Dermal Absorption and Toxicity Assessment, 2nd ed. Roberts MS, Walters KA, eds., New York: Informa Healthcare, 2007 Dec; :563-574
Recently, Spalt et al. (1) critically reviewed the available (English language) literature describing dermal absorption from soil. The earliest entry in that review is a paper by Swiss investigators concerning oral and dermal absorption of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in multiple formulations including soil (2). That investigation was inspired by dioxin contamination events in Germany and Italy in the 1970s. All but one of the subsequent studies identified in the review were conducted in the United States. Given the universality of English as the language of science, this observation presumably reflects research funding priorities stemming from political attention to hazardous waste sites and other contaminated lands, and the relative importance of quantitative risk assessment in the regulatory environment in the United States, ra her than mere language bias. Regardless, the total body of research is quite limited [Spalt et al. (1) found fewer than 50 distinct studies] and represents the efforts of a relatively small group of investigators. In addition to its limited scope, significant shortcomings of the extant dermal-absorption-from-soil literature include (i) a lack of uniformity of methodology, which greatly hinders systematic comparison across compounds and laboratories, (ii) frequently inadequate reporting of experimental details, and (iii) obvious flaws in some experimental approaches. As a consequence, commonly used procedures for estimation of dermal absorption of chemical contaminants from soil are not well developed. Current U.S. Environmental Protection Agency (USEPA) guidance for use in investigations of the worst uncontrolled and abandoned toxic waste sites in the United States, those designated Superfund sites, presents recommendations for estimation of absorption of contaminants from both soil and water (3). The soil protocol is relatively primitive and depends heavily upon literal acceptance of results, expressed as fraction of initial dose absorbed, for the limited number of chemicals (shown in Table 1) for which experimental results are available. In some cases, measurements made on one chemical were extended to the entire class of chemicals [e.g., benzo(a)pyrene as a surrogate for all polyaromatic hydrocarbons, TCOD for all dioxins, and Arodor 1242 or 1254 for all polychlormated biphenyls (PCBs)]. For contamination of soil by semivolatile organic compounds (SVOCs) not otherwise listed, a default availability of 10% is recommended. For unlisted chemicals that cannot be characterized as SVOCs, no default is stipulated and a qualitative approach is recommended. . . In contrast, a relatively well-founded protocol for estimation of dermal absorption of chemicals from water is described in the same guidance. This disparity reflects the fact that many more studies with much greater uniformity of methodology are available for water. Specifically, data describing absorption from water obtained in vitro using human cadaver skin were available for about 90 organic compounds at the time the USEPA guidance was written, whereas only about one-third as many compounds have been studied In experimental investigations of absorption from soil by all methods. A theoretically more rigorous approach for soil, based on the water permeation data, has been proposed by Bunge and Parks (4,5), but has not been adequately tested due to lack of suitable data and has not been widely adopted. In a limited comparison with results from a set of experiments that were sufficiently described (6), the Bunge and Parks approach (5) over-estimated dermal absorption of lindane and 2,4-dichlorphenoxyacetic acid (2,4-0) from two soils. . . All soils have a limited capacity to interact with a given contaminant, which is essentially the saturation limit (7). If the amount of contaminant in soil exceeds this limit, neat chemical will be present. Based on rudimentary chemical and physical principles, an organic chemical sorbed to soil at a concentration less than saturation of the soil would be expected to be less available for dermal absorption than it would be in neat form. Sorption on soil should lower fugacity and hence reduce the thermodynamic driving force for dermal absorption and might also reasonably be expected to increase mass transfer resistance. In concert, these effects should reduce flux into skin. Recent results for two different soils contaminated with methyl paraben (7,8), which included determinations of soil saturation, are consistent with the fugacity argument. The results reported in the early paper by Poiger and Schlatter (2) noted above are also generally in accord with this basic concept. In those experiments, TCOD was apparently less well absorbed from soil than from neat compound and even less well absorbed from activated carbon than from soil. However, the intervening literature is not consistent on even this fundamental point as several investigators have reported that they did not observe reduced availability from soil in the experiments they conducted. A sampling of those reports is described briefly below.