People are encouraged to wear sunscreens because of their effectiveness at reducing the risk of skin cancer. The dermal penetration of the herbicide 2,4-D can be enhanced by commercial formulations containing chemical ultraviolet (UV) absorbers, the absorbers themselves and the insect repellent DEET. This work has been extended to determine whether commercially available sunscreens containing the physical UV absorbers titanium dioxide (TiO2) or zinc oxide (ZnO) enhance the transdermal absorption of pesticides. Hairless mouse skin was pretreated with either commercially available sunscreens or the UV absorbers themselves, dissolved in phenyl trimethicone. In vitro permeability studies were performed with the pesticides 2,4-D, paraquat, parathion or malathion. The data demonstrate that pretreatment with five of the nine sunscreens tested increased the transdermal absorption of 2,4-D (P<0.05). Transdermal studies using paraquat, parathion and malathion pretreated with a representative sunscreen all demonstrated significant penetration enhancement when compared to controls (P<0.05). Repeated 2,4-D and sunscreen applications resulted in either no change between pulses or an increase in absorption after the second pulse depending on the washing regimen. Examining penetration of individual UV absorbers formulated in phenyl trimethicone showed that that ZnO can impede 2,4-D penetration and TiO2 had no effect. Combining UV absorbers in the presence of trimethicone resulted in 'sunscreens' that could actually inhibit 2,4-D penetration. Inert ingredients therefore control the increased absorption seen in commercial sunscreen products and this enhancement can be eliminated by substituting phenyl trimethicone as the solvent. Sunscreen use must still be encouraged even with the undesirable side effect of increased penetration through the skin.
Topical ethanol is used as a dermal penetration enhancer in some commercial products. Previous studies have demonstrated that chronic ethanol consumption can also disrupt skin barrier function, leading to increased transdermal penetration. This observation becomes much more relevant if a single drinking episode induces similar changes. The purpose of this study was thus to examine the transdermal penetration of three model chemicals after acute ethanol consumption. Wistar rats were gavaged with either 10, 6, 4.3, 3, 1.5 g/kg ethanol or saline and allowed to recover for 2 or 24 h. Blood and skin ethanol levels were determined and in vitro penetration experiments performed. The herbicide paraquat, industrial solvent N,N-dimethylformamide (DMF), and insect repellent N,N-diethyl-m-toluamide (DEET) were used as is model chemicals. Absorption was determined and directly compared between ethanol- and saline-treated skin by calculating enhancement ratios. Blood ethanol levels range from 0.25 to 0.015% at 2 h with skin levels at 12-18% of blood values. Ethanol enhances the absorption of paraquat, DMF, and DEET in a dose-dependent fashion. Paraquat and DEET showed no appreciable reduction in enhancement between 2 and 24 h postgavage for the 10-g/kg dose, but DMF did. Enhancement ratios were higher at 24 h for 10 than for 6 g/kg animals, demonstrating a dose-response relationship for recovery time. These studies imply that increased absorption of topical chemical occurs after alcohol ingestion. Both acute and chronic ethanol consumption can compromise the dermal barrier.
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