Absorption - Pesticide Biotransformation and Disposition - page 36

absorption of p -nitrophenol and paraoxon. In other related absorption studies, pre-
treatment with 3% fenvalerate decreased subsequent absorption of parathion, increased
subsequent lindane absorption, and had no effect on subsequent fenvalerate or carba-
ryl absorption ( Chang et al., 1995 ). These results underscore the chemical specificity
of these interactions and reinforce the concept that the percutaneous absorption of a
mixture cannot be predicted from individual component studies.
These data suggest that other mechanisms in addition to vehicle and surfactant
effects must be operating simultaneously; hence further investigation is required. The
data reinforce the concept that the permeability of a mixture cannot be predicted from
individual component studies. Many of the mechanisms of pesticide mixture interac-
tions are not well understood and are not easy to model, although a biophysical model
for parathion has been attempted ( Williams et al., 1996 ). It should also be recognized
that it is more often the formulation additives and other environmental factors rather
than the active ingredient that compromise the skin barrier and eventually enhance
pesticide absorption. There is epidemiological evidence that agricultural pesticides can
cause dermatoses ( Abrams et al., 1991; Cellini and Offidani, 1994; Guo et al., 1996 ),
and there is experimental evidence that UV irradiation can enhance skin reactions to
topical agricultural chemical treatment ( Kimura et al., 1998 ). In the latter study, sig-
nificant reactions were observed for several herbicides. Maibach and Feldmann (1974)
demonstrated that dermal absorption of pesticides such as parathion, azodrin, and
diquat occurs more readily (ninefold) through damaged skin than through normal skin.
It is, therefore, plausible to assume that the formulation additive can inflict local revers-
ible or irreversible damage to the skin structure and physiology and that it is these
interactions that modulate dermal absorption of most pesticides.
Recent in vivo animal studies have demonstrated that oral consumption of alcohol
can significantly increase the dermal absorption of the herbicides 2,4-D, paraquat, and
atrazine and the insect repellent DEET by as much as 1.6- to 2.3-fold ( Brand et al.,
2004, 2007 ). The authors of these studies proposed that alcohol solvates the polar head
regions of the lipid in the stratum corneum and thus disrupts the interactions between
the polar head group and alkyl chains. Further work by these investigators demon-
strated that oral ingestion of alcohol significantly enhanced skin lipid peroxidation and
transepidermal water loss and this is a more plausible explanation for the increased
dermal absorption of these pesticides ( Brand and Jendrzejewski, 2008 ).
Environmental Factors
(a) Temperature
Changes in ambient air temperature can alter lipid fluidity in the intercellular lipid
domain of the stratum corneum. This alteration in the intercellular pathway can the-
oretically alter pesticide penetration through the stratum corneum. Previous in vivo
studies have demonstrated that increased percutaneous absorption of a cholinesterase