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. 3
Tulve et al., 2010. Methodologies for estimating cumulative human exposures to current

-use

pyrethroid pesticides. Journal of Exposure Science and Environmental Epidemiology, 1-11.

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Buist et al., 2009. Relative absorption and dermal loading of chemical substances: Consequences

for risk assessment. Regulatory Toxicology and Pharmacy, 54:221-228.

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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.

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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.

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Physical 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. 13
Quaranta et al., 2009. Why Amphibians Are More Sensitive than Mammals to

Xenobiotics. PLOS One, e7699. 14
Permeability coefficient data

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Quaranta 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. 17
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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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