MONITORING OF MALATHION AND ITS IMPURITIES AND ENVIRONMENTAL TRANSFORMATION PRODUCTS ON SURFACES AND IN AIR FOLLOWING AN AERIAL APPLICATION

Concentrations in air and on surfaces of malathion and impurities [malaoxon, isomalathion, O,O,S-trimethyl phosphorodithioate, O,O,O-trimethyl phosphorothioate, O,O,S-trimethyl phosphorothioate, and diethyl fumarate (DEF)] were determined at three sites during and for 9 days after a 1990 aerial spraying in California. Malathion spray was transformed via oxidation to malaoxon, hydrolysis to DEF, and selective volatilization. Malathion half-lives were 1.6 to >9 days, and depositions at three sites ranged from 1100 to 2413 mug/ft2, while malaoxon ranged from 2.9 to 6.0 mug/ft2. Malaoxon increased fastest on filter paper surfaces, e.g., initial deposition of 7 mug/ft2 increasing to 315 mug/ft2 after 9 days. Malathion air concentrations increased to 80 ng/m3 within 1 day, and malaoxon to 64 ng/m3 at 24-48 h. Both were detectable after 9 days. DEF surface levels increased over 9 days; air concentrations persisted for 72 h. The three phosphoro(di)thioate trimethyl esters were undetectable on surfaces within 24 h, producing an air burst apparently via rapid volatilization. Accurate human exposure assessment during malathion spraying must consider environmental transformations.