Highly
Exposed
Population
Background
Document
I.
Estimation
of
the
Number
of
Highly
Exposed
Individuals
in
the
United
States
from
the
Land
Application
of
Sewage
Sludge
The
number
of
people
estimated
to
be
in
the
modeled
highly
exposed
individual
(
HEI)

population
and
the
lifetime
and
annual
excess
potential
cancer
cases
predicted
for
this
population
were
derived
using
two
approaches.
The
first
approach
was
initially
presented
in
the
June
12,

2002
Notice
of
Data
Availability
(
NODA)
(
EPA,
2002)
and
is
reviewed
below.
The
second
approach
is
essentially
the
same
as
the
initial
approach
but
different
assumptions
were
made
regarding
the
amount
of
agricultural
land
that
receives
biosolids
(
400,000
hectares/
year
instead
of
800,000
hectares/
year)
and
the
portion
of
the
HEI's
diet
that
contributes
the
greatest
amount
to
the
total
dioxin
exposure
(
a
combination
of
beef
and
dairy
products
instead
of
only
beef).

The
HEI
population
estimate
initially
presented
in
the
NODA
was
derived
as
follows:

EPA's
Exposure
Factors
Handbook
(
EFH)
(
USEPA,
1997),
Table
13­
4,
estimates
that
approximately
four
percent
of
the
households
in
the
United
States
farm.
Assuming
that
the
number
of
people
per
household
is
the
same
for
both
farm
and
non­
farm
households,
this
equates
to
four
percent
of
the
United
States
Population
(
280
million)
living
on
farms
(
11.2
million).
Table
13­
18
of
the
EFH
estimates
that
52.4
percent
of
the
households
that
farm
consume
"
total
meats"

from
farm
raised
animals.
"
Total
meats"
include
beef,
pork,
veal,
lamb,
mutton,
goat,
game,
lunch
meat,
mixtures,
and
poultry.
The
multi­
pathway
probabilistic
risk
assessment's
results
indicate
that
approximately
95
percent
of
the
HEI's
daily
exposure
to
dioxins
is
attributable
to
the
consumption
of
beef
and
dairy
products
produced
on
the
farm
with
the
majority
of
that
exposure
attributed
to
the
consumption
of
beef.
Therefore,
in
order
to
be
conservative
for
the
purposes
of
2
estimating
the
number
of
people
in
the
modeled
HEI
population,
it
was
assumed
that
approximately
50
percent
of
the
farm
population
of
11.2
million
consumed
a
significant
portion
of
its
annual
"
total
meats"
diet
from
farm
raised
animals.
The
actual
fractions
of
food
intake
that
is
home
produced
(
from
Table
13­
71
from
the
EFH)
are
as
follows:

Beef
49%

Dairy
25%

Poultry
16%

Eggs
15%

Exposed
Fruits
33%

Exposed
Vegetables
42%

Belowground
Vegetables
17%

Multiplying
the
farm
population
of
11.2
million
by
50%,
which
approximates
the
percentage
of
people
who
consume
total
meats
(
52.4%)
from
their
farm,
equates
to
a
population
of
5.6
million.

Annual
production
of
sewage
sludge
in
the
United
States
is
estimated
to
be
approximately
eight
million
metric
tons
(
dry
weight
basis).
In
this
estimation,
it
was
assumed
that
all
8
million
metric
tons
are
applied
to
agricultural
lands
where
an
HEI
lives.
However,
this
assumption
is
an
over
estimation
since
sewage
sludge
is
also
disposed
of
through
incineration,
surface
disposal,
and
used
in
land
reclamation.
The
second
population
estimation
approach
adjusts
this
assumption
and
further
discusses
the
rationale
for
doing
so.
By
assuming
an
annual
agronomic
application
rate
of
10
metric
tons
(
dry
weight
basis)
of
sewage
sludge
per
hectare
on
agricultural
land,
the
maximum
number
of
hectares
with
land
applied
sewage
sludge
at
any
one
time
across
the
U.
S.
is
estimated
3
to
be
800,000
hectares.
According
to
the
1997
United
States
Department
of
Agriculture
(
USDA)

