Assessing Dermal Exposure from Soil

Region 3 Technical Guidance Manual, Risk Assessment

Dermal absorption from soil is one of the routes of exposure that may be addressed during risk assessment at Superfund sites. One factor necessary to estimate dose, and therefore risk, via this route is the absorption factor of a chemical from soil. This document is intended to provide default assumptions for this factor in the assessment of dermal soil exposure.

Assessing Dermal Contact With Soil; Existing Guidance

Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part A) ("RAGS"; EPA, 1989) presents an equation used to estimate exposure from dermal contact with soil:

Where:

• AD = (CS x CF x SA x AF x ABS x EF x ED)/(BW x AT)
• AD = Absorbed dose (mg/kg/day)
• CS = Chemical concentration in soil (mg/kg)
• CF = Conversion factor (10-6 kg/mg)
• SA = Skin surface area available for contact (cm2/event)
• AF = Soil-to-skin adherence factor (mg/cm2)
• ABS = Absorption factor (unitless)
• EF = Exposure frequency (events/year)
• ED = Exposure duration (years)
• BW = Body weight (kg)
• AT = Averaging time (period over which exposure is averaged-days)

(RAGS, Exhibit 6-15)

RAGS then states: "Absorption factors (ABS) are used to reflect the desorption of the chemical from soil and the absorption of the chemical across the skin and into the bloodstream. Consult the open literature for information on chemical-specific absorption factors. In the absence of chemical-specific information, use conservative assumptions to estimate ABS." The use of conservative assumptions is appropriate when determining Reasonable Maximum Exposure (RME), and reflects EPA's policy that protection of human health should be ensured.

Assessment of dermal exposure is important for a complete risk assessment. This document summarizes chemical-specific and general (for classes of compounds) absorption factors that have been found in the limited database available. The factors were compiled from existing national guidance and peer-reviewed scientific literature. It is recommended that these numbers be used as defaults for the ABS parameter when calculating RME soil exposure in the absence of chemical-specific, site-specific information. These defaults are presented in order to facilitate performance of risk assessments by compiling these factors in one place, and to promote consistency in risk assessment.

Polychlorinated Biphenyls (PCBs)

A review of studies assessing the dermal absorption of 3,3',4,4'-tetrachlorobiphenyl (TCB) from soil appeared in EPA, 1992. The range of absorption was reported to be 0.6% to 6%. Region 3 recommends accepting the 6% value as a conservative assumption of ABS for polychlorinated biphenyls, in keeping with RAGS.

• Learn more from the IRIS Assessment of Polychlorinated Biphenyls (PCBS).

Chlorinated Dioxins

A review of studies assessing the dermal absorption of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from soil appeared in EPA, 1992. The range of absorption was reported to be 0.1% to 3%. Region 3 recommends accepting the 3% value as a conservative assumption of ABS for chlorinated dioxins, in keeping with RAGS.

• Learn more from the IRIS Assessment of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Cadmium

A review of studies assessing the dermal absorption of cadmium from soil appeared in EPA, 1992. The range of absorption was reported to be 0.1% to 1%. Region 3 recommends accepting the 1% value as a conservative assumption of ABS for cadmium, in keeping with RAGS.

• Learn more from the IRIS Assessment of Cadmium.

Arsenic

In vivo studies from Wester et al, 1993a, report 3.2% for a dose of 0.6 ug/cm2. Region 3 recommends accepting this as a default ABS for arsenic.

• Learn more from the IRIS Assessment of Arsenic, inorganic.

Other Metals

Suggested ABS factors based on the pharmacokinetic properties of chemicals appeared in Ryan et al, 1987. The proposed range for dermal absorption of inorganics from soil was 0.1% to 1%. This was also consistent with a review of the studies for cadmium, an inorganic, as assessed in EPA, 1992. Region 3 recommends accepting the 1% value as a conservative assumption of ABS for inorganics, in keeping with RAGS.

Volatile Organic Compounds

Volatile organics are especially difficult to assess, because most studies to date have involved occluding the skin, which may give artificially high ABS values, since these compounds would also be expected to volatilize from the skin. Suggested ABS factors based on the pharmacokinetic properties of chemicals appeared in Ryan et al , 1987. The proposed range for dermal absorption of volatile organics from soil was 10% to 25%.

However, experimental data show even lower ABS values for volatile organics. For volatile organics such as benzene (vapor pressure approximately 95.2 mm Hg), Region 3 recommends accepting the 0.05% value based on Skowronski et al , 1988, and Franz, 1984. This would include chemicals such as 1,1- dichloroethane 1,1,1-trichloroethane, and other volatiles with vapor pressure similar to or greater than that of benzene. For volatiles such as ethylbenzene, tetrachloroethene, toluene, and xylenes, which have vapor pressures lower than that of benzene (and less volatilization from the skin may occur), a default ABS of 3% is recommended.

These numbers only apply to non-occluded skin, which would be the scenario expected for most environmental exposures. If, however, the skin is occluded for any reason, higher ABS values (up to 100%) should be used.

