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

MALATHION 35
3. HEALTH EFFECTS
Studies in animals support the findings in humans regarding cholinesterase inhibition. Exposure of
rabbits to 123 mg malathion/m 3 as an aerosol generated from a technical malathion formulation
(95% pure) for 6 hours inhibited plasma cholinesterase activity by 37% at 24 hours postexposure and 41%
at 72 hours postexposure (Weeks et al. 1977). This exposure also resulted in inhibition of erythrocyte
cholinesterase by 38, 48, and 48% at 24 hours, 72 hours, and 7 days postexposure, respectively. The
NOAEL was 65 mg/m 3 . Exposure to an aerosol formulation containing 6% malathion and a fuel oil
mixture resulted in 38% inhibition of plasma cholinesterase with a 66 mg/m 3 concentration 72 hours after
exposure and 71% inhibition with a 128 mg/m 3 concentration 10 minutes postexposure. With this
formulation, erythrocyte cholinesterase was inhibited 61 and 46% with the 128 mg/m 3 concentration
10 minutes and 24 hours, respectively, postexposure. Exposure to the 6% formulation caused lethality,
but no signs of toxicity or deaths were seen among rabbits exposed to the 95% malathion formulation. By
comparing these results to those obtained after oral exposure in a parallel experiment, Weeks et al. (1977)
estimated that it took 15–20 times more malathion by ingestion to cause an effect similar to that seen by
inhalation. The NOAEL of 65 mg/m 3 was used to derive an intermediate inhalation MRL of 0.02 mg/m 3
for malathion.
In a 13-week study, Sprague-Dawley rats were exposed whole body to up to 2,010 mg/m 3 of malathion
(96.4% pure) 6 hours/day, 5 days/week (Beattie 1994). At termination, the effects on cholinesterase
activities were found to be exposure concentration-related and effects on females seemed more
pronounced that in males. Plasma cholinesterase activity was decreased 30% at 450 mg/m 3 and 70% at
2,010 mg/m 3 in females, respectively. RBC cholinesterase activity was decreased 22 and 27% at
450 mg/m 3 in males and females, respectively, and 43 and 44% at 2,010 mg/m 3 in males and females,
respectively. Brain cholinesterase activity was decreased 41% at 2,010 mg/m 3 in females. Excess
salivation was seen mostly in rats from the high-exposure group, although it occurred sporadically in the
other exposed groups.
NOAEL and LOAEL values from the Beattie (1994), Golz (1959), and Weeks et al. (1977) studies are
presented in Table 3-1 and plotted in Figure 3-1.
3.2.1.5 Reproductive Effects
Reproductive outcomes were investigated in a group of 7,450 women who were confirmed as pregnant
during periods of malathion spraying to control an infestation by the Mediterranean fruit fly in the San