Farm
Census
(
USDA,
1997),
the
United
States
has
approximately
377
million
hectares
of
agricultural
land.
The
probability
of
the
5.6
million
people
that
farm
and
consume
a
significant
portion
of
their
total
meats
diet
from
farm
raised
animals
that
also
use
sewage
sludge
to
fertilize
their
crops
and
amend
their
soils
is
equal
to
the
ratio
of
the
potential
800,000
hectares
of
agricultural
land
receiving
sewage
sludge
annually
to
the
total
hectares
of
agricultural
land
in
the
United
States
(
377
million
hectares)
which
is
approximately
0.2
percent.
This
assumption
is
valid
if
both
the
modeled
HEI
farm
population
and
the
use
of
sewage
sludge
on
agricultural
lands
are
distributed
in
the
same
random
manner
throughout
the
United
States.
A
final
modeled
HEI
population
is
obtained
by
multiplying
the
5.6
million
people
by
0.2
percent
to
yield
a
final
HEI
population
of
11,200.

II.
Estimation
of
Cancer
Risk
and
Incidence
to
the
HEI
For
the
probabilistic
risk
model
(
Monte
Carlo)
simulations
that
were
run
on
the
HEI,
as
described
in
the
Technical
Background
Document
(
TBD)
(
USEPA,
2003a),
the
95%
exposure
as
expressed
by
a
lifetime
average
daily
dose,
LADD,
was
0.11
pg/
kg­
day.
An
upper­
bound
lifetime
excess
cancer
risk
for
this
95%
exposure
is
calculated
as
the
product
of
0.11
pg/
kg­
day
and
the
cancer
slope
factor
of
1.56x10­
4
(
pg/
kg­
day)­
1.
This
results
in
an
individual
lifetime
excess
cancer
risk
of
2x10­
5.
If
it
is
conservatively
assumed
that
this
95%
individual
cancer
risk
estimate
applies
to
the
entire
(
emphasis
added)
HEI
population
of
11,200,
then
an
estimate
of
0.2
cases
of
cancer
over
a
70­
year
lifetime
is
made,
and
an
annual
estimate
of
0.003
cases
(
i.
e.,
0.2/
70)
is
made.

However,
to
assess
the
risks
to
a
highly
exposed
population
(
not
individual)
such
as
the
4
HEI
population,
the
Agency's
Risk
Characterization
Guidance
(
USEPA
1992)
recommends
that
the
mean
(
or
median)
risk
estimate
be
used
to
estimate
excess
cancer
cases
within
the
HEI
population
as
a
whole.
Applying
the
median
cancer
risk
from
exposure
to
land
applied
biosolids,

which
is
1
x
10­
6,
to
the
population
of
11,200
estimate
for
the
HEI,
results
in
0.01
cases
of
cancer
in
a
lifetime
and
0.0001
cases
of
cancer
on
an
annual
basis.

III.
Consideration
of
Alternative
Assumptions
for
the
Size
and
Behaviors
of
the
HEI
Population
As
mentioned,
based
on
further
review
of
information
in
the
EFH,
a
second
alternative
approach
was
used
to
estimate
the
HEI
population
and
subsequent
potential
cancer
cases.
This
second
approach
yielded
lower
estimates
of
the
HEI
population
and
potential
cancer
cases.
As
indicated
above,
in
the
model
used
to
estimate
HEI
risk,
the
total
daily
intake
of
dioxins
for
the
HEI
is
driven
by
the
daily
consumption
of
dioxin
contaminated
beef
and
dairy
products
raised
on
the
farm
that
uses
sewage
sludge
for
fertilizer
or
soil
amendment.
The
risk
assessment
did
not
model
the
other
"
total
meats"
producing
farm
animals
because
animals
such
as
swine,
goat,
and
lamb
would
be
expected
to
be
raised
on
a
grain
diet
and
not
on
dioxin­
contaminated
pasture
or
forage
which
is
the
food
chain
source
for
dioxin
contamination
in
the
farm
animals.
As
a
note,

poultry
is
considered
and
modeled
separately
in
the
risk
assessment
as
a
contribution
to
the
dioxin
exposure
to
the
HEI
population.
Therefore,
by
only
considering
exposure
to
dioxin­
contaminated
beef
and
dairy,
lower
HEI
population
and
cancer
cases
are
obtained
as
follows.

