Dermal exposure to pyrethroid pesticides can occur during manufacture and application. This study
examined the in vitro dermal absorption of pyrethroids using rat and human skin. Dermatomed skin from
adult male Long Evans rats or human cadavers was mounted in flow-through diffusion cells, and
radiolabeled bifenthrin, deltamethrin or cis-permethrin was applied in acetone to the skin. Fractions of
receptor fluid were collected every 4 h. At 24 h, the skins were washed with soap and water to remove
unabsorbed chemical. The skin was then solubilized. Two additional experiments were performed after
washing the skin; the first was tape-stripping the skin and the second was the collection of receptor fluid for
an additional 24 h. Receptor fluid, skin washes, tape strips and skin were analyzed for radioactivity. For rat
skin, the wash removed 53–71% of the dose and 26–43% remained in the skin. The cumulative percentage of
the dose at 24 h in the receptor fluid ranged from 1 to 5%. For human skin, the wash removed 71–83% of the
dose and 14–25% remained in the skin. The cumulative percentage of the dose at 24 h in the receptor fluid
was 1–2%. Tape-stripping removed 50–56% and 79–95% of the dose in rat and human skin, respectively, after
the wash. From 24–48 h, 1–3% and about 1% of the dose diffused into the receptor fluid of rat and human
skin, respectively. The pyrethroids bifenthrin, deltamethrin and cis-permethrin penetrated rat and human
skin following dermal application in vitro. However, a skin wash removed 50% or more of the dose from rat
and human skin. Rat skin was more permeable to the pyrethroids than human skin. Of the dose in skin, 50%
or more was removed by tape-stripping, suggesting that permeation of pyrethroids into viable tissue could
be impeded. The percentage of the dose absorbed into the receptor fluid was considerably less than the dose
in rat and human skin. Therefore, consideration of the skin type used and fractions analyzed are important
when using in vitro dermal absorption data for risk assessment.