The mismeasure of dermal absorption

The results of dermal absorption experiments are routinely and often exclusively reported in terms of fractional absorption. However, fractional absorption is not generally independent of skin loading conditions. As a consequence, experimental outcomes are commonly misinterpreted. This can lead in turn to poor estimation of exposures under field conditions and inadequate threat assessment. To aid interpretation of dermal absorption-related phenomena, a dimensionless group representing the ratio of mass delivery to plausible absorptive flux under experimental or environmental conditions is proposed. High values of the dimensionless dermal number (N(DERM)) connote surplus supply (i.e., flux-limited) conditions. Under such conditions, fractional absorption will generally depend on load and should not be assumed transferable to other conditions. At low values of NDERM, dermal absorption will be delivery-limited. Under those conditions, high fractional absorption is feasible barring maldistribution or depletion due to volatilization, washing, mechanical abrasion or other means. Similar logic also applies to skin sampling and dermal toxicity testing. Skin surface sampling at low NDERM is unlikely to provide an appropriate measure of potential dermal dose due to depletion, whereas dermal toxicity testing at high NDERM is unlikely to show dose dependence due to saturation. Journal of Exposure Science and Environmental Epidemiology (2011) 21, 302-309; doi:10.1038/jes.2010.22; published online 28 April 2010