1
Response
to
OMB
Issues
Population
risk
estimates
using
NATA
Estimated
NATA­
Derived
Population
Cancer
Risk
Estimated
Cancer
Risk
at
Least:
100
in
a
million
10
in
a
million
1
in
a
million
All
Sources
of
Perchloroethylene
0
13,649
8,589,338
Area
source
Perc
Dry
Cleaners
0
0
963,992
To
assess
the
risks
from
typical
area
sources,
we
first
analyzed
readily
available
data.
The
1999
National
Air
Toxics
Assessment
(
NATA)
provides
census
tract
level
estimates
of
inhalation
cancer
risk
and
non­
cancer
risk
across
the
entire
United
States
for
a
subset
of
the
188
HAP.
Using
this
assessment,
we
were
able
to
generate
a
course­
scale
estimate
of
population
risk
for
PCE
by
scaling
the
cancer
and
non­
cancer
risk
for
PCE
according
to
the
relative
contribution
of
area
source
dry
cleaners
to
the
total
emission
inventory
of
PCE.
See
table
xxx
below
for
a
summary
of
these
risks
from
both
area
source
cleaners
and
all
sources
of
PCE.

This
estimate
provided
a
screening­
level
estimate
of
PCE
risk
to
the
general
population.
In
estimating
this
population­
level
estimate
of
risk,
the
assessment
makes
certain
assumptions
about
personal
activity
patterns.
In
doing
so,
it
is
somewhat
less
conservative
than
an
assessment
that
considers
MIR
which
assumes
continuous
exposure
for
70
years.

Because
this
screening­
level
assessment
suggested
potentially
high
population
risk,
we
decided
to
pursue
a
more
refined
risk
assessment.

APEX
Assessment
To
account
for
the
fact
that
individuals
may
move
through
areas
(
microenvironments)
of
differing
concentrations
during
their
daily
activities,
EPA
conducted
an
exposure
variability
analyses
in
which
it
used
the
Total
Risk
Integrated
Methodology
Exposure
model
(
TRIM.
Expo,
also
known
as
the
Air
Pollutant
Exposure
Model
3,
or
APEX3).
The
TRIM.
Expo
model
uses
a
personal
profile
approach
in
which
it
stochastically
simulates
exposures
for
individuals
of
differing
demographic
characteristics
and
associated
daily
activity
patterns.
The
model
output
provides
a
distribution
of
exposure
estimates
which
are
intended
to
be
representative
of
the
study
population
with
respect
to
their
demographically
based
behavior,
in
terms
of
the
microenvironments
through
which
they
move
during
a
day
and
throughout
a
year
(
see
http://
www.
epa.
gov/
ttn/
fera
for
more
information
regarding
the
model).
To
estimate
cancer
risk,
EPA
assumes
that
this
one
year
exposure
scenario
continues
for
70­
years.
2
The
table
below
contrasts
the
ISCST­
3
and
TRIM
estimates
of
population
risk
for
the
worst­
case
major
source
facility:

Comparison
of
ISCST­
3
Exposure
Estimates
with
Activity­
patterned/
day,
lifetime
exposure
(
ISC+
TRIM.
Expo)­­
Cancer
Risk
Total
Population
at
Cancer
Risk
Model
>
1E­
4
>
1E­
5
>
1E­
6
Maximum
Value
ISCST­
3
900
14,000
75,000
3.3E­
04
(
MIR)

TRIM.
Expo
400
9,000
80,000
3.24E­
04
APEX
provides
a
more
central
tendency
estimate
of
risk
by
accounting
for
variability
in
personal
exposure.
The
table
above
shows
that
a
smaller
number
of
individuals
exposed
at
the
higher
levels
of
cancer
risk
and
a
slightly
larger
number
of
individuals
exposed
at
a
cancer
risk
of
at
least
one
in
one
million.
While
we
performed
this
analysis
for
the
worst­
case
facility,
it
is
reasonable
to
infer
that
the
risk
distribution
above
would
be
similar
to
the
remainder
of
the
major
source
facilities.
One
limitation
of
this
analysis
is
that
we
assume
continuous
70­
year
exposure
when
calculating
cancer
risk,
and
some
individuals
are
likely
to
move
away
from
the
facility.
However,
given
the
large
number
of
dry
cleaners
nation
wide,
and
the
consequent
ubiquity
of
exposure,
it
is
unlikely
that
individuals
moving
out
of
the
APEX
study
area
would
receive
lower
exposure
than
the
model
estimates.

Co­
located
exposure
assumptions
To
better
characterize
inhalation
cancer
risk
among
residents
of
apartments
co­
located
with
area
source
cleaners,
we
performed
a
sensitivity
analysis
in
which
it
varied
the
assumed
duration
of
exposure.
The
table
below
illustrates
the
results
of
this
analysis.
3
Estimated
Cancer
Risk
for
Residents
of
Co­
located
Apartments:
Exposure
Duration
Sensitivity
Analysis
Assumed
Exposure
Duration
70
Years
50
Years
30
Years
20
Years
10
Years
Risk
per
million
(
CalEPA)
11998
8588
5179
3474
1770
Risk
per
million
(
OPPTS)
1444
1034
623
418
213
Hazard
Quotient
(
ATSDR)
8
5
3
2
1
Risk
calculated
assuming:
(
1)
apartment
residents
receive
exposure
at
95th
percentile
of
monitored
concentrations
from
area
source
cleaners
for
duration
noted;
and
(
2)
exposure
falls
to
New
York
urban
background
(
11
ug/
m3)
for
PCE
during
years
in
which
residents
move
away
from
area
source
cleaner
The
results
above
indicate
that
even
when
EPA
relaxes
the
70­
year
exposure
duration
to
calculate
inhalation
cancer
risk,
individual
risk
remains
very
high.