Table
13­
36
of
the
EFH
estimates
that
38.9
percent
of
the
United
States
farm
population
consumes
a
significant
portion
of
their
beef
diet
from
beef
cattle
raised
on
their
farms.
Table
5
13­
28
estimates
that
13.9
percent
of
the
United
States
farm
population
consumes
a
significant
portion
of
their
dairy
products
from
dairy
cattle
raised
on
their
farms.
Therefore,
the
number
of
people
on
the
farm
that
consume
both
(
emphasis
added)
beef
and
dairy
products
from
their
farms
(
as
is
modeled
in
the
risk
assessment)
can
be
no
higher
than
the
number
of
people
who
consume
dairy
products
from
their
farms
(
13.9
percent).
Therefore,
13.9
percent
of
the
11.2
million
people
who
farm
equates
to
approximately
1.6
million
people.

In
the
initial
analysis,
it
was
assumed
that
100
percent
of
the
8,000,000
dry
metric
tons
of
sewage
sludge
produced
annually
in
the
United
States
are
land­
applied
only
to
farm
land
where
the
HEI
lives.
However,
only
54
percent
of
the
United
States'
annual
production
of
sewage
sludge
would
be
assumed
to
be
land­
applied
to
agricultural
lands
to
raise
both
food
for
direct
human
consumption
and
animal
feeds
for
raising
domesticated
animals
for
human
consumption,
as
well
as
non­
food
chain
lands
such
as
forests
and
disturbed
lands
such
as
strip
mines
for
reclamation,
parks,
and
lawns.
Thus,
substantially
less
than
50
percent
of
sewage
sludge
produced
annually
in
the
United
States
would
actually
be
applied
to
farms
modeled
in
the
risk
assessment.
However,
conservatively
assuming
that
50
percent
of
the
annual
production
of
sewage
sludge
(
4
million
metric
tons
(
dry
matter
basis))
is
applied
to
agricultural
lands
annually
at
the
assumed
agronomic
rate
of
up
to
10
dry
metric
tons
per
hectare
yields
a
total
sewage
sludge
amended
area
of
400,000
hectares.
This
then
would
represent
a
fraction
of
approximately
0.1
percent
of
all
agricultural
lands
amended
with
sewage
sludge.
Multiplying
this
by
the
1.6
million
people
that
live
on
farms
and
consume
a
significant
portion
of
their
dairy
diet
from
farm
raised
dairy
cattle
yields
what
could
be
considered
a
lower­
bound
estimate
of
the
HEI
modeled
population
of
1,600
people.
As
before,
the
analogous
upper­
bound
estimate
of
the
excess
lifetime
6
cancer
risk
due
to
exposure
to
dioxin
in
biosolids
to
the
HEI
(
using
the
95th
percentile
exposure)

is
2x10­
5.
The
corresponding
excess
cancer
cases
over
a
lifetime
and
annually
to
the
modeled
population
of
1,600
are
0.002
and
0.00003
cases,
respectively.
These
values
are
based
on
the
median
risk
to
the
HEI
population.
If
the
lifetime
and
annual
cancer
cases
for
the
HEI
population
are
calculated
using
the
conservative
95th
percent
exposure,
the
values
are
0.03
cases
and
0.00046
cases
respectively.

