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

MALATHION 183
6. POTENTIAL FOR HUMAN EXPOSURE
In surface waters, malathion degrades by hydrolysis and microbially mediated biodegradation (Mulla et
al. 1981). Hydrolysis is considered to be the predominant degradation process, and occurs more rapidly
at alkaline pHs, while the compound is stable to hydrolysis at acidic pHs (Wolfe et al. 1975). Malathion
appears to be relatively stable to direct aqueous photolysis, but may be transformed in the water by
indirect photolysis. Adsorption to sediment and suspended particulate matter is not expected to be a
significant factor in the fate of the compound in the environment.
In soil and sediments, the major degradation process of malathion is microbially mediated biodegradation,
which occurs mainly through enzyme-catalyzed hydrolysis (Mulla et al. 1981). The predominant
pathway for the biodegradation of malathion has been reported to be carboxylesterase activity (Laveglia
and Dahm 1977). Malathion degrades rapidly in soil, with reported half-lives ranging from hours to
approximately 1 week (Gibson and Burns 1977; Howard 1991; Konrad et al. 1969). Half-life values in
soil of 3–7 days have been reported for the degradate malaoxon (Bradman et al. 1994). Based on limited
data, malathion does not appear to photodegrade on soils (Chukwudebe et al. 1989; EPA 2000a). Little
malathion appears to volatilize from soil (EPA 2000a) and, while malathion is moderately to highly
mobile in soils (EPA 2000a), leaching of malathion through the soil and into groundwater is unlikely due
to the rapid degradation of the compound in the environment.
6.3.1 Transport and Partitioning
Data indicate that malathion may be transported in the air following application to either agricultural or
urban/residential areas (LeNoir et al. 1999; Majewski et al. 1998). Malathion may be transported in the
atmosphere as a vapor or adsorbed onto particulate matter (Bossan et al. 1995). In a review paper, Mulla
et al. (1981) stated that the occurrence of malathion in the atmosphere is generally localized. However, in
a non-U.S. study of malathion adsorbed to fly ash (particulate matter), Bosson 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. These study results indicate that malathion adsorbed to kaolin
may be transported over long distances, while malathion adsorbed to fly ash will be rapidly
photodegraded and, therefore, will not be transported far in the atmosphere (Bosson et al. 1995).
Additionally, malathion has been detected in the fog of remote pristine areas, indicating that long-range
transport may occur under some conditions (Rice 1996).
Transport of malathion residues in air may follow aerial applications or result from spray drift or
postapplication volatilization; however, the latter source may not be significant for this compound. The