title: A new method for estimating dermal absorption from chemical exposure. 3. Compared with steady-state methods for prediction and data analysis.
abstract: OBJECTIVE
  This paper compares unsteady-state and steady-state methods for estimating dermal absorption or analyzing dermal absorption data. The unsteady-state method accounts for the larger absorption rates during short exposure times as well as the hydrophilic barrier which the viable epidermis presents to lipophilic chemicals.
  
  
  METHODS
  Example calculations for dermal absorption from aqueous solutions are presented for five environmentally relevant chemicals with molecular weights between 50 and 410 and log10Kow between 0.91 and 6.8: chloromethane, chloroform, chlordane, 2,3,7,8-TCDD, and dibenz(a,h)anthracene. Also, the new method is used to evaluate experimental procedures and data analyses of in vivo and in vitro permeation measurements.
  
  
  RESULTS
  In the five example cases, we show that the steady-state approach significantly underestimated the dermal absorption. Also, calculating permeability values from cumulative absorption data measured for exposure periods less than 18 times the stratum corneum lag time will overestimate the actual permeability.
  
  
  CONCLUSIONS
  In general, steady-state predictions of dermal absorption will underestimate dermal absorption predictions which consider unsteady-state conditions. Permeability values calculated from data sets which include unsteady-state data will be incorrect. Strategies for analyzing in vitro diffusion cell experiments and confirming steady state are described.
mesh_terms: D000818:Animals; D002463:Cell Membrane Permeability; D004781:Environmental Exposure; D004785:Environmental Pollutants / Q000493:pharmacokinetics*; D006801:Humans; D008432:Mathematical Computing; D008954:Models, Biological; D011237:Predictive Value of Tests; D012869:Skin Absorption
chemical_list: D004785:Environmental Pollutants