Drifted States
Environmental Protection Agency
Region ill
Philadelphia, PA 19107
EPA/903/R-93-001
January 1993
Region III
Technical Guidance Manual
Risk Assessment
U.S. r:/!
Selecting Exposure Routes ahc
Contaminants of Concern by Risk-Based
Screening
EPA Contact: Dr. Roy L Smith
EPA
Region III
Hazardous Waste Management Division
Office of Super-fund Programs
January 1993
Human health risk assessment includes effort-intensive steps which require many detailed calculations by experts. Most
baseline risk assessments are dominated by a few chemicals and a few routes of exposure. Effort expended on minor
contaminants and exposure routes, i.e.jhose which do not influence overall risk, is sssentiaJty wasted. This guidance
is intended to identify and focus on dominant contaminants of concern and exposure routes at the earliest feasible point
in the baseline risk assessment. Use of these methods will decrease effort and time spent assessing risk, without loss
of protectiveness. This guidance is not intended for other risk assessment activities, such as determining preliminary
remediation goals.
SELECTING CONTAMINANTS AND EXPOSURE ROUTES
OF CONCERN
Most samples from hazardous waste sites are analyzed
for 103 target compounds and analytes recommended
by tf?e EPA Superfund program. Semi-volatile analysis
can detect additional tentatively identified compounds
not on the target lists. Special analytical services
procedures, if used, may find still more contaminants.
The combined number of contaminants detected at a
site sometimes exceeds one hundred.
While EPA considers it necessary to gather information
on many contaminants, very little of this data actually
influences the overall quantitative assessment of health
risk. For most sites, baseline risk assessments are
dominated by a few contaminants and a few routes of
exposure. The remaining tens, or hundreds, of
detected contaminants have a minimal influence on total
risk. This small impact is lost by rounding. Entire
environmental media may contain not a single
contaminant at a concentration which could adversely
affect public health. Quantitative risk calculations using
data from such 'risk-free* media have no effect on the
overall risk estimate for the site.
The EPA baseline risk assessment process at several
points requires careful data evaluation by scientific
experts. These evaluations, which are contaminant-
specific, include: (1) statistical comparisons between
site-related and background samples, (2) special
handling of undetected contaminants, (3) calculation of
toxicity equivalence, (4) evaluation of frequency of
detection, and (5) comparison with ARARs. Because
overall risk is usually driven by a few contaminants and
exposure routes, effort spent in detailed evaluation of
minor contaminants and routes of exposure is
essentially wasted. For some sites, this wasted effort
exceeds 90% of the total.
The baseline risk assessment process can be made
more efficient by focusing on dominant contaminants
and routes of exposure at the earliest feasible stage.
The mechanisms recommended for this are (1) a re-
ordering of the process of eliminating contaminants and
routes of exposure, and (2) use of a risk-based
concentration screen. Appropriately used, this process
can dramatically reduce the effort of risk assessment,
while not changing the result significantly.
EXISTING GUIDANCE
Chapter 5 of 'RAGS IA' (Risk Assessment Guidance for
Suoerfund. Volume I. Human Health Evaluation Manual
(Pan A): EPA, 1989) provides a detailed procedure for
evaluating data for a baseline risk assessment. This
903/R-
93-001
-------
procedure includes steps by which the risk assessor
selects contaminants of concern in each exposure
medium. These steps are summarized in Table 1.
There are two major limitations to the RAGS procedure.
First, the eliminating step (a concentration toxicity
screen) comes late in the process. Many of the
preceding steps (e.g.. evaluation of quantitation limits,
comparison with background, calculation of toxicity
equivalence, and evaluation of frequency of detection)
are contaminant- and medium-specific. They require
the sustained attention of an expert, and cannot be
automated. If the contaminant is eliminated, this work
is wasted.
The second limitation is that the concentration toxicity
screen compares only relative risk among contaminants
in the same medium. While very efficient at selecting
dominant contaminants in each medium, this method
does not evaluate significance of total risk for the
medium. Thus, the concentration toxicity screen can
eliminate contaminants, but not routes of exposure.
RECOMMENDED METHODOLOGY
This guidance makes two changes intended to remove
the limitations in existing guidance. These
recommendations are intended for baseline risk
assessments.
1. Re-ordering of steps. The eliminating screen is
moved forward in trie data evaluation process to a point
immediately following dam quality evaluation. The new
process is shown in Table 2. Effort-intensive steps such
as evaluation tf quantitation limits and comparison with
background now follow the eliminating screen. The
steps are divided into four categories: data quality
evaluation, initial data set reduction, re-inclusion of
special cases, and optional final data set reduction.