• Learn more from the IRIS Assessment of Arsenic.

Pentachlorophenol

In vivo studies from Wester et al, 1993b, report 24.4% for a dose of 0.7 ug/cm2 in soil. Region 3 recommends accepting this as a default ABS for pentachlorophenol.

Other Semivolatile Organic Compounds

Suggested ABS factors based on the pharmacokinetic properties of chemicals appeared in Ryan et al, 1987. The proposed range for dermal absorption of semivolatile organics from soil was 1% to 10%. The reported absorption of topically applied pure benzo[a]pyrene in studies in EPA, 1992, ranged from 1% to 13%. Kao et al, 1985, reported approximately 3% for absorption of topically applied pure benzo[a]pyrene by in vitro human skin. The absorption from soil would be expected to be lower and indicates that the range in Ryan et al , 1987, may be conservative with respect to this particular compound but not necessarily unreasonable. Region 3 recommends accepting the 10% value as a conservative assumption of ABS for semivolatile organics, in keeping with RAGS.

Pesticides

Suggested ABS factors based on the pharmacokinetic properties of chemicals appeared in Ryan et al, 1987. The proposed range for dermal absorption of pesticides from soil was 1% to 10%. The reported absorption of topically applied pesticides and herbicides in acetone to in vitro human skin was reported to be within this range for lindane, aldrin, dieldrin, malathion, parathion, and 2,4-D in Feldmann and Maibach, 1974. DDT absorption from soil in monkey and human skin was reported to range from 1.04 to 3.3% in EPA, 1992. These studies indicate that the range in Ryan et al , 1987, may be conservative but not necessarily unreasonable. Region 3 recommends accepting the 10% value as a conservative assumption of ABS for pesticides, in keeping with RAGS.

Applications of this Guidance

This document represents a summary of best professional judgment at this time. It is not intended to be a detailed technical analysis of dermal exposure experimentation. As a summary of best professional judgment and default parameters, the recommendations herein may be superseded by newer, chemical-specific and route-specific studies, or by site-specific studies of acceptable quality.

These factors apply to absorption from soil or sediment. Dermal absorption of chemicals from water is discussed in RAGS and EPA, 1992.

It should be noted that when estimating absorbed doses for chemicals, dose-response parameters such as Reference Doses (RfDs) and Cancer Slope Factors (CSFs) should be adjusted accordingly, where possible and appropriate, as per RAGS Appendix A.

Summary

Dermal absorption from soil is one of the routes of exposure that may be addressed during risk assessment at Superfund sites. One factor necessary to estimate dose, and therefore risk, via this route is the absorption factor of a chemical from soil. This document recommends default assumptions for this factor in the assessment of dermal soil exposure, based on the limited available information and best professional judgment.

References

• EPA, 1989. Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part A). Office of Emergency and Remedial Response, December, 1989. EPA/540/1-89/002.
• EPA, 1992. Dermal Exposure Assessment: Principles and Applications. Interim Report. Office of Health and Environmental Assessment, January, 1992. EPA/600/8-91/011/B.
• Feldmann, R.J. and H.I. Maibach, 1974. Percutaneous penetration of some pesticides and herbicides in man. Toxicol. Appl. Pharmacol. 28:126-132.
• Franz, T.J., 1984. Percutaneous absorption of benzene. Adv. Modern Environ. Toxicol. Vol. 6. Applied Toxicology of Petroleum Hydrocarbons. Princeton Scientific Publishers, Inc., Princeton, N.J. pp. 61-70.
• Kao, J.K., F.K. Patterson, and J. Hall, 1985. Skin penetration and metabolism of topically applied chemicals in six mammalian species, including man: An in vitro study with benzo[a]pyrene and testosterone. Toxicol. Appl. Pharmacol. 81:502-516.
• Ryan, E.A., E.T. Hawkins, et al. 1987. Assessing Risk from Dermal Exposure at Hazardous Waste Sites. In Bennett, G. and J. Bennett, eds. Superfund '87: Proceedings of the Eighth National Conference; November 16-18; Washington, D.C. The Hazardous Materials Control Research Institute. pp. 166-168.
• Skowronski, G.A., R.M. Turkall, and M.S. Abdel-Rahman. 1988. Soil adsorption alters bioavailability of benzene in dermally exposed male rats. Am. Ind. Hyg. Assoc. J. 49(10):506-511.
• Wester, R.C., H.I. Maibach, et al. 1993a. In vivo and in vitro percutaneous absorption and skin decontamination of arsenic from water and soil. Fundamental and Applied Toxicology. Vol. 20, No. 3, pp. 336-340.
• Wester, R.C., H.I. Maibach, et al. 1993b. Percutaneous absorption of pentachlorophenol from soil. Fundamental and Applied Toxicology. Vol. 20, No. 1, pp. 68-71.

For additional information, call (215) 814-3041.

Approved by:   
Thomas C. Voltaggio, Director
Hazardous Waste Management Division