toxicological profile for malathion - Agency for Toxic Substances and ...

MALATHION 109
3. HEALTH EFFECTS
4.6-cm 2 area). On day 8, the same cycle began for the second week. The absorption estimated on the
basis of urinary excretion from the first radiolabeled malathion dose was 4.48% and that from the second
dose was 3.53%, a value not significantly different.
A similar study was carried out with guinea pigs of both sexes by using 14 C-malathion (label position
unspecified) (Bucks et al. 1985). Daily doses of 22.7 mg were administered to the bald area behind the
ear at 24-hour intervals for 15 days, with labeled doses on days 1, 8, and 15. In the group in which the
application site was washed with soap and water 1 hour before daily application, absorption rates of the
three radiolabeled doses were 1.63, 3.52, and 5.34% of the applied doses for 1, 8, and 15 days,
respectively. This suggested a decrease of barrier function of the skin by washing. Without the washing,
absorption rates were fairly steady at 2.28, 2.13, and 3.67% of the applied doses, respectively. In all
cases, more than half of the excretion occurred within 24 hours. Absorption from a single dermal dose of
15 µg of labeled malathion was 6.8% of the applied dose for nonwashed animals and 7.5% for washed
animals, comparable to human data obtained by Feldmann and Maibach (1974).
Absorption of malathion was examined in 32 healthy volunteers (17 male and 15 female, 18–61 years of
age; mean age, 34.1 years) treated with one of four head lice preparations containing malathion (Dennis
and Lee 1999). Typical doses of 0.1–0.2 g malathion were applied to the scalp, and urinary excretion was
determined after alkaline hydrolysis. A total of 0.2–3.2% of the dose was excreted over 96 hours,
indicating low rates of dermal absorption.
14
C-Malathion (label position unspecified) was used to estimate dermal absorption in humans and rats by
using a curve-fitting model (Dary et al. 1994). In humans, absorption rate constants estimated from
urinary excretion ranged from 0.007 to 0.028/hour (absorption half-time of 95–25 hours) for pure
malathion and from 0.003 to 0.020/hour (absorption half-time of 232–35 hours) for 10% aqueous
solution. In rats, the average absorption rate constant and absorption half-time were 0.029/hour and
23.9 hours, respectively; however, comparison with human data is difficult due to a large variation among
the latter.
14 C-Methoxy malathion was used to trace technical malathion and a 50% emulsifiable concentrate
following a dermal application onto the shaved backs of male Sprague-Dawley rats at one-tenth the LD50
(410 mg/kg) (Abou Zeid et al. 1993). The 14 C in blood was higher for the unformulated malathion than
for the formulation; in the latter case, 14 C in blood increased steadily over 7 days. Most of the excretion
occurred via urine (>90%) in the first day, while some 14 C appeared in the feces. Excretion during the