United States
Environmental Protection Agency
Region III
Philadelphia, PA 19107
EPA903-F-94-001
February 1994 -
Region III
Technical Guidance Manual
Risk Assessment
Use of Monte Carlo Simulation in Risk
Assessments
EPA
Region III
EPA Contact: Dr. Roy L Smith
Hazardous Waste Management Division
Office of Superfund Programs
February 1994
EPA's current risk assessment methods express health risks as single numerical values, or •single-point* estimates of
risk. This technique provides little information about uncertainty and variability surrounding the risk estimate. Recent
EPA guidance (EPA, 1992) recommends developing 'multiple descriptors' of risk to provide more complete information
to Agency decision-makers and the public. Monte Carlo simulation is a highly effective way to produce these multiple
risk descriptors. This document recommends guidelines under which Region III risk assessors may accept the optional
use of Monte Carlo simulation to develop multiple descriptors of risk. The Region will continue to require single-point
risk estimates, prepared under current national guidance, in conjunction with optional Monte Carlo simulations.
SINGLE RISK ESTIMATES VS. MULTIPLE
DESCRIPTORS
EPA designed its human health risk assessment
guidance (e.g., EPA, 1991,1989 and 1988) to produce
protective, rather than best, estimates of risk. EPA is
aware that true risks are probably less than its
estimates, but has chosen a regulatory policy of giving
the benefit of uncertainty surrounding the risk
assessment to the exposed public.
These protective risk estimates sometimes create
difficulty for Agency decision-makers and the public.
Site-specific Regional risk assessments usually present
risk as a single number, or single-point estimate,
accompanied by a qualitative discussion of uncertainty.
The public tends to focus on the single-point estimate
and to overtook the uncertainty, which may span
several orders of magnitude. EPA risk managers,
though aware of the uncertainty, must still justify their
decisjon to either accept or reduce the single-point risk.
If the risk is close to the maximum acceptable level, it
is likely that different assumptions would have
produced a different risk number, leading to a different
decision. In this way, single-point risk assessment
methods place the risk assessor in an inappropriate
risk management role.
Recent1 EPA guidance on risk characterization (EPA,
1992) discusses this problem in depth, and
recommends the use of multiple risk descriptors in
addition to protective single-point risk estimates.
Inclusion of these additional risk descriptors provides
the public with more complete information on the
likelihood of various risk levels, and risk managers with
multiple risk-based cleanup goals from which to
choose. This guidance mentions Monte Carlo
simulation as an effective source of multiple risk
descriptors.
MONTE CARLO SIMULATION
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Monte Carlo simulation is a statistical technique by
which a quantity is calculated repeatedly, using
randomly selected •what-if* scenarios for each
calculation. Though the simulation process is internally
complex, commercial computer software performs the
calculations as a single operation, presenting results in
simple graphs and tables. These results approximate
the full range of possible outcomes, and the likelihood
of each. When Monte Carlo simulation is applied to
risk assessment, risk appears as a frequency
distribution graph similar to the familiar bell-shaped
curve, which non-statisticians can understand
intuitively.
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Monte Carlo simulation also has important limitations,
which have restrained EPA from accepting it as a
preferred risk assessment tool:
1. Available software cannot distinguish between
variability and uncertainty. Some factors, such as
body weight and tap water ingestion, show well-
described differences among individuals. These
differences are called •variability*. Other factors,
such as frequency and duration of trespassing, are
simply unknown. This lack of knowledge is called
•uncertainty*. Current Monte Carlo software treats
uncertainty as if it were variability, which may
produce misleading results.
2. Ignoring correlations among exposure variables
can bias Monte Carlo calculations. However,
information on possible correlations is seldom
available.
3. Exposure factors developed from short-term
studies with large populations may not accurately
represent long-term conditions in small
populations.
4. The tails of Monte Carlo risk distributions, which
are of greatest regulatory interest, are very
sensitive to the shape of the input distributions.
Because of these limitations, Region III does not
recommend Monte Carlo simulation as the sole, or
even primary, risk assessment method. Nevertheless,
Monte Carlo simulation is clearly superior to the
qualitative procedures currently used to analyze
uncertainty and variability. For baseline risk
assessments at NPL sites, Region III recommends that
uncertainty and variability surrounding single-point risk
estimates rely on multiple descriptors of risk (EPA,
1992). Monte Carlo simulation will be an acceptable
method for developing these multiple descriptors.
The following example (from Smith, in press) illustrates
the advantages of Monte Carlo simulation in risk
assessment:
At a Superfund site in Region III, volatile organic
compounds migrated to residential wells. The single-
point RME estimate of lifetime cancer risk to exposed
residents, based on ingestion of tap water and
inhalation while showering, was 1.14e-3.
