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.
Sunscreen use can reduce the incidence of certain skin cancers. However, a number of commercially available formulations have been shown to enhance the transdermal penetration of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Most of the active ingredients used in these compounds can individually act as penetration enhancers. Commercial sunscreens frequently contain multiple active ingredients in order to provide broad sunscreen protection. The purpose of this study was therefore to examine the effect of these active ingredient combinations on the transdermal absorption of 2,4-D in vitro. All six of the combinations tested resulted in increased cumulative penetration (P <0.01) and faster lag times (P <0.05). The 2,4-D cumulative penetration in the presence of the OFF! Deepwoods combination was significantly greater than the absorption with either the individual ingredients or their average (P <0.05). A systematic study designed to isolate the chemicals responsible for this enhancement demonstrated that with UV absorbers DEET synergistically increased the 2,4-D penetration and that DEET's cumulative enhancement properties correlate with its concentration. By contrast, octocrylene significantly slowed the lag time when used in combinations and was the only active ingredient that showed any antagonistic effects on 2,4-D penetration. Because none of the active ingredient combinations were able to inhibit dermal uptake of 2,4-D, it seems that proper selection of inert ingredients may be the most feasible solution for reducing penetration enhancement.
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