EPA
has
chosen
to
present
a
range
for
the
number
of
the
people
in
the
HEI
population
as
well
as
the
estimated
excess
lifetime
and
annual
cancer
cases.
One
could
view
the
lower­
bound
estimate
as
a
less
conservative
estimate
of
the
number
of
people
in
the
HEI
population
and
the
subsequent
cancer
cases
because:
1.
this
estimate
does
not
take
into
account
the
greater
incremental
exposure
to
dioxin
for
the
portion
of
the
farm
population
that
consumes
only
beef
from
their
farms
(
as
opposed
to
the
lower
percentage
that
consume
both
beef
and
dairy;
the
consumption
of
beef
creates
the
most
dietary
exposure
to
dioxin
for
the
HEI)
and
2.
it
does
not
account
for
the
farm
population
that
raises
and
consumes
other
animals
on
their
farms
besides
dairy
and
beef
(
even
though
the
consumption
of
those
animals
do
not
create
a
significant
additional
dioxin
exposure).

The
lower
estimates
also
do
not
account
for
the
potential
growth
in
the
use
of
sewage
sludge
on
agricultural
lands.
With
the
difficulties
in
building
new
sewage
sludge
incinerators
to
replace
current
units
that
have
reached
the
end
of
their
operational
life
or
cannot
meet
Part
503
Standards
or
State
standards
and,
therefore,
must
close,
and
with
the
difficulties
encountered
in
siting
and
building
new
landfills
as
a
method
of
sewage
sludge
disposal
(
Note:
some
landfills
refuse
to
accept
sewage
sludge),
there
is
a
potential
for
a
significantly
greater
percentage
of
7
sewage
sludge
generated
in
the
United
States
to
be
used
on
agricultural
lands
in
the
coming
decades
than
the
amount
that
is
currently
utilized
in
this
manner.

IV.
How
did
EPA
Analyze
the
Relative
Importance
of
Inputs
to
the
Risk
Model?

In
addition
to
the
revised
risk
assessment,
EPA
conducted
a
sensitivity
analysis
to
identify
the
effects
of
variability
and
uncertainty
in
the
risk
model
on
the
risk
estimates.
These
steps
are
performed
on
the
inputs
and
outputs
of
the
probabilistic
risk
(
Monte
Carlo)
analysis.
Please
refer
to
the
NODA
(
or
TBD?)
for
details
on
the
sensitivity
analysis.

As
previously
noted,
EPA
developed
a
revised
risk
assessment
using
a
probabilistic
approach
as
a
basis
for
the
Agency's
decision
of
no
action
on
development
of
a
numerical
standard
for
dioxins
in
sewage
sludge
applied
to
agricultural
land.
In
order
to
protect
the
general
public
from
adverse
health
impacts
from
dioxins
in
land­
applied
sewage
sludge
with
an
adequate
margin
of
safety,
the
risk
assessment
calculates
the
risk
to
the
most
highly
exposed
population
(
i.
e.,
a
farm
family
consuming
up
to
50
percent
of
their
diet
from
products
grown
on
sewage
sludge
amended
soil).
The
following
discussion
characterizes
the
key
elements
of
EPA's
revised
risk
assessment
and
compares
them
according
to
the
principles
in
EPA's
guidance
for
exposure
assessment
and
for
risk
characterization
(
USEPA,
2003a).

Approximately
95
percent
of
the
U.
S.
population's
exposure
to
dioxins
results
from
the
consumption
of
animal
products
in
the
diet
where
dioxin
is
concentrated
in
the
fatty
portion
of
the
meats
and
dairy
products.
EPA
chose
the
farm
family
as
the
highly
exposed
population
to
be
modeled,
using
a
key
assumption
that
their
diets
have
significant
percentages
of
meat
and
dairy
products
from
their
own
farms
where
sewage
sludge
is
land
applied
as
a
fertilizer
or
soil
amendment.
Members
of
such
a
farm
family
are
at
greater
risk
from
exposure
to
dioxins
8
associated
with
the
land
application
of
sewage
sludge
as
compared
with
the
overall
U.
S.

population
because
their
diets
are
based
on
farm
products
derived
from
products
grown
on
sewage
sludge
amended
fields
and
pastures.
A
decision
that
is
protective
of
this
highly
exposed
family
is
likely
protective
of
the
general
population,
assuming
the
same
pathways
of
dioxins
exposure.
This
is
because
the
diet
of
the
general
population
contains
only
a
small
fraction
of
meat
and
dairy
products
grown
on
farms
with
land­
applied
sewage
sludge.

