United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington, DC 20460
Research and Development
EPA/600/S6-90/003 May 1991
vxEPA Project Summary
Methodology for Assessing
Health Risks Associated with
Indirect Exposure to Combustor
Emissions
This document provides risk asses-
sors with the guidance necessary to
estimate the health risks that result
from indirect (other than inhalation)
human exposure to toxic pollutants in
combustor emissions. These indirect
exposures can result from the transfer
of emitted pollutants to soil, vegeta-
tion, and water bodies. To character-
ize the exposure and risk posed by
pollutants in various pathways, this
methodology guides risk assessors
through the four-step process of risk
assessment: hazard identification,
dose-response assessment, exposure
assessment, and risk characterization.
Analytical procedures and computer
models are described that can be used
to estimate exposure and risk by a va-
riety of environmental pathways. Also,
this document serves as a preliminary
source of data for carrying out the risk
calculations.
This Project Summary was developed
by EPA's Environmental Criteria and
Assessment Office, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The indirect exposure pathways ana-
lyzed in this methodology result from am-
bient air concentrations and wet and dry
deposition of pollutants emitted from the
stacks of combustion facilities. This meth-
odology can be applied to numerous emit-
ted pollutants (multipollutant).
This methodology is not intended to be
prescriptive; that is, it does not comprise a
set of guidelines or recommended ap-
proaches that the U.S. EPA thinks should
be applied in all circumstances. Rather, it
provides a set of procedures that the risk
assessor can draw upon, where applicable,
for a given assessment. Example calcu-
lations for two chemicals are provided to
demonstrate the exposure pathways and
risk evaluation.
Analytical procedures and computer
models that can be used to estimate ex-
posure and risk by a variety of environ-
mental pathways are described. In addi-
tion, this document serves as a prelimi-
nary source of data for the development
of risk calculations. The degree of scien-
tific support or uncertainty attendant to
each calculation varies widely. Therefore,
the appropriate use of these procedures
and the discussion of uncertainties sur-
rounding the results remain important re-
sponsibilities of the risk assessor.
Exposure Scenarios
Pollutants emitted to the atmosphere
from stationary combustion facilities may
be deposited on soil, water and vegeta-
tion, near the combustor. Humans or
other organisms can be exposed to the
emitted pollutants by various pathways,
such as dermal contact with the pollutants
in soil or water, consumption of the ex-
posed environmental media, consumption
of animals that have ingested exposed
soil, water, or vegetation, or consumption
of fish living in polluted waters. For hu-
mans, an important part of a multimedia,
multipollutant exposure assessment is de-
fining the individual and/or population in
the vicinity of the combustor and the po-
tential pathway(s) by which the individual(s)
may be exposed to the emitted pollutants.
Once the individual and environment
are defined and the potential pathways of
exposure chosen, mathematical models
of environmental fate and transport can
be used to estimate the concentration of
pollutants in the various environmental
media in the vicinity of the combustor, or
the area within a 50 km radius of the
source.
Printed on Recycled Paper
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Three hypothetical exposure scenarios
are developed to illustrate possible ap-
proaches to the risk analysis, and to illus-
trate calculations for the exposure path-
ways. Scenario A represents the most
likely occurrence for an exposed popula-
tion anywhere within a 50 kin radius of
the facility. Typical or average values of
input variables are used for this scenario.
In contrast, Scenario C illustrates the hy-
pothetical case in which all variables are
maximized so that the highest potential
exposure to an individual is determined.
While most persons in the vicinity of a
combustion facility are not likely to have
such an exposure, this possibility may ex-
ist. Scenario B strikes an eipproximata
midpoint between Scenarios A and C; ex-
posures will be higher than those in Sce-
nario A but not as infrequent as those in
Scenario C. The scenarios constructed
are examples only; other exposure sce-
narios can be defined/constructed by a
risk assessor for any site.
Air Dispersion Modeling
Combustion of materials produces re-
sidual amounts of pollution that may be
released to the environment. Estimation
of potential human health risks associated
with these emissions requires knowledge
of atmospheric pollutant concentrations
and annual deposition rates in the areas
around the combustion facility. Values for
these quantities are most often estimated
through the use of atmospheric dispersion
models.
Complex-terrain deposition models are
not currently available through the U.S.
