Human exposure to DEF/merphos

The harvest-aid herbicides DEF (S,S,S-tributylphosphorotrithioate) and merphos(S,S,S-tributylphosphorotrithioite) have been employed for defoliation of cotton for over 20 years. These compounds are readily taken up by the plant leaf and induce and accelerate abscission. There is little information available on the chemical characteristics of these compounds or on their environmental fate. DEF, however, appears to have very low vapor pressure and is also apparently quite stable. For the most part, environmental fate and human exposure estimates for these compounds have been based on data derived from parathion studies. While these estimates may prevail to some degree, the compounds are not completely comparative, as for example, the fact that parathion hydrolyzes readily under alkaline conditions while DEF reportedly does not.