DOSE-RESPONSE ASSESSMENT OF AIRBORNE METHYL ISOTHIOCYANATE (MITC) FOLLOWING A METAM SODIUM SPILL

A tank car derailment in northern California in 