Predicting dermal absorption of gas-phase chemicals: transient model development, evaluation, and application

A transient model is developed to predict dermal absorption of gas-phase chemicals via direct air-to-skin-to-blood transport under non-steady-state conditions. It differs from published models in that it considers convective mass-transfer resistance in the boundary layer of air adjacent to the skin. Results calculated with this transient model are in good agreement with the limited experimental results that are available for comparison. The sensitivity of the modeled estimates to key parameters is examined. The model is then used to estimate air-to-skin-to-blood absorption of six phthalate esters for scenarios in which (A) a previously unexposed occupant encounters gas-phase phthalates in three different environments over a single 24-h period; (B) the same as 'A', but the pattern is repeated for seven consecutive days. In the 24-h scenario, the transient model predicts more phthalate absorbed into skin and less absorbed into blood than would a steady-state model. In the 7-day scenario, results calculated by the transient and steady-state models converge over a time period that varies between 3 and 4 days for all but the largest phthalate (DEHP). Dermal intake is comparable to or larger than inhalation intake for DEP, DiBP, DnBP, and BBzP in Scenario 'A' and for all six phthalates in Scenario 'B'.

Practical implications: Dermal absorption from air has often been overlooked in exposure assessments. However, our transient model suggests that dermal intake of certain gas-phase phthalate esters is comparable to, or larger than, inhalation intake under commonly occurring indoor conditions. This may also be the case for other organic chemicals that have physicochemical properties that favor dermal absorption directly from air. Consequently, this pathway should be included in aggregate exposure and risk assessments. Furthermore, under conditions where the exposure concentrations are changing or there is insufficient time to achieve steady-state, the transient model presented in this study is more appropriate for estimating dermal absorption than is a steady-state model.