EPA. Two existing complex terrain mod-
els, COMPLEX I and RTDM, were modi-
fied to realistically account for deposrtional
effects. COMPDEP, a modification of
COMPLEX I, and RTDMDEP, a modifica-
tion of RTDM, are the two new models
described that account for both wet and
dry deposition, respectively. Using
COMPDEP and RTDMDEP, hourly pollut-
ant concentrations can be calculated dur-
ing periods of precipitation and no precipi-
tation.
The results of these new models are
also compared with the Industrial Source
Complex-Short Term (ISCST) model, a
widely used flat-terrain model.
Deposition values were derived from the
COMPDEP model using unit (1 g/s) emis-
sion rate and were then multiplied by
chemical-specific emission rates to arrive
at a chemical-specific deposition rate.
These values can be used to determine
annual average deposition for various ar-
eas (areal averages) surrounding the com-
bustor, and are presented for the three
exposure scenarios.
Determining Concentrations of
Contaminants in Soil
Following deposition on soil, contami-
nants may be incorporated into the upper
layers of soil where crops or other vegeta-
tion are grown. The cumulative concen-
tration of a given contaminant in the soil
following deposition from a combustor is
estimated. The concentrations are used
to estimate the risk to humans who may
ingest soil directly or consume vegetation
and animals that have been exposed to
contaminated soil.
Calculating cumulative soil concentra-
tion requires the input of site-specific data,
which must be calculated or derived from
the available literature. The input vari-
ables required for the calculations are the
dry and wet deposition rates, the time
period of deposition, the soil depth, the
bulk density of the soil, and the soil con-
stant.
Human Daily Intake of
Pollutants via the Terrestrial
Food Chain
The Terrestrial Food Chain Model cal-
culates pollutant concentrations in plant
and animal tissue and the human expo-
sure from consuming plant and animal
foods.
Plant Pathways
Pollutants can bioaccumulate in plants
by root uptake, direct deposition on ex-
posed plant surfaces, and air-to-plant
transfer of vapor-phase pollutants. The
relative importance of each mechanism
depends in part on which of three broad
categories the plant belongs to: leafy
vegetables (including forage), exposed pro-
duce (fruits, fruiting vegetables and le-
gumes), or protected produce (grains, po-
tatoes, and root vegetables).
The food chain model calculates each
of three different concentrations in the
plant, each concentration based on one of
the plant's three different bioaccumulation
mechanisms, and sums the concentrations
to obtain the total pollutant concentration
in the plant. The plant-soil biocon-
centration factor (root uptake) is a mea-
sure of the ability of a pollutant to accu-
mulate in plant tissue and is defined as
the ratio of the pollutant concentration in
the plant to the pollutant concentration in
the soil.
Bioaccumulation due to direct deposi-
tion on exposed plant surfaces requires
several input variables. The fraction of
wet deposition determines the amount of
pollutant that adheres to the plant sur-
faces. The interception fraction accounts
for the fact that not all of the airborne
material depositing within a unit area will
initially deposit on edible vegetation sur-
faces. The plant-surface loss coefficient
is a measure of the amount of contami-
nant loss to several environmental pro-
cesses, such as wind removal, over time.
The amount of time the edible part of the
plant is exposed to direct deposition limits
the length of potential exposure. The
standing crop biomass is estimated by
productivity.
In addition to the root uptake and depo-
sition routes, plants can accumulate va-
por-phase contaminants by air-to-plant
transfer. The input variables required for
this calculation are the concentration of
pollutant in the air due to direct emis-
sions; the fraction of pollutant air concen-
tration present in the vapor phase; the air-
to-plant biotransfer factor; and the con-
centration of pollutant in the air due to
volatilization from the soil.
Animal Pathways
The animals that humans usually con-
sume as food take up pollutants by in-
gesting plants and ingesting soil while graz-
ing. The Terrestrial Food Chain Model
calculates the concentration of pollutant in
animal tissues by considering the concen-
tration of pollutant in plants and soil, the
quantity of plants and soil that the animals
consume, and the biotransfer factor of
each type of animal tissue. The biotransfer
factor is the ratio of pollutant concentra-
tion in animal tissue to the daily intake of
pollutant by the animal.
Human daily intake of the pollutant from
consumption of contaminated plants is cal-
culated by multiplying the concentration of
pollutant in each plant group by the amount
of contaminated plant group consumed
daily and the fraction of food consumed
that originates from contaminated soil.