The data quality evaluation steps (evaluating
appropriateness of methods and qualifiers, significance
of blank contamination, and need for special analyses)
should be done e» described in RAGS IA, Chapter 5.
Next, the risk assessor should consult wtto tf» RPM to
discuss the use of the risk-besed concentration table
(described in item [2] below} as • screening
mechanism. With tr» RPM's approval, the risk sssessor
should reduce the data set and document the rationale
for eliminating contaminants and routes of exposure
from further analysis.
After the initial data set reduction, the risk assessor and
RPM should consider re-including specific
contaminants on the basis of historical data, toxicity,
mobility,persistence, bioaccumuiation, special exposure
routes, special treatability problems, or exceedance of
ARARs. These activities should proceed as described
in Section 5.9 of RAGS IA.
Finally, optional further reductions in the data set may
be justified, based on the status of a contaminant as an
essential nutrient, low frequency of detection, or no
statistical difference between site and background
levels. These evaluations, the most complicated and
contaminant-specific, are saved for last.
2. Screening by risk-based concentrations. The
screening method is changed from the relative
concentration toxicity screen of RAGS lAtoan absolute
comparison of risk. This is done by means of a table of
risk-based concentrations (Appendix I). This table
contains levels of nearly 600 contaminants in air,
drinking water, fish tissue, and soil, which correspond
to a systemic hazard quotient of 0.1 or a lifetime cancer
risk of 10*. The risk-based concentrations were
developed using protective default exposure scenarios
suggested by EPA (1991) and the best available
reference doses and carcinogenic potency slopes (see
the table for sources), and represent relatively protective
environmental concentrations at which EPA would
typically not take action.
The risk-based concentration screen is used as follows:
(a) The risk assessor extracts the maximum
concentration of each substance detected in each
medium.
(b) If the maximum concentration exceeds the risk?
based concentration for that medium, the
contaminant is retained for risk assessment, for all
routes of exposure involving that medium.
Otherwise the contaminant is dropped for that
medium.
(c) If a specific contaminant does not exceed its risk-
based concentration for any medium, the
contaminant is dropped from the risk assessment
(d) If no contaminant in a specific medium exceeds its
risk-based concentration, the medium is dropped
from the risk assessment
(e) All contaminants and exposure routes which are
dropped are kept on a sub-list and considered for
re-inclusion, based on special properties.
(f) If the risk assessor wants to include a route of
exposure not covered in the risk-based
concentration table, the equations provided in
Appendix I can serve as the basis for new risk-
-------
based concentrations. Similarly, the risk assessor
can use the same equations to calculate alternate
risk levels (i.e., other than a systemic hazard
quotient of 0.1 and lifetime cancer risk of 10*) to be
the basis for screening.
SUMMARY
The process by which contaminants and exposure
routes are selected in quantitative risk assessment can
be made less effort-intensive by two simple changes.
First, high-effort steps should be postponed until later in
the selection process, because performing these
operations on trivial contaminants and exposure routes
is pointtess. Second, changing from a relative
concentration toxicity screen to an absolute risk-based
concentration screen improves the risk assessor's
ability to focus on dominant contaminants and exposure
routes at an earlier stage.
REFERENCES
EPA, 1991. Human Health Evaluation Manual,
Supplemental Guidance: 'Standard Default Exposure
Factors'. OSWER Directive 9285.6-03, Office of
Emergency and Remedial Response, March 25,
1991.
EPA, 1989. Risk Assessment Guidance for Superiund,
Volume I, Human Health Evaluation Manual (Pan A).
Office of Emergency and Remedial Response,
December, 1989. EPAJ540I1-891002.
For additional information, call (215) 597-6682.