The
following
discussion
characterizes
the
three
principal
components
of
the
revised
risk
assessment:
(
1)
the
exposure
scenario;
(
2)
key
assumptions
and
data
used
in
the
exposure
assessment
modeling;
and
(
3)
the
cancer
slope
factor
(
Q1*
or
potency
factor).
Each
of
these
components
is
characterized
as
either
"
high
end"
or
"
central
tendency."
While
these
are
considered
"
high
end"
factors,
it
is
important
to
note
that
even
though
the
assumption
is
"
high
end,"
in
that
it
assumes
a
large
or
inordinate
consumption
or
exposure,
the
effect
it
may
have
in
"
driving"
the
risk
assessment
may
not
be
great.
As
discussed
earlier,
the
largest
components
of
dioxin
exposure
(
the
"
drivers"
of
the
risk
assessment)
are
due
to
beef
and
dairy
product
consumption.

Sewage
sludge
is
assumed
to
be
applied
at
agronomic
rates
to
tilled
crop
land
used
for
the
production
of
vegetables,
fruits,
and
root
crops,
and
to
pasture
land
which
is
not
tilled.
Fifty
percent
of
the
farm
family's
agricultural
land
is
assumed
to
be
tilled
crop
land
and
the
other
fifty
percent
untilled
pasture.
An
assumption
in
terms
of
characterizing
the
risk
is
that
the
dioxins
in
each
application
of
sewage
sludge
to
pasture
is
assumed
to
permanently
remain
in
the
top
two
centimeters
of
the
land
surface
and
is
not
diluted
over
time.
Volatilization
from
soil
to
the
leaf
surfaces
of
crops
consumed
by
animals
and
humans
is
the
principal
mechanism
by
which
dioxins
9
are
transported
from
sewage
sludge
applied
to
the
land.
This
high­
end
assumption
predicts
a
maximum
amount
of
transport
of
dioxins
for
subsequent
consumption
by
pastured
animals.
In
addition,
this
pasturing
scenario
is
not
varied.
EPA
assumes
that
the
farmer
does
not
rotate
the
pasture
to
grow
row
crops
where
tilling
of
sewage
sludge
in
the
soil
would
mitigate
dioxin
volatilization
transport.
Thus,
this
assumption
is
likely
to
contribute
to
an
overestimation
of
risk.

Another
important
high
end
assumption
contributing
to
the
risk
estimate
is
that
the
family
is
simultaneously
exposed
to
a
combination
of
agricultural
products
produced
on
the
farm.
For
the
purpose
of
the
exposure
assessment
and
risk
assessment,
all
pathways
of
exposure
to
dioxins
are
summed.

The
cancer
slope
factor
used
in
the
revised
risk
assessment
is
1.56
x
10­
4/
pg
TEQ/
kg­
d.

This
value
is
characterized
as
the
upper
bound
(
approximating
a
95th
percentile
confidence
level)

on
the
slope
of
the
dose­
response
curve
in
the
low­
dose
region
and
is
assumed
to
be
linear.

As
described
above
in
the
description
of
the
revised
risk
assessment,
most
of
the
parameters
used
in
the
probabilistic
risk
model
(
Monte
Carlo
simulations)
were
distributions
of
a
range
of
observed
values
for
each
parameter.
Where
a
range
of
data
was
not
available,
"
fixed"

data
points
or
assumptions
were
used
(
USEPA,
2003a).
The
sources
of
information
for
the
fixed
point
inputs
necessary
to
conduct
the
revised
risk
assessment
include
the
EPA
Exposure
Factors
Handbook
(
USEPA,
1997),
peer
reviewed
scientific
literature,
and
other
assumptions
specifically
related
to
land
application
of
sewage
sludge
based
on
actual
practice
(
refer
to
TBD
for
details,

USEPA,
2003a).