Similarly, daily pollutant intake from the
consumption of meat, dairy products, or
eggs from animals that have consumed
contaminated forage, grain or soil is de-
termined from the concentration of pollut-
ant in the animal tissue, the amount of
each contaminated animal tissue that is
consumed daily, and the fraction of ani-
mal tissue group assumed to originate
from contaminated soil.
The human daily intakes of pollutant
from plant and animal ingestion are then
compared with health-based criteria to de-
termine if a potential human health risk
exists.
Human Daily Intake of
Pollutants from Exposure to
Soil
Humans can be exposed to pollutants
from soil by directly ingesting contami-
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nated soil, by the skin contacting contami-
nated soil, and by inhaling resuspended
contaminated dust.
Soil Ingestion
Contaminants from combustor emissions
deposited on soil can be directly ingested
by humans who eat soil intentionally or
incidentally by hand-to-mouth transfer.
Human daily intake resulting from inges-
tion of contaminated soil is a function of
soil concentration and soil ingest ion rate.
Humans could possibly be exposed to
combustor-emrtted contaminants soon af-
ter deposition on soil and before contami-
nants are incorporated into soil layers.
Therefore, for this pathway, 100% of the
deposited contaminant is assumed to be
incorporated in the uppermost 1 cm of
soil, and ingested soil is assumed to origi-
nate from the same 1 cm layer.
Dermal Exposure
The Dermal Exposure Model determines
exposure from human skin contact with
contaminants in the soil. The daily dermal
intake represents the increase above back-
ground in daily human dermal intake due
to the contaminant from combustor emis-
sions. The daily dermal intake is calcu-
lated using the following input variables:
the duration of daily contact with contami-
nants in the soil, the exposed skin surface
area, the amount of soil accumulating on
the skin, the fraction of the applied dose
of the compound absorbed by human skin
in one day, and the soil concentration of
pollutant. For many chemicals, however,
data for the fraction of compound adsorbed
by the skin are not available. Thus, it
may not be possible to quantitatively de-
termine daily dermal intake.
Dust Resuspension
Pollutants in the soil can be resu-
spended on dust by wind erosion. Par-
ticles less than 10 u,m in diameter may be
inhaled.
When determining exposure to pollut-
ants resuspended in the soil, the amount
of vegetation, soil concentration, size of
the source, wind velocity, moisture con-
tent, and soil roughness height are in-
cluded in the calculations.
Both the annual emission rate and the
worst-case 24-hour emission rate of con-
taminant as respirable particles are the
product of the area of the field, the pollut-
ant concentration in the soil, and the emis-
sion factor of respirable particulate matter.
The difference between the worst-case
and mean annual emission rate is the
emission factor. The worst-case emission
factor is derived using the maximum 6-hr
wind speed, whereas the mean annual
emission factor is based on the average
annual wind speed.
After the exposures from soil ingestion,
dermal contact and dust inhalation are
calculated, the results are then compared
with health-based criteria to determine if a
potential human health risk exists.
Determining Concentrations of
Contaminants in Water
The concentration of a contaminant in
water can be calculated for surface water,
collected precipitation, and groundwater.
The Surface Runoff Model is used to
calculate the concentration of contaminant,
both dissolved and adsorbed to suspended
particles, that accumulates in a surface
water body from deposition both directly
onto the water body and onto the water-
shed followed by surface runoff. These
predicted concentrations are used to esti-
mate human risk that may result from
drinking water, swimming and bathing in
water, or eating fish from a contaminated
surface water body. The surface water
model follows a three-tiered approach,
beginning with a simple, extremely con-
servative calculation (Tier 1) and proceed-
ing to more detailed calculations when
necessary (Tier 2 and Tier 3). Tier 1 of
the Surface Runoff Model is a conserva-
tive screening method that assumes that
all contaminant deposited onto a water-
shed/water body is transported to the re-
ceiving water body during the loading pe-
riod.
Contaminants in combustor emissions
may dissolve, infiltrate the ground, and
become available for recharging the
groundwater. If contaminated groundwa-
ter is used as a source of drinking water,
individuals will be exposed. A Groundwa-
ter Infiltration Model was developed and
formulated in three successive tiers. Tier
1 is an extremely conservative approach,
in which projected leachate concentrations
are compared with health-based criteria.
Leachate concentrations are predicted on
the basis of annual fallout and recharge
rates. If the contaminant concentration in
the leachate exceeds the health-based cri-
terion, the contaminant is carried forward
to Tier 2 and 3 analysis, which allow for
site-specific inputs to predict dispersion,
degradation, and retardation effects.
