Advective and diffusive dermal processes for estimating terrestrial amphibian pesticide exposure

Background/Question/Methods

Dermal exposure presents a potentially significant but understudied route for pesticide uptake in terrestrial amphibians.  Historically, evaluation of pesticide risk to both amphibians and reptiles has been achieved by comparing ingestion and inhalation estimates to bird and/or mammal toxicity values.  Although some toxicity data exist for amphibians, the focus has largely centered around early aquatic life stages.  Beyond providing protection and minimizing desiccation, amphibian skin has unique characteristics for gas and water exchange that may make this group very susceptible to contaminants relative to amniotes.  Amphibian skin is very thin, porous, lacks an outer hydrophobic layer and has little keratin relative to reptiles and birds.  Hydrophilic pesticides are very likely to be taken up through amphibian skin and this includes uptake from contaminated soil and foliage by terrestrial amphibians at any life stage.   To expand on current pesticide screening tools, we surveyed the literature using ISI Web of Knowledge for pesticide toxicity studies in terrestrial amphibians.  We used 5 published data sets to parameterize competing dermal exposure models.  Parameters used include pesticide specific K_ow (octanol-water partitioning coefficient), K_p (permeability coefficient), pesticide soil concentration, species body weight, skin thickness, surface area of exposure, water flux and bioavailability factor. 

Results/Conclusions

We examined two alternative dermal uptake process models with the available datasets on pesticide exposure in terrestrial amphibians.  As amphibians became increasingly dehydrated, the advective model incorporating water potential between soil and dermis had more support relative to the diffusion model.  Juvenile and adult amphibians living in terrestrial habitats may experience higher pesticide exposures in agricultural environments due to specialized adaptations for maintaining hydration levels.  Diffusion-based methods used to inform regulatory agencies of contaminant risk to amphibians have the potential to underestimate dermal risk.  The paucity of published data sets relating contaminant loads to terrestrial amphibian uptake and mortality is a limiting factor in constructing and validating reliable models.  To further improve modeling efforts and policies related to terrestrial amphibian pesticide exposure, laboratory and field studies evaluating species and contaminant specific endpoints are greatly needed.