Updated Dermal Exposure Assessment Guidance
United States Environmental Protection Agency
Philadelphia, PA 19107
Region 3 Technical Guidance Manual, Risk Assessment
EPA Contact: Jennifer Hubbard (email@example.com)
This memorandum describes EPA Region 3's recommended approach to dermal risk assessment at Superfund sites. This document does not substitute for any statutory provisions or regulations, nor is it a regulation itself. Thus, it cannot impose legally binding requirements on EPA, States, or the regulatory community, and may not apply to a particular situation based upon the circumstances. Any decision regarding cleanup of a particular site will be made based on the statutes and regulations, and EPA decision-makers retain the discretion to adopt approaches on a case-by-case basis that differ from this guidance to a particular situation. Region 3 welcomes public input on this document at any time.
This guidance is based on the state of knowledge at present. The practices discussed herein may be refined, updated, or superseded by future advances and discoveries.
EPA Region 3 has relied primarily on two documents (the 1992 document Dermal Exposure Assessment: Principles and Applications and the 1995 regional guidance "Assessing Dermal Exposure from Soil") for the assessment of dermal risk at Superfund sites. New dermal guidance (Risk Assessment Guidance for Superfund Volume I: Human Health Evaluation Manual (Part E, Supplemental Guidance for Dermal Risk Assessment, here abbreviated "RAGS E") has been under development. EPA now has a draft of this guidance that is publicly available (the interim draft dated September 2001). Therefore, the recommendations in RAGS E will be followed by Region 3, but there are some Region-specific recommendations presented below.
The following section highlights the major changes to Region 3 practices, and also clarifies the cases in which this Regional guidance will supplement the national RAGS E guidance.
Selection of chemicals to evaluate
EPA Region 3 typically quantifies risk from the dermal route for all chemicals of potential concern (COPCs) in soil, sediment, and water. RAGS E suggests that this may not be necessary for many chemicals, because the dermal route usually contributes far less than, e.g., the ingestion route, to overall risk. RAGS E recommends not quantifying dermal risk unless the dermal dose is at least 10% of the oral dose.
This would reduce the number of chemicals that receive a quantitative assessment, and on the face of it, would seem to save resources. However, in the era of automated spreadsheets and databases, it is actually more troublesome in some instances to try to remove chemicals from part of a risk assessment.
The RAGS E recommendation also could lead to underestimation of risk for chemicals when the dermal dose is less than 10% of the oral dose, but dermal toxicity is greater than oral toxicity. For these reasons, EPA Region 3 will generally recommend quantifying dermal risks for all COPCs, although in specific instances, the quantitation may be eliminated. Also, for media where the other exposure routes have been eliminated for some reason, the dermal route will need to be quantitated because it will be the only source of risk.
Quantitation of risks from organic chemicals in water
New equations are available for the non-steady-state model. For t <= t*, a factor called "FA" has been introduced. ("FA" is the systemically available fraction. It is especially important for compounds with large lag times, such as highly lipophilic compounds.) For t > t*, "FA" has also been introduced, but in addition, there is a change to the quantity (1 + 3B)/(1 + B). The latter expression is now (1 + 3B + 3B2)/(1 + B)2. The latter change does not appear to significantly alter risk computations. The addition of "FA" has a proportional change in the risk; i.e., an FA of 0.7 means the risk is now 70% of the risk as it would have been calculated formerly. However, FA is 1 for most chemicals; only risks for highly lipophilic chemicals (those outside the effective predictive domain) are affected. EPA Region 3 is adopting both of these changes to the risk equations.
Child total-body skin surface area
RAGS E introduces a slight reduction to the former default skin surface area for young children. The change from 7200 cm2 to 6600 cm2 will only slightly decrease whole-body dermal risks, and EPA Region 3 is recommending the new factor of 6600 cm2.
Shower and bathing time
EPA Region 3 has used approximately median values for showering and bathing times (adult shower, 12 minutes with 20 minutes total in shower room; child bath, 20 minutes). RAGS E now recommends 95th percentile values (adult shower, 35 minutes; child bath, 60 minutes). This tripling of exposure time would triple the risk.
Reasonable Maximum Exposure (RME) estimates of risk are typically generated using a combination of high-end and average inputs. In this case, the exposure time factor has been a central estimate used in combination with other factors, some of them high-end values. EPA Region 3 has not yet seen evidence that the exposure time should be a high-end value in order to meet this goal. At this time, and until the RAGS E guidance is final, EPA Region 3 is retaining the use of median showering and bathing time values as defaults.