After the exposures to surface water,
collected precipitation and groundwater are
calculated, the results are compared with
health-based criteria to determine if a po-
tential human health risk exists.
Human Exposure from Water
Humans can be exposed to pollutants
in water by ingesting contaminated water,
eating fish from contaminated surface wa-
ter bodies, and by the skin contacting
contaminated water.
Drinking Water Sources
Once contaminants from combustor
emissions are transported to surface wa-
ter bodies, they may be incorporated into
the aquatic food chain. Humans may
then be exposed to contaminants when
they eat fish from contaminated surface
water bodies. Total daily intake of con-
taminants from fish is calculated from the
contaminant concentration in the water and
the water consumption rate. The long-
term Tier 2/3 water concentrations of
benzo[a]pyrene and cadmium are used
for the example calculations. Water con-
sumption rates utilized are 1.4 L/day for
adults, 0.5-0.8 L/day for children aged 5-
14, and 0.2 L/day for children with body
weights <10 kg and aged « 5 years.
Aquatic Food Chain
Once contaminants from combustor
emissions are transported to surface wa-
ter bodies, they may be incorporated into
the aquatic food chain. Humans may
then be exposed to contaminants when
they eat fish form contaminated surface
water bodies. Total daily intake of con-
taminants from fish is calculated from the
contaminant concentration in the water,
the fish ingestion rate, and the biocon-
centration factor, which is the ratio of con-
taminant concentration in fish to contami-
nant concentration in water. The fish in-
gestion rate was adjusted downward by
50% to account for the fact that in most
instances -50% of total fish consumption
is marine, rather than freshwater or estua-
rine.
Dermal Exposure
Once contaminants in combustor emis-
sions reach water sources, humans may
be exposed by absorbing the contami-
nants through the skin while swimming or
bathing. The daily dermal intake from
water is affected by the exposed skin sur-
face area, and the amount of time each
day and the number of days per year the
skin is in contact with the contaminated
water. A quantitative approach to esti-
mating dermal exposure to contaminants
in water is precluded by the paucity of
data available concerning dermal absorp-
tion from water.
After the exposure due to water and
fish ingestion and dermal contact of water
are calculated, the results are compared
with health-based criteria to determine if a
potential human health risk exists.
Example Calculations
Three example scenarios are con-
structed and used to illustrate calculation
•&U. S. GOVERNMENT PRINTING OFFICE: 1991/548-028/20219
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of exposures. Risk assessors can vary
the set of conditions to create exposure
scenarios for a site-specific assessment.
A hypothetical municipal waste combustor
in western Fbrida is used as an example
combustor facility. Benzo[a]pyrene arid
cadmium are used as examples for an
organic and an inorganic chemical, re-
spectively, as well as for compounds that
the U.S. EPA considers to be non-thresh-
old and threshold-acting, respectively.
Scenario A represents the most likely
occurrence for an exposed population any-
where within a 50 km radius of the facility.
This scenario is represented by an adult
living in a metropolitan area (i.e., central
city) where an MWC has been operating
continuously for 30 years. Ground level
concentrations of the emitted pollutants
are relatively bw, being represented as
the average concentration for a 50 km
(radial distance) ring around the source.
The individual resides in this area for 16
years, but works (8 hours/day) away from
the area receiving fallout, input variables
for the exposure pathways are generally
average values.
Scenario C illustrates a case in which
all variables are maximized so that the
highest potential exposure can be deter-
mined. In this scenario, the individual is a
child living in close proximity to the facility
(0.2 km away) in a non-metropolitan area
after the combustion source has been op-
erating for 100 years. The child grows up
and spends his lifetime (70 years) in this
area. It is assumed the individual's daily
activity is confined to the area of maximal
deposition of pollutants.
Scenario B strikes a midpoint between
Scenarios A and C and is represented by
a child living in the suburbs, 5 km from the
combustion facility, where the area! aver-
age ground-level concentrations of emis-
sions (average for the 5 km ring surround-
ing the source) have been deposited for
the 60 years of operation of the MWC.
The child grows up and remains in this
area for a total of 30 years. During child-
hood, the individual stays at home during
the day, but as an adult works 8 hours/
day away from the residence.
Randall Bruins is the EPA Project Officer (see below).
The complete report, entitled "Methodology for Assessing Health Risks Associated
with Indirect Exposure to Combustor Emissions," (Order No. PB90-187 055/
AS;Cost: $45.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S6-90/003
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