Absorption - Pesticide Biotransformation and Disposition - page 33

in monkey foreheads is twice that in monkey forearms ( Moody and Franklin, 1987;
Moody et al., 1987; Sidon et al., 1988 ).
However, Moody et al. (1990, 1992) demonstrated that there is no difference
between the absorption of the acid and the amine forms of 2,4-dichlorophenoxyacetic
acid (2,4-D) in rhesus monkey forearm and forehead and forearm and palm regions. The
palmar absorption data conflict with the accepted dogma that absorption through palmar
skin should theoretically be less than that through forearm skin because of the thickness
of the stratum corneum in palmar skin ( Maibach et al., 1971 ). It is proposed that because
of the hydrophilic nature of 2,4-D-amine, absorption can occur through polar routes
such as eccrine glands, which are more frequent in the palmar skin than in the forearm
skin. This anatomical difference does not explain the discrepancy with lindane, which is
more lipophilic than 2,4-D and least likely to be absorbed via a polar route.
Despite a 3-fold range in follicle area in the marmoset, no differences in absorp-
tion rates of paraquat, mannitol, water, and ethanol were observed between various
body sites ( Scott et al., 1991 ). However, among the various species examined in this
study, there was an 80-fold range in follicle area, which correlated with observed dif-
ferences in the rates of mannitol and paraquat absorption. The authors concluded that
this correlation was possible only with relatively slowly absorbed test penetrants such
as paraquat and mannitol. Further work is needed to determine the extent to which
the unique anatomical features at different body sites play a role in the absorption and
penetration of both lipophilic and hydrophilic pesticides.
Pesticide Formulation and Mixtures
Insecticide efficacy, the stability of active ingredients, and programmed release of
active ingredients from the vehicle/device are the most important characteristics con-
trolled for when pesticides are formulated ( Krenek and Rohde, 1988 ). Environmental
Protection Agency (EPA) registration does not always require percutaneous absorp-
tion studies. For this reason, more efficacy data than dermal pharmacokinetic data are
available in the literature. Furthermore, most of the available pesticide absorption data
pertain to binary mixtures (pesticide  vehicle). Technical grade formulations are, how-
ever, complex mixtures of formulation additives and, therefore, risk assessment based
on data from exposure to binary mixtures may be inappropriate. Pesticides are usually
formulated to contain active and inactive or inert ingredients. The latter can enhance
the rate and extent of absorption or slow the release of the active ingredient and thus
reduce the rate and extent of absorption ( Walters and Roberts, 1993 ). These “inert”
ingredients are often classified as adjuvants, surfactants, preservatives, solvents, diluents,
thickeners, and stabilizers. These pesticide additives were first covered by the Food and
Drug Administration and now are covered by EPA regulation 40 CFR 180.1001 and
also TSCA and FIFRA ( Seaman, 1990 ). This increasing list of inerts as well as the pro-
hibitive cost of obtaining 40 CFR 180.1001 clearance of new inerts strongly supports