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

MALATHION 190
6. POTENTIAL FOR HUMAN EXPOSURE
demethylation reaction that leads to the formation of demethyl-malathion. Delayed phosphatase activities
leading to phosphorylated derivatives, which act as cholinesterase inhibitors in fish, were also cited
(Bourquin 1977). Complete mineralization of malathion was observed to be more rapid (initial
appearance of CO2 at 2 versus 7 days) with a mixed microbial culture than with the individual cultures.
A study was conducted to determine the ability of fungi to degrade malathion in aquatic environments
using a species of fungus (Aspergillus oryzae) isolated from a freshwater pond (Lewis et al. 1975).
Although the study authors stated that the rate of transformation of malathion in the lab could not be
extrapolated to the field, it was determined, based on comparisons with data obtained previously, that
malathion was degraded 5,000 times more rapidly by bacteria versus the fungus. The main degradate of
the fungal degradation of malathion, β-malathion monoacid, was the same degradate produced by
bacterial degradation of malathion (Lewis et al. 1975).
6.3.2.3 Sediment and Soil
In soils and sediments, microbial degradation (mainly through enzyme-catalyzed hydrolysis) and
hydrolysis are important degradation processes for malathion. Studies have demonstrated that this is
particularly true at higher pH values and soil moisture contents (Miles and Takashima 1991). Malathion
degrades rapidly in soil, with reported half-lives in soil ranging from hours to approximately 1 week
(Gibson and Burns 1977; Howard 1991; Konrad et al. 1969). Bradman et al. (1994) reported a range of
half-life values of