Absorption - Pesticide Biotransformation and Disposition - page 30

and plasma, and 97% of pentachlorophenol was associated with plasma. In plasma, lin-
dane (64%) and DDE (92%) were mainly associated with lipoproteins, pentachlorophe-
nol was mainly associated with “other” plasma proteins (81%), and hexachlorobenzene
was nearly equally distributed. A very different pattern of distribution was observed in
human blood. Hexachlorobenzene and lindane in plasma were nearly equally distributed
between lipoproteins and “other” plasma proteins, whereas 60% of DDE was associated
with “other” proteins. Other investigators have shown that dieldrin is over 99% bound
to human serum proteins ( Garrettson and Curley, 1969 ), and diflubenzuron is 40-50%
bound to plasma proteins in chickens ( Opdycke and Menzer, 1984 ). The organophos-
phate diazinon is 89% bound to proteins in rat plasma ( Wu et al., 1996 ).
ABSORPTION
Human exposure to pesticides is typically by oral, dermal, and inhalation routes.
Occupational exposure to pesticides is more likely to occur by dermal contact, and
this is the focus of the section Percutaneous Absorption. Percutaneous absorption is
reported as the possible route of entry in 65-85% of all cases of occupational exposure
to pesticides ( Galli and Marinovich, 1987 ). Spray or dusting of pesticides can result in
disposition of 20-1700 times the amount deposited in the respiratory tract ( Feldmann
and Maibach, 1974 ). Epidemics of pesticide poisoning following cutaneous exposure
have been reported for nonoccupational uses ( Ferrer and Cabral, 1993 ). These cases
have often involved accidental contamination of infant clothing or exposure to tal-
cum powder with pesticides ( Martin-Bouyer et al., 1983 ). These anecdotal case reports,
coupled with dermal exposure estimated from various direct and indirect dosimetric
experiments, are often the only available human data with which to perform der-
mal absorption assessment. Despite such limited data, it is possible to estimate dermal
absorption by extrapolation from dermal exposure data. Algebraic equations that take
into account exposure time and the chemical nature of the compound (lipophilicity
and molecular weight) have been presented for estimating dermal absorption ( Cleek
and Bunge, 1993; Potts and Guy, 1992 ).
Exposure of small children is more likely to be by oral and/or dermal routes.
Absorption from the gastrointestinal and respiratory tracts is discussed under
Absorption from the Gastrointestinal Tract and Absorption from the Respiratory Tract,
respectively. Other routes of exposure that are used primarily in the laboratory (sub-
cutaneous, intravenous, intraperitoneal, and intramuscular) are discussed only briefly
under Absorption after Exposure by Other Routes.
A chemical is considered to be absorbed when it reaches the bloodstream. For
routes other than intravenous administration, which bypasses the process of absorption,
a chemical is absorbed when it crosses the epithelial layers in the skin, small intestine,
or alveoli in the lungs and enters the bloodstream from an external site of exposure.