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

MALATHION 187
6.3.2.1 Air
6. POTENTIAL FOR HUMAN EXPOSURE
Pesticides may be transformed and degraded in the atmosphere due to photolysis and by reaction with
ozone, hydroxyl, and nitrate radicals. Very little definitive information was found in the literature on the
transformation and degradation of malathion in air. Bradman et al. (1994) stated that both malathion and
malaoxon are unstable in the atmosphere and that reported half-lives for the two compounds in air range
from a few hours to 1 day; however, a review of the original papers cited in Bradman et al. (1994) do not
directly support this statement for malathion and malaoxon specifically (Atkinson et al. 1989; Goodman
et al. 1988). In a review paper, Mulla et al. (1981) stated that although the effects of light (including
ultraviolet [UV] radiation) on both malathion and its degradate malaoxon have been studied, little is
understood of the transformation of the parent to malaoxon through photolysis. Howard (1991) reported
an estimated atmospheric photooxidation half-life of 1.5 days based on the reaction of malathion with
photochemically produced hydroxy radicals. Based on the scarce data available in the literature, it
appears that malathion degradation in the atmosphere occurs mainly due to indirect photolysis (photooxidation)
rather than resulting from direct photolysis. In a photodegradation study of malathion exposed
to natural sunlight and UV irradiation (maximum wavelength of 360 nm) as thin films on glass, the
compound was relatively stable to sunlight and exhibited only slight degradation (16%) by 25 hours under
UV light; none of the six degradates were present at >0.01% (Chukwudebe et al. 1989). In a non-U.S.
study of malathion adsorbed to fly ash (particulate matter), Bossan et al. (1995) determined that adsorbed
malathion is photodegraded when exposed to irradiation of >290 nm for up to 1.5 hours, but does not
degrade when adsorbed to kaolin. Malathion on fly ash decreased by up to 70% of the adsorbed
compound following 40 minutes of irradiation. However, the study authors attributed the degradation of
malathion on fly ash to the presence of metals and metal oxides in the fly ash.
6.3.2.2 Water
Malathion in water undergoes chemical and microbial degradation. The rate and extent of its degradation
is dependent on the chemical and physical properties of the water system, particularly temperature and the
solution pH, in addition to the composition of the microbial population present in the system.
Malathion is degraded rapidly by hydrolysis at more alkaline pH levels, but is stable to hydrolysis at
acidic pHs (Wolfe et al. 1975, 1977). In a review paper, Mulla et al. (1981) noted that malathion was
instantaneously degraded by hydrolysis at pH 12; had a half-life of approximately 1 day at pH 11; was