Absorption - Pesticide Biotransformation and Disposition - page 32

Percutaneous Absorption
The skin is a complex tissue with a large surface area, whose primary functions
are to protect the body from physical or chemical insult, to thermoregulate, and to
simultaneously prevent water loss from the body. Dermal absorption of any chemi-
cal requires movement from the environment across this barrier, which is a biochemi-
cal milieu of complex lipids and proteins. Experimentally, there are several in vitro, ex
vivo, and in vivo models that have been used to estimate dermal absorption of pesti-
cides in humans. Although in vivo methods are the gold standard, each of these meth-
ods has its respective weaknesses and strengths for accurately predicting the dermal
absorption of pesticides. Dermal absorption assessment is further complicated by spe-
cies, age, and sex differences and differences between anatomical sites within a species.
More importantly, dermal absorption in rodent skin is not always equivalent to that in
human skin.
Dermal absorption is dependent on the physicochemical properties of the pesticide,
the formulation, and the environmental conditions. The pesticide applicator is often
clothed and operating in extreme environments, not standard laboratory conditions.
This section of the chapter focuses on the differences in absorption between anatomi-
cal body sites and the effects of formulation chemistry and environmental factors that
influence percutaneous absorption of pesticides.
Anatomical Site Differences
Regional variation in skin permeability at different body sites may be related to skin
thickness, number of cell layers, cell size of the epidermis and stratum corneum, and
distribution of hair follicles and sweat pores. Because of thick layers of stratum cor-
neum, permeability in palmar and plantar skin is expected to be less than that in the
scalp or forearm ( Feldmann and Maibach, 1974 ). Data from several studies suggest
that regional variation in vascular anatomy and blood flow should also be considered
( Monteiro-Riviere et al., 1990; Qiao et al., 1993 ).
Various studies have demonstrated regional variation in penetration of drugs and
pesticides in pig skin ( Qiao and Riviere, 1995; Qiao et al., 1993 ), rat skin ( Bronaugh,
1985 ), and rhesus monkey skin ( Wester et al., 1980 ). These studies further demon-
strated that parathion penetrates nonoccluded pig skin in the decreasing order of back,
shoulder, buttocks, abdomen; for occluded skin, the order is back, abdomen, buttocks,
shoulder. Wester et al. (1994) also demonstrated that pyrethrin absorption through the
human forearm is less than the predicted absorption in the human scalp. This anatom-
ical difference is somewhat consistent with lindane absorption through the forearm
(18%), forehead (34%), and palm (34%) of rhesus monkeys ( Moody and Ritter, 1989 ).
This anatomical range for lindane is similar to that for dermal absorption of DEET
(diethyl- m -toluamide) in rhesus monkeys ( Moody et al., 1989 ). There are also data
to suggest that dermal absorption of permethrin, aminocarb, DEET, and fenitrothion