The
following
is
a
listing
of
some
of
the
key
fixed
parameters
used
in
the
Monte
Carlo
simulations
and
their
characterizations.
Some
of
the
fixed
assumptions
characterized
as
"
high
1
USEPA,
1998a.
Methodology
for
Assessing
Health
Risks
Associated
with
Multiple
Pathways
of
Exposure
To
Combustor
Emissions.
These
values
were
gathered
from
various
sources
and
are
either
mean
values
or
representative
ranges
(
not
high
end).

10
end"
have
the
greatest
impact
on
the
risk
estimate
based
on
the
results
of
the
sensitivity
analysis.

These
assumptions
include
the
farm
family
simultaneously
exposed
to
multiple
pathways
including
a
certain
percentage
of
their
own
products,
dioxins
remaining
in
the
top
two
centimeters
on
pasture
lands,
and
use
of
the
Q1*
(
cancer
slope
factor).
The
following
"
fixed"
parameters
are
important
to
note,
but
have
a
lesser
impact
on
the
risk
estimate.

Other
"
High
End"
Assumptions
°
Exposure
Frequency
­
350
days
per
year.

°
Fraction
of
diet
for
home­
caught
fish
­
100%.

°
Fraction
of
soil
ingested
that
is
contaminated
­
100%.

°
Fraction
of
ingested
dioxin
absorbed
by
the
mother
­
100%.

°
Use
of
potential
dose
rather
than
applied
or
internal
dose
Mean
or
Central
Tendency
Values
from
EPA
Exposure
Factors
Handbook
°
Fraction
of
food
preparation
loss
for
exposed
fruit,
exposed
vegetables,
and
root
vegetables.

°
Percent
cooking
and
percent
post­
cooking
loss
for
beef
and
poultry.

°
Fraction
of
home­
caught
fish
that
are
at
trophic
levels
3
and
4
(
high
dioxin
bioaccumulating
fish).

°
Soil
ingestion
rates
for
children
and
adults.

Values
from
Scientific
Literature1
°
Biological
half­
life
of
dioxin
in
lactating
women.
11
°
Concentration
of
dioxins
in
maternal
milk.

°
Fraction
of
fat
in
maternal
breast
milk
(
mean
value).

°
Fraction
of
ingested
dioxin
absorbed
by
the
infant.

°
Fraction
of
mother's
weight
that
is
fat
(
mean
value).

°
Proportion
of
dioxin
stored
in
maternal
fat.

The
probabilistic
methodology
facilitates
risk
estimates
for
individuals
in
any
percentile
of
the
assessed
population.
The
revised
land
application
risk
assessment
reports
estimates
of
risks
for
individuals
at
the
50th,
75th,
90th,
and
95th
percentiles
of
exposure
within
farm
family
individuals
defined
for
this
analysis
as
"
highly
exposed."
USEPA,
2002b.

The
incremental
cancer
risk
for
land
application
of
sewage
sludge
was
estimated
considering
all
exposure
pathways.
The
estimated
lifetime
risks
for
HEI
adults
using
the
current
cancer
slope
factor
of
1.56
x
10­
4/
pg
TEQ/
kg­
d
range
from
2
x
10­
5
at
the
95th
percentile
to
1
x
10­
6
at
the
50th
percentile
for
multi­
pathway
exposure
to
dioxins
through
land­
applied
sewage
sludge
(
see
Table
2
in
the
Federal
Register
Notice).
As
indicated
in
Table
2,
the
estimated
lifetime
excess
cancer
risks
for
HEI
people
born
on
the
farm
due
to
land
applied
sewage
sludge
are
less
than
or
equal
to
the
estimated
lifetime
excess
cancer
risks
for
adults
not
born
on
the
farm.

