# Dermal contact, movement, and amphibian pesticide exposure Tom Purucker, Tanja Crk , Ed Odenkirchen

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### Mammalian dermal contact

Hydrophilic and lipophilic molecules have separate pathways for dermal exposure in humans, with lipophilic molecules getting the most attention and a subsequent focus on non

- ionic (neutral, lipophilic) forms of contaminants. 3Tulve et al., 2010. Methodologies for estimating cumulative human exposures to current

-use pyrethroid pesticides. Journal of Exposure Science and Environmental Epidemiology, 1

- 11. 4Buist et al., 2009. Relative absorption and dermal loading of chemical substances: Consequences for risk assessment. Regulatory Toxicology and Pharmacy, 54:221

- 228. 5

### Fundamental structural differences Empirical data Modeling results comparing amphibian dermal contact dose to other taxa

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# Physical differences between amphibian and mammal skin
Thin stratum corneum No external hydrophobic barrier Less keratinized Seat patch preferential pathway Potential for dermal gas exchange Physiological metamorphosis changes properties over life history course.

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# The seat patch Amphibian seat patch is crenulated, effective surface area for uptake is much larger than the actual size of the patch. Amphibian seat patch is a preferential path for osmotic water uptake. Water potential dependent. Movement exposes seat patch to pesticides sprayed on bare soil and leaf surfaces in terrestrial environments.

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# Burrowing behavior

Irrigated farm systems are preferential habitats during droughts (or even after dry days), terrestrial amphibians prefer soils with high moisture content and may burrow into them daily for rehydration purposes. Amphibians (often) burrow, overnight or overwinter, to rehydrate and this period where they are in 100% contact with soil can be a significant portion of exposure. Also while burrowing for long periods of time they ingest shedded skin, adding a dermal component to the ingestion scenario. 9

### A number of studies comparing dermal contact with other exposure pathways have indicated that amphibian dermal exposures can be a significant fraction of contributions to the total dose (Hall and Swineford 1979, James et al. 2004, Johnson et al. 1999).

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# What does this mean for dermal pesticide exposure? The more terrestrial an amphibian’s life cycle is, the more likely it is to use the skin to regulate its water content in order to maintain hydration. Therefore, amphibian dermal contact from enhanced skin permeability can be a key exposure pathway compared to non

- amphibian receptors. This effort is an attempt to quantify the contribution of dermal for amphibians, reptiles, birds, and mammals to total dose.

11Physical differences in skin causes differences in permeability rates

– cm/hour Number of studies available for mammals, few for non- mammals. Function of a number of variables, but primarily hydrophobicity

- log( Kow) 

# Permeability coefficients

Weir et al. 2010. Ecological risk of anthropogenic pollutants to reptiles: Evaluating assumptions of sensitivity and exposure. Environmental Pollution, 158:3596

- 3606. 12

# Permeability coefficient data

Walker et al. 2003. Quantitative structure

- activity relationships for predicting percutaneous absorption rates. Environmental Toxicology and Chemistry, 22(8):1870

- 1884. 13Quaranta et al., 2009. Why Amphibians Are More Sensitive than Mammals to Xenobiotics . PLOS One, e7699. 14

# Permeability coefficient data

15Quaranta et al., 2009. Why Amphibians Are More Sensitive than Mammals to Xenobiotics . PLOS One, e7699. 16

# Model exposure profiles

Calculate exposure for diet and dermal for amphibians, birds, mammals, reptiles. Solve for range of appropriate body weights for insectivores in each class (instead of representative receptor). Range of Kows
. Compare total dose between classes and relative contribution of dermal. No tox comparisons, just looking at exposure. 17181920212223

# Dermal approaches: Finite source and Exposure rate Kp dermal exposure rate
Proposed Dust refinement. Exposure associated with movement. “Finite” source exposure

Part of DUST Tier 1 Finite source in vicinity of aestivating animal.

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# Conclusions Common exposure methods applied to 4 Classes, parameterized for Amphibians, Birds, Mammals, Reptiles Amphibians/Reptiles get significant dose from dermal, Birds/Mammals not at all. Implications for use of birds to be protective of amphibians/reptiles for some contaminants. Need to compare dermal exposure rate method and finite source method to lab and field data sets for amphibians, apply parameter estimation techniques.

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