Absorption - Pesticide Biotransformation and Disposition - page 43

Nolan et al., 1984; Timchalk et al., 2002 ). The commonly used chlorophenoxy herbi-
cide 2,4-D is rapidly absorbed by the human gastrointestinal tract ( Kohli et al., 1974;
Sauerhoff et al., 1977 ); however, a more recent rodent study suggests that clearance can
be uniquely sex-dependent ( Griffin et al., 1997 ).
The quaternary nitrogen herbicide paraquat has a limited bioavailability in labo-
ratory animals of about 22% and less than 5% absorption during the first 6 h ( Chui
et al., 1988; Meredith and Vale, 1987 ), although it has often been associated with
human poisonings following accidental or suicidal ingestion. Paraquat absorption
across the GI tract can be rapid, with the peak plasma concentration occurring within
the first hour after exposure ( Heylings, 1991; Nagao et al., 1993 ) and it is plausible to
assume that the presence of herbicide performance surfactants may play a significant
role in paraquat uptake across the GI tract. There have been recent successful efforts to
include additives such as alginate in paraquat formulations to reduce paraquat uptake
by the GI tract of experimental animals. The mechanism involves delaying gastric emp-
tying, which prevents early high lethal dose delivery to the lungs and thus enhances
human survival following acute exposure to this lethal herbicide ( Heylings et al., 2007 ).
Atrazine, which belongs to the class of triazine herbicides, displays unique absorp-
tion kinetics that are not often observed with many pesticides. While one rodent study
has estimated that oral absorption may be slow, with an absorption half-life of 3 h, and
limited to approximately 57% based on 24-h urine excretion data ( Timchalk et al.,
1990 ), more recent rodent studies demonstrated a double-peak phenomenon as evi-
denced by a single first-order absorption process and a longer plateau ( McMullin
et al., 2003, 2007 ). These studies also estimated that limited solubility in the intestine
and presystemically metabolized atrazine in the intestine may account for limited oral
bioavailability. However, it should be emphasized again that oral absorption is signifi-
cantly greater through oral exposure than by dermal exposure, as exemplified here by
atrazine, for which only as much as 5.6% of the topical dose is expected to be absorbed
across the skin of humans and as much as 10 times this amount could be absorbed by
the gastrointestinal tract. Risk assessors should be wary of formulation and dose effects,
which will alter bioavailability across both routes of exposure.
Absorption from the Respiratory Tract
A chemical must be in the form of a gas, vapor, or particulate (e.g., aerosol) to be
absorbed in the respiratory tract. Although the anatomy of the respiratory tract var-
ies widely within mammalian species, the respiratory system can be generally com-
partmentalized into the nasopharyngeal, tracheobronchial, and pulmonary regions
( Kennedy and Valentine, 1994 ). The function of the nasopharyngeal region is to con-
dition inspired air and to remove large inspired particles before they reach the tra-
cheobronchial and pulmonary regions. The tracheobronchial region is lined by
mucus-secreting and ciliated cells, which together make up the mucociliary escalator.