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Appendix F - Dermal Uptake of Chemicals from Soil (Technical Support Document for Exposure Assessment and Stochastic Analysis)

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Overview

This September 2000 OEHHA Technical Support Document Appendix F provides California’s recommended dermal absorption fractions (ABS) for soil-bound chemicals used in the Air Toxics Hot Spots program. The chemicals reviewed include the inorganic salts of arsenic, beryllium, cadmium, mercury, lead, nickel, and hexavalent chromium, plus several organic compounds (TCDD/PCDD/PCDF, PCBs, BaP/PAHs, hexachlorocyclohexanes, DEHP, pentachlorophenol). Per-chemical ABS recommendations are point estimates derived from review of in-vivo human, in-vivo animal, and in-vitro percutaneous-absorption studies. OEHHA values diverge from the prior 1% Clement Associates (1988) default for inorganics when chemical-specific evidence supports a different value.

Key numbers

OEHHA recommended dermal ABS point estimates from soil (per metal; section references are to Appendix F):

  • Arsenic: 4% (F.1.3.1, p. F-8). Based on Wester et al. 1993b in vivo Rhesus monkey study (n=4 per dose group): 4.5 ± 3.2% absorption at low dose (0.0004 µg/cm²) and 3.2 ± 1.9% at high dose (0.6 µg/cm²), nonsignificant difference; arithmetic mean = 4%. Soil load 0.04 g/cm², 24 h exposure. In-vitro human cadaver-skin replicate of same study gave 0.8%. DTSC and EPA Region IX use 3%.
  • Beryllium: 1% (F.1.3.2, p. F-9). No quantitative dermal-absorption data located; OEHHA adopts the Clement Associates 1988 default for inorganics (consistent with Cal/EPA DTSC, EPA Region IX, CAPCOA 1993).
  • Cadmium: 0.1% (F.1.3.3, pp. F-11 to F-12). Based on Wester et al. 1992a in vitro human cadaver skin: ¹⁰⁹Cd-labeled CdCl₂ adsorbed to soil at 13 ppb, soil loads 0.02 and 0.04 g/cm², 16 h exposure; mean of 12 samples (2 soil concentrations × 2 skin sources × 3 replicates) was 0.1%; individual values ranged 0.08-0.2% (Table 6.12, p. F-11). USEPA 1992 recommends 0.1-1.0%.
  • Chromium (VI): 1% (F.1.3.4, p. F-14). No chemical-specific number derivable from available studies; OEHHA adopts the Clement Associates 1988 default. Available studies (Baranowska-Dutkiewicz 1981 in humans 3.4-23% from aqueous, with disappearance technique caveats; Wahlberg and Skog 1965; Czernielewski et al. 1965 1.3% in guinea pig) suggest 1% may be too low but data are insufficient to derive a chemical-specific value.
  • Lead: 1% (F.1.3.5, p. F-16). OEHHA adopts the Clement Associates 1988 default consistent with Cal/EPA DTSC. USEPA does not currently recommend a number for dermal Pb absorption. Quantitative human data are limited: Moore et al. 1980 (n=8, lead-203 acetate from cream/lotion to forehead, 12 h) reported mean whole-body uptake of 0.058 ± 0.081% (range 0-0.3%), rising to 0.18 ± 0.339% when preparation was dried and skin slightly abraded; Lilly et al. 1988 reported sweat/saliva increases without deriving a percent absorbed; Stauber et al. 1994 estimated 29% absorbed by disappearance technique (24 h occluded lead nitrate to forearm), flagged as overestimate.
  • Mercury: 10% (F.1.3.6, p. F-19). Order-of-magnitude default based on Baranowska-Dutkiewicz 1982 (n=8 male volunteers, aqueous HgCl₂ to forearm, 0.01-0.2 M, 22 cm² occluded; absorption 20%, 29%, 37%, 60%, 64% at 5, 10, 15, 30, 60 min) and Wester et al. 1995 abstract (in vitro ²⁰³HgCl₂ in soil at 24 h: skin content 10.4 ± 6.8%, 6.1 ± 2.0%, 7.2 ± 10.8% of dose at 5, 10, 40 mg soil loads). Replaces prior 1% Clement default.
  • Nickel: 4% (F.1.3.7, p. F-20). Based on Fullerton et al. 1986 in vitro human skin, nickel chloride hexahydrate under occlusion (3.6% in experiment 1; 9-16% in experiment 2, attributed to donor-skin variability); nickel sulfate gave 0.3% under occlusion. OEHHA selects 4% from the chloride data as a protective default. Prior Cal/EPA DTSC and CAPCOA 1993 default was 1%.