The Region does anticipate performing a probabilistic analysis to determine the best combination of exposure factors, and may adopt the 95th percentile values in the future if the probabilistic analysis so indicates.
Skin surface area exposed to soil
Lacking default recommendations, Region 3 typically relied on professional judgment or site-specific values when estimating skin surface area available for soil exposure. The 1997 Exposure Factors Handbook provided some guidance. RAGS E recommends 5700 cm2 for adult residents; 2800 cm2 for child residents, and 3300 cm2 for adult workers. The use of the values in the guidance will promote consistency from site to site. EPA Region 3 recommends use of the RAGS E surface areas as defaults.
Soil-to-skin adherence factors
The 1992 dermal guidance provided justification for a range of adherence factors from 0.2 to 1 mg/cm2. When the 1997 Exposure Factors Handbook appeared, it emphasized activity- and soil-specific numbers, but the numbers were also consistent with the use of 0.2 mg/cm2 as a conservative default. The 0.2 mg/cm2 value has frequently been used at Region 3 sites. RAGS E recommends this value for child residents and adult workers, but introduces a default of 0.07 mg/cm2 for adult residents. EPA Region 3 proposes accepting this recommended change in the adult resident default adherence factor.
Dermal PAH cancer risk
Following the recommendations in Risk Assessment Guidance for Superfund, Volume I, Part A ("RAGS A") and EPA's Superfund Technical Support Center (STSC) Issue Papers, EPA Region 3 has typically not quantified dermal risks from polycyclic aromatic hydrocarbons (PAHs). However, RAGS E recommends performing a quantitative assessment. It is acknowledged that direct-contact dermal risks cannot be quantified using the oral-to-dermal adjustment; however, at least the dermal risks from absorbed PAHs can be quantified in this manner. In accordance with RAGS E, Region 3 now recommends quantifying the dermal PAH cancer risk. The risk assessor should, however, acknowledge (e.g., in the uncertainty section) that the direct effect of PAHs on the skin is not included in this quantitative assessment.
Adjustment of oral toxicity factors for dermal risk
RAGS A, Appendix A, gave equations for adjusting administered doses (typically used in oral assessments) to absorbed doses (typically used in dermal assessments). EPA Region 3 compiled lists of adjustment factors from various sources, usually STSC Issue Papers and ATSDR Toxicological Profiles. RAGS E presents a compilation of adjustment factors in Exhibit 4-1, and also recommends not making this adjustment unless there will be at least a two-fold difference in the toxicity factor (i.e., if the adjustment factor is approximately 0.5 or less). The presence of centrally documented values will improve consistency across sites. EPA Region 3 recommends using RAGS E Exhibit 4-1 and the "two-fold" rule.
Dermal absorption from soil
The 1992 EPA dermal guidance included a very short list of recommendations for specific chemicals. Region 3's 1995 guidance specified chemical-class-specific defaults that could be used in the absence of chemical-specific values. RAGS E includes an expanded, but still limited, list of chemical-specific factors (Exhibit 3-4). The only class-specific defaults recommended are for semivolatiles (10%), which is consistent with Region 3's default for that class.
EPA Region 3 agrees that non-occluded skin exposed to VOCs will not absorb a large amount of material (defaults were up to 3%); however, occluded skin can have significant absorption. Furthermore, although metals are often not well-absorbed through the skin, their high absorbed-toxicity factors can make this a significant exposure route. Therefore, EPA Region 3 recommends retaining the class-specific defaults identified in the 1995 Regional guidance, in the absence of chemical-specific numbers. Region 3 recommends use of RAGS E, Exhibit 3-4, for chemical-specific defaults, superseding the 1995 guidance in that respect. As always, both chemical-specific and class-specific defaults can be superseded by site-specific data.
Filtered and unfiltered water samples
RAGS E recommends not using data from filtered water samples for the estimation of the dermal dose. Region 3 agrees that surface water, no matter how turbid, is generally available for dermal exposure, and has typically used only unfiltered data for surface water exposure. However, for groundwater, the EPA Region 3 practice has been to evaluate both filtered and unfiltered data, and to use the set that is believed to best represent the water quality in the aquifer. Highly turbid groundwater may not be representative of the exposure to actual well users, and EPA Region 3 continues to recommend that both filtered and unfiltered groundwater data should be evaluated for the criterion of representativeness. The most representative data should then be used in the risk assessment.
EPA Region 3 provides this guidance as a helpful tool to those who perform and review risk assessments in Region 3. As always, site-specific decisions or new information may supersede default practices. Questions about this document can be directed to firstname.lastname@example.org.