Demographic
data
indicate
that
people
born
on
a
farm
tend
to
leave
that
farm
at
an
earlier
age
in
life
than
people
who
move
to
that
farm
after
birth.
Therefore,
the
former
group
is
exposed
to
dioxins
from
living
on
the
farm
for
a
shorter
period
than
members
of
the
latter
group.

Continual
application
of
sewage
sludge
with
significantly
higher
concentrations
of
dioxins
than
currently
measured
would
be
necessary
to
predict
quantifiable
increases
in
risk.
However,

comparison
of
data
from
the
1988
National
Sewage
Sludge
Survey
(
NSSS)
(
USEPA,
1990)
and
12
the
EPA
2001
dioxin
update
survey
(
USEPA,
2002b)
indicate
that
"
spikes"
(
i.
e.,
higher
concentrations
of
dioxins
in
sewage
sludge)
appear
to
be
decreasing
over
time.
Specifically,
all
ten
sewage
sludge
samples
with
the
highest
concentrations
of
dioxins
and
furans
measured
in
the
1988
Survey
(
concentrations
ranging
from
97
ppt
TEQ
to
827
ppt
TEQ)
had
greatly
reduced
concentrations
of
dioxins
and
furans
in
the
2001
dioxin
update
survey
(
concentrations
ranging
from
2
ppt
TEQ
to
53
ppt
TEQ).
The
four
sewage
sludge
samples
with
the
highest
concentrations
of
dioxins
and
furans
measured
in
the
2001
dioxin
update
survey
(
concentrations
ranging
from
93
ppt
TEQ
to
682
ppt
TEQ)
had
markedly
lower
concentrations
of
dioxins
and
furans
in
the
1988
Survey
(
concentrations
ranging
from
2
ppt
TEQ
to
41
ppt
TEQ)
(
USEPA,

2002a
and
USEPA,
1990).
[
Note:
These
comparisons
are
based
on
dioxin
and
furan
concentrations
since
only
dioxins
and
furans
were
measured
in
the
1988
Survey.]
Thus,
it
is
highly
unlikely
that
a
single
family
would
be
exposed
to
sewage
sludge
with
a
high
concentration
of
dioxins
long
enough
to
produce
a
quantifiable
increase
in
risk
References
Cited:

USEPA,
1990.
National
Sewage
Sludge
Survey;
Availability
of
Information
and
Data,
and
Anticipated
Impacts
on
Proposed
Regulations;
Proposed
Rule.
Federal
Register
55
(
218):

47210­
47283.

USEPA,
1992,
Risk
Characterization
Guidance,
updated
1995
Agency
Risk
Guidance
for
Characterization
for
Risk
Managers
and
Risk
Assessors
USDA,
1997.
Census
of
Agriculture,
Washington,
DC
USEPA,
1997.
Exposure
Factors
Handbook.
National
Center
for
Environmental
Assessment.

Washington,
D.
C.
EPA/
600/
P­
95/
002F(
a­
c).
Vol.
I:
208
pp.
Vol.
II:
336
pp.
Vol.
III:
340
13
pp.
Also
available
at
NTIS
(
Vol.
I
PB98­
124225,
Vol.
II
PB98­
124233,
Vol.
III
PB98­
124241,
The
Set
PB98­
124217).
See
also
http://
www.
epa.
gov/
ncea/
exposfac.
htm
USEPA,
2002a,
Notice
of
Data
Availability,
June
12,
2002,
67
FR
40554
USEPA,
2002b.
Statistical
Support
Document
for
the
Development
of
Round
2
Biosolids
Use
or
Disposal
Regulations,
Office
of
Science
and
Technology,
Washington,
DC.

USEPA,
2003a.
Exposure
Analysis
for
Dioxins,
Dibenzofurans,
and
Coplanar
Polychlorinated
Biphenyls
in
Sewage
Sludge,
Technical
Background
Document,
The
Office
of
Water,

Washington,
DC.
