Absorption - Pesticide Biotransformation and Disposition - page 40

Interactions between soil and the phenoxy herbicides (e.g., 2,4-D acid, 2,4-D
amine) are unique. One study demonstrated that dermal absorption of the herbicide
2,4-D acid is nonlinear with respect to soil load or skin contact time ( Wester et al.,
1996a ). Percutaneous absorption in an acetone vehicle (8.6%) was not different from
absorption of soil loads of 1 mg/cm 2 (8.6%) and 40 mg/cm 2 (15.9%) in rhesus mon-
key in vivo . Further in vitro experiments with human skin demonstrated that increas-
ing the soil load from 5 to 40 mg/cm 2 did not affect 2,4-D absorption, which ranged
from 1.4 to 1.8%. During the first 24 h of in vitro exposure, absorption was linear
with respect to time for an acetone vehicle (3.2%); however, there was an apparent lag
time of about 8 h with absorption from a soil vehicle (0.03-0.05%). This early lag time
may be related to chemical partitioning from soil and may be beneficial if the skin
is decontaminated within 24 h. The investigators proposed that because of complex
interactive forces between pesticides and soil, dermal absorption calculations based on
assumed linearity can incorrectly estimate the threat to human health. Mathematical
extrapolation from high soil loads to low soil loads may significantly underestimate
2,4-D absorption. These studies also demonstrated that soil release kinetics may limit
dermal absorption and that more data are needed to make valid predictions. It is there-
fore plausible to assume that only pesticides in the soil layer that is in direct contact
with skin are bioavailable and heavy soil loads may not necessarily increase dermal
absorption.
In contrast to DDT, chlordane absorption in rhesus monkeys in acetone (6.0%
dose) was similar to absorption in soil (4.2% dose) 6 days after exposure ( Wester et al.,
1992b ). Although human skin in vitro experiments demonstrated similar partition-
ing into receptor fluid for acetone (0.07%) and soil vehicles (0.04%), there was greater
penetration into skin with acetone (10.8%) than with soil (0.34% dose) at 24 h. It is
possible that chlordane adsorption to soil delayed percutaneous absorption during the
initial 24 h and an extrapolation to 6 days would reveal no vehicle differences as dem-
onstrated in the in vivo study. The octanol:water partitioning coefficients (log P ) of
chlordane and DDT are 5.58 and 6.91, respectively, and, therefore, dermal disposition
should be similar. The high lipophilicity of these pesticides explains the higher propor-
tion of pesticide in the skin than in the receptor phase, but it does not explain why
the differences between acetone and soil for DDT are greater than those for chlor-
dane; it only suggests that factors other than lipophilicity influence absorption of these
organochlorines.
Various studies have demonstrated that the very ubiquitous pesticide, PCP, is very
readily absorbed through human, monkey, and pig skin ( Qiao et al., 1997; Wester et al.,
1993b ). In vivo absorption of PCP in rhesus monkeys with acetone vehicle (29.2%)
was similar to absorption with soil vehicle (24.4%) after a 24-h exposure period
( Wester et al., 1993b ). However, in vitro absorption in human skin appears to under-
estimate in vivo absorption because only 0.6-1.5 and 0.01-0.07% dose were detected