OEHHA recommended dermal ABS point estimates from soil for organics (Appendix F also covers these; included for cross-reference):

  • PCBs (Aroclor 1242/1254): 14% (F.1.4.1, p. F-23). Wester et al. 1993a in vivo Rhesus monkey.
  • PCDD/PCDF (as TCDD): 2% (F.1.4.2, p. F-27). Shu et al. 1988 in vivo rat.
  • BaP/PAHs: 13% (F.1.4.3, p. F-31). Wester et al. 1990 in vivo Rhesus monkey, 24 h dermal absorption factor 13.2 ± 3.4% (n=4).
  • Hexachlorocyclohexanes (lindane): 10% (F.1.4.5, p. F-32). Clement Associates 1988 default for organics; only one soil study located (Duff and Kissel 1995, in vitro, 0.45-2.35%).
  • DEHP: 10% (F.1.4.6, p. F-33). No soil studies located; default for organics.

Methodological notes:

  • Soil loading: USEPA 1992 recommends 0.2-1.0 mg/cm² as typical human exposure (F.1.2.4, p. F-3). Many cited studies use 20-40 mg/cm², which can overestimate fractional absorption relative to typical exposure.
  • In vivo > in vitro absorption for most chemicals; OEHHA prefers in vivo data for exposure assessment.
  • Species rank for in-vivo uptake: rabbit > rat > pig ≈ monkey ≈ humans (Wester and Maibach 1975, 1983), based on chemicals not including the inorganics or BaP; species-extrapolation caution noted.
  • Occlusion can substantially increase absorption (5-10×) for some chemicals.

Methods (brief)

OEHHA-issued regulatory technical support document; no primary measurement. Per-chemical ABS values derived from systematic review of percutaneous-absorption literature: in-vivo human, in-vivo animal (rabbit, rat, monkey), and in-vitro (human cadaver skin, isolated skin flaps, diffusion cells). Selection criteria favor in-vivo human data over animal; non-occluded over occluded; soil-matrix experiments over solvent-only; urinary/fecal excretion endpoints over disappearance from skin surface. Each chemical section has “Studies Considered” (Key Study + Other Studies) followed by “Selection of Absorption Value” with Discussion / Other Agencies’ Default Values / Recommendation. Where chemical-specific data are insufficient, OEHHA defaults to Clement Associates (1988) 1% for inorganics or 10% for organics. Limitations stated: dermal Pb and Hg data are quite limited; in-vitro studies tend to underestimate vs. in-vivo; disappearance technique overestimates; occlusion, particle size, soil organic content, and soil loading all modify ABS.

Implications

  • Certification (HMTc): Contributes per-metal soil-pathway dermal absorption point estimates (As 4%, Be 1%, Cd 0.1%, Cr-VI 1%, Pb 1%, Hg 10%, Ni 4%) covering the inorganic salts of seven metals. The Hg value is a tenfold increase over the prior 1% Clement Associates 1988 inorganic default, reflecting Baranowska-Dutkiewicz 1982 and Wester 1995 evidence; the As, Cd, and Ni values are chemical-specific derivations, while Be, Cr-VI, and Pb retain the 1% Clement default in the absence of chemical-specific data.
  • Courses: Foundational reading for any exposure-science course module on dermal absorption of metals from environmental media. Demonstrates how regulatory ABS values are constructed from limited primary evidence, and how species, occlusion, soil loading, and matrix interact to modify the underlying fraction.
  • App: Not relevant to ingredient contamination_profile. The ABS values feed any future dermal-route exposure annex but do not affect ingredient-level food contamination data.

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