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

MALATHION 154
3. HEALTH EFFECTS
exposed to several pesticides have also documented inhibition of plasma and RBC cholinesterase, clinical
signs, and neurophysiological alterations (Baker et al. 1978; Ernest et al. 1995; Peedicayil et al. 1991;
StDlberg et al. 1978). A study of pesticide applicators also investigated the possible role of
organophosphate insecticides and retinal degeneration, but a definite conclusion could not be reached
(Kamel et al. 2000). Information is lacking on potential effects of long-term, low-level exposure to
malathion on many end points. This information can only be obtained from evaluation of cohorts
exposed only to malathion, but data from subjects exposed to a limited number of organophosphates
would also be helpful. No studies were located that evaluated the effects of chronic inhalation or dermal
exposure in animals, but because malathion is relatively rapidly degraded in the environment, no such
exposures are expected to occur for the general population or for people living near waste sites. Several
long-term bioassays have been conducted in rats and mice that provided information on noncancer effects
(death, systemic, and neurological effects) (Daly 1996a; NCI 1978, 1979a; Slauter 1994). The lowest
LOAEL was 29 mg malathion/kg/day for 29% inhibition of plasma cholinesterase in male Fischer-344
rats in a study by Daly (1996a); the NOAEL was 2 mg/kg/day and was used to derive a chronic oral
MRL.
Several studies have examined the possible association between occupational exposure to pesticides and
certain types of cancer, particularly non-Hodgkin’s lymphoma (NHL) and leukemia. However, because
workers are usually exposed to multiple pesticides, it is difficult to establish associations with specific
pesticides. Still, information relevant to malathion is available in some of these studies. Exposure to
malathion was associated with increased risk of NHL in a study of men in Iowa and Minnesota (Cantor et
al. 1992), of women in Nebraska (Zahm et al. 1993), and of men in Canada (McDuffie et al. 2001). A
retrospective cohort study of 32,600 employees of a lawn care company found no specific association
between deaths due to NHL and exposure to malathion (Zahm 1997) and neither did a study of pesticide
users from Iceland (Zhong and Rafnsson 1996). The study of men in Iowa and Minnesota also found an
elevated risk of leukemia in men who used malathion as an animal insecticide for >10 days/year (Brown
et al. 1990). A slight non-significant increased risk for multiple myeloma was observed among farmers
who used malathion in an additional study among men in Iowa, but failure to use protective equipment
was not associated with increased risk (Brown et al. 1993). In spite of some positive associations, there is
no clear evidence that exposure to malathion causes cancer in humans. As previously mentioned, the
magnitude of the excesses is generally small, exposure assessment is unreliable, and mixed exposure
always occurs.