Approved by:
Thomas C. Vo
Hazardous Waste Mbnagffnent Division
-------
Tabte 1. Summary of existing EPA guidance on selecting contaminants of concern (EPA, 1989, chapter 5)
Section 5.1: Combining data from site investigations
1. Determine if methods are appropriate
2. Evaluate quantitation limits
3. Determine if qualifiers are appropriate
4. Determine if significant blank contamination exists
5. Determine if special analyses for tentatively identified compounds are needed
6. Compare site samples to background
Section 5.9: Further reduction in the number of chemicals (optional)
7. Consult with RPM
8. Document rationale for eliminating chemicals
9. Examine historical information
10. Consider exceptional toxidty, mobility, persistence, or bfoaccumuiatfon
11. Consider special exposure routes
12. Consider special treatabfflty problems
13. Determine if contaminants exceed ARARs
14. Group chemicals by class, evaluate toxidty equivalence
15. Evaluate frequency of detection
16. Evaluate essentiality
17. Use a concentration toxidty screen
-------
Table Z EPA Region III guidance on selecting contaminants and exposure routes of concern
A. Data quality evaluation
1. Determine if methods are appropriate
2. Determine if qualifiers are appropriate
3. Determine if significant blank contamination exists
4. Determine if special analyses for tentatively identified compounds are needed
B. Reduce data set using risk-based concentration screen
5. Consult with RPM
6. Use risk-based concentration table to screen contaminants and exposure routes of concern
7. Document rationale for eliminating chemicals and exposure routes
C. Consider re-including eliminated chemicals and routes, based on:
8. Historical information
9. Exceptional toxtcity, mobility, persistence, or bioaccumulation
10. Special exposure routes
11. Special treatabiltty problems
12. ARARs exceedance
13. Toxicity equivalence of chemical class (e.g., CDD/CDFs, PAHs)
D. Make further specific reductions in data set (optional)
14. Evaluate essentiality
15. Evaluate frequency of detection
16. Compare site samples to background
-------
Appendix I:
EPA Region in Risk-Based Concentration Table
Background Information
The risk-based concentrations were calculated as follows:
GENERAL; Separate risk-based concentrations were calculated for carcinogenic and non-
carcinogenic effects of each compound for each pathway. The concentration in the table is
the lower of the two, rounded to two significant figures. For non-carcinogenic effects, the
averaging time equals the exposure duration, so the exposure duration term has been used
for both. The following terms were used in the calculations:
General:
Oral carcinogenic slope factor (mg/kg/d)"1: SFe
Inhaled carcinogenic slope factor (mg/kg/d)'1: SF,
Oral reference dose (mg/kg/d): R£D0
Inhaled reference dose (mg/kg/d): RfD,
Target cancer risk: TR
Target hazard quotient: THQ
Body weight, adult (kg): BW.
Body weight, child age 1-6 (kg): BW.
Averaging time (years of life): AT
Air breathed (mVd): IR,
Drinking water ingestion (L/d): IR,
Fish ingestion (g/d): ERf
Soil ingestion - age adjusted (mg/d) IRS*
Soil ingestion - age 1-6 (mg/d): IRS,
Soil ingestion - adult (mg/d): IRS.
Residential:
Exposure frequency (d/y): EF,
Exposure duration (y): EDr
Volatilization factor (L/mJ): VF
Commercial/Industrial:
Exposure frequency (d/y): EF0
Exposure duration (y): ED0
The priority among sources of lexicological constants was as follows: (1) IRIS, (2) HEAST,
(3) HEAST alternative method, (4) ECAO-Cincinnati, (5) other EPA documents, (6)
withdrawn from IRIS, and (7) withdrawn from HEAST. Each source was used only if
numbers from higher-priority sources were unavailable.
ALGORITHMS:
6
-------
1. Residential water use (^g/L). Volatilization terms were calculated only for compounds
with "y" in the "Volatile" column. Compounds having a Henry's Law constant greater than
10"5 were considered volatile. The list may be incomplete, but is unlikely to include false
positives. The equations and the volatilization factor (VF, above) were obtained from the
draft RAGS IB. Oral potency slopes and reference doses were used for both oral and
inhaled exposures for volatile compounds lacking inhalation values. Inhaled potency slopes
were substituted for unavailable oral potency slopes only for volatile compounds; inhaled
RfDs were substituted for unavailable oral RfDs for both volatile and non-volatile
compounds.
a. Carcinogenic effects:
TR • BWa • AT • 365^ • 10002
EF, • EDr - ([VF - IR. • CPS} + [flT • SFJ)
b. Non-carcinogenic effects:
THQ -BW -ED • 365f • 10002
*• « ' r ««
EF -ED
RfDi RfD
2. Air (jug/m3). Oral potency slopes and references were used where inhalation values were
not available.
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TR • BW • AT • 365i • 10002
• r
_
• £D, • IRt • SF.
b. Non-carcinogenic effects:
THQ • RfD. • BWt - ED • 365i • 10002
££>
3. Fish (mg/kg):
a. Carcinogenic effects:
TR -BW -AT -365f
• y
EF -ED • '— - SF
' 10001
-------
b. Non-carcinogenic effects:
THQ • RfD. • BWt - EDr
IK
EF -ED - '
' 10001
4. Soil commercial/industrial (mg/kg): The default exposure assumption that only 50% of
incidental soil ingestion occurs at work has been omitted.
a. Carcinogenic effects:
77?- BW: AT • 365f
__
EF • ED • L • SF
' 10*2
b. Non-carcinogenic effects:
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S. Soil residential (mg/kg):
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• r
IRS
EF -ED ± • CPS
'10*2
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IRS
EF -ED - 1
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