Figure 1 shows the output of a PC-based Monte Carlo
simulation program for the risk assessment. Each
exposure parameter was entered as a frequency
distribution (i.e., a 'bell-shaped* curve showing the
range of possible values, and the likelihood of each)
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Fig 1. Probability distribution of upper bound lifetime cancer risk.
rather than as a single number. Carcinogenic potency
slopes were entered as fixed values rather than
frequency distributions, so the variability in risk was
due entirely to the exposure assumptions.
Risk was calculated 5000 times, with each calculation
based on a different randomly-selected exposure
scenario. The figure lists the RME, average, and four
percentiles of risk, and shows the entire risk
distribution. The RME risk estimate fell between the
95th and 99th percentiles in this example, appropriately
protective as intended. This figure clearly provides
more complete risk information than the
numerical RME estimate.
GUIDELINES FOR USING MONTE CARLO SIMULATION
Region III risk assessors believe that Monte Carlo
simulation requires more development before it can
serve as the primary risk assessment method, for
reasons described above. However, the technique has
clear advantages over the qualitative analyses of
uncertainty and variability currently in use. Region III
will accept Monte Carlo simulations submitted as
uncertainty/variability analyses in risk assessments,
under the following guidelines:
1. Include only human receptors. This guidance
excludes environmental receptors.
2. Submit a work plan for EPA review before doing
the Monte Carlo simulation, to ensure the work will
be acceptable to EPA. The workplan should
describe the software to be used, the exposure
routes and models, and input probability
distributions and their sources. EPA expects that
peer-reviewed literature and site-specific data will
be used whenever possible. Use professi
judgment only as a last resort, and only in the
of triangular or uniform distributions. Describe how
correlations among input variables will be handled.
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3. Include only exposure variables in the Monte Carlo
simulation. Enter reference doses and
carcinogenic slope factors as single numbers,
except for specific contaminants for which the EPA
Office of Research and Development has already
approved frequency distributions.
4. Include only significant exposure scenarios and
contaminants in the Monte Carlo simulation. First,
calculate RME risks for all exposure routes under
current guidance. Select exposure routes for
which RME risk exceeds either 1e-6 cancer risk or
a non-carcinogenic hazard index of 1. Include only
contaminants which contribute 1% or more of the
total RME risk or hazard index.
5. Use Monte Carlo simulation only to analyze
uncertainty and variability, as a 'multiple descriptor*
of risk. Include standard RME risk estimates in all
graphs and tables of Monte Carlo results. Generate
deterministic risks using current EPA national
guidance (EPA 1992, 1991, 19B9, and 1988).
6. Include graphs and tables showing and describing
each input distribution, distributions of risk for each
exposure route, and distributions of total risk
(summed across exposure pathways and age
groups, as appropriate under current guidance).
Region III will not accept Monte Carlo simulations which
are not approved beforehand, or do not adhere to
these guidelines.
SUMMARY
Region III will accept Monte Carlo simulations that
conform to the guidelines in this document, as part of
baseline human health risk assessments. The most
important guideline is that all risk assessments must
include single-point RME risk estimates prepared under
current EPA national guidance. The Region will accept
Monte Carlo simulation only as an optional addition to,
not a substitute for, current risk assessment methods.
REFERENCES
EPA, 1992. Guidance on Risk Characterization for Risk
Managers and Risk Assessors, (U.S. Environmental
Protection Agency, Office of the Administrator,
Washington, DC, memorandum from F. Henry
Habicht on 26 February 1992).
EPA, 1991. Standard Default Exposure Factors, Risk
Assessment Guidance for Superfund, Volume I:
Human Health Evaluation Manual Supplemental
Guidance, (U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response,
Toxics Integration Branch, Washington, DC,
OSWER Directive 9285:6-03).
EPA, 1989. Risk Assessment Guidance for Superfund,
Volume I: Human Health Evaluation Manual (Part A),
(U.S. Environmental Protection Agency Office of
Solid Waste and Emergency Response, Toxics
Integration Branch, Washington, DC, EPA/540/1-
89/002).
EPA, 1988. Exposure Factors Handbook, (U.S.
Environmental Protection Agency Office of Health
and Environmental Assessment, Washington, DC,
EPA/600/8-89/043).
Smith, R.L In press. Use of Monte Carlo simulation
for human exposure at a Superfund site.
Submitted to Risk Analysis, May 1993.
For additional information, call (215) 597-6682.
Approved by:
Thomas C. V<
Hazardous Wi
Division
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