Determination of methyl isothiocyanate in air downwind of fields treated with metam-sodium by subsurface drip irrigation.

Air concentrations of methyl isothiocyanate (MITC) were determined near two fields treated with metam-sodium (MS) by subsurface drip irrigation. The two study fields showed measurable airborne MITC residues during application of MS and for periods up to 48 h postapplication. Using a Gaussian plume dispersion model, flux values were estimated for all of the sampling periods. On the basis of the flux estimates, the amount of MITC that volatilized within the 48 h period was about 1.4% of the applied material. Compared to other studies, MITC residues in air measured during application by subsurface drip irrigation were up to four orders-of-magnitude lower than those previously published for applications involving delivery through surface irrigation water. Our measured concentrations of MITC in field air were at levels below current regulatory guidance and thresholds for adverse human health effects.