Absorption - Pesticide Biotransformation and Disposition - page 37

inhibitor (VX) was a function of skin temperature ( Craig et al., 1977 ). In humans topi-
cally exposed to parathion at various ambient temperatures (11, 25, and 40°C), the
urinary excretion of the metabolite p -nitrophenol paralleled the increase in ambient
temperature ( Hayes et al., 1964 ). Several in vitro experiments with pig skin also dem-
onstrated that increasing the air temperature from 37 to 42°C significantly increased
parathion absorption ( Chang and Riviere, 1991 ).
Increased ambient temperatures can also increase the evaporation of volatile pes-
ticides from the skin, thereby reducing the topical dose available for absorption.
Increasing air flow over the skin increases evaporative loss and significantly decreases
dermal residues in the upper skin layer of pigs for DDT (dichlorodiphenyltrichloro-
ethane), malathion, parathion, and DEET ( Reifenrath et al., 1991 ). Wester et al. (1992a)
demonstrated that isophenfos concentrations on the human skin surface in vivo were
less than 1% dose at 24 h and that evaporation from the skin surface during absorption
reduced the dose available for penetration and absorption. Finally, it should be recog-
nized that skin surface conditions in vitro are more easily controlled than in vivo, and
data from in vitro studies can significantly underestimate evaporation in vivo.
(b) Humidity and Occlusion
Skin hydration can be increased by occlusion, with high relative humidity or immer-
sion conditions (e.g., swimming or bathing). Although previously it was assumed that
hydration changes affect dermal absorption of only polar compounds, there are signifi-
cant data that suggest that at high relative humidity, this hydration effect becomes more
important for nonpolar molecules such as pesticides, and is most probably secondary to
an increase in diffusivity of the penetrating molecule ( Behl et al., 1980 ). Under relative
humidity conditions greater than 80%, parathion absorption was significantly increased
in pig skin in vitro by as much as two to three times the value under standard condi-
tions of 60% relative humidity ( Chang and Riviere, 1991, 1993 ).
The practical application of occlusion is when pesticides get into and under the
clothing of workers; this creates the ideal reservoir for penetration and absorption
into the skin. Occlusion can change dermal absorption by various mechanisms, such
as reduction in evaporation from the skin surface, enhancement of skin hydration,
changes in cutaneous metabolism, dermal irritation, and altered cutaneous blood cir-
culation (e.g., vasodilation). Occlusion can increase hydration of the stratum corneum
from as little as 5-15% to as much as 50% ( Bucks et al., 1989a ), thereby modulating
the absorption profile for the pesticide. One in vivo study with pigs ( Qiao et al., 1997 )
demonstrated that occlusion significantly enhanced pentachlorophenol (PCP) absorp-
tion from 29.1 to 100.72% dose and changed the shape of the absorption profile in
blood and plasma. The study also suggested that occlusion changed the local metabo-
lism of PCP and, as a result, the 14 C partitioning between plasma and red blood cells.
Occlusion was also kinetically related to modification of cutaneous biotransformation