
Research
Triangle
Park,
NC
27711
10.
PROGRAM
ELEMENT
NO.

11.
CONTRACT/
GRANT
NO.

12.
SPONSORING
AGENCY
NAME
AND
ADDRESS
13.
TYPE
OF
REPORT
AND
PERIOD
COVERED
14.
SPONSORING
AGENCY
CODE
15.
SUPPLEMENTARY
NOTES
16.
ABSTRACT
This
document
provides
modeling
guidance
to
support
risk
assessments
as
applied
to
stationary
sources
of
hazardous
air
pollutants.
The
guidance
focuses
on
procedures
which
may
be
used
in
support
of
the
petition
processes
described
in
Title
III
of
the
Clean
Air
Act
Amendments
of
1990.
The
analysis
approach
described
herein
is
a
tiered
one,
in
which
each
subsequent
modeling
tier
requires
additional
site­
specific
information
to
produce
a
less
conservative
estimate
of
the
risk
associated
with
a
given
stationary
source
(
or
group
of
sources).
The
modeling
approach
begins
with
Tier
1
screening
tables
which
require
only
source
emission
rates,
stack
heights,
and
nearest
fenceline
distances
to
estimate
maximum
cancer
and/
or
noncancer
risks.
Tier
2
utilizes
additional
source
parameters
(
including
stack
diameter,
exit
gas
temperature
and
velocity,
and
nearby
building
dimensions)
with
the
SCREEN
computer
program
to
develop
more
refined
estimates
of
maximum
risks.
Tier
3
utilizes
site­
specific
meteorological
data,
plant
layout
information,
and
release
frequency
data
with
the
TOXST
and
TOXLT
computer
models
to
provide
additional
refinement
to
these
assessments.

17.
KEY
WORDS
AND
DOCUMENT
ANALYSIS
a.
DESCRIPTORS
b.
IDENTIFIERS/
OPEN
ENDED
TERMS
c.
COSATI
Field/
Group
Air
Pollution
Atmospheric
Dispersion
Modeling
Risk
Assessment
18.
DISTRIBUTION
STATEMENT
Release
Unlimited
19.
SECURITY
CLASS
(
Report)

Unclassified
21.
NO.
OF
PAGES
20.
SECURITY
CLASS
(
Page)

Unclassified
22.
PRICE
EPA
Form
2220­
1
(
Rev.
4­
77)
PREVIOUS
EDITION
IS
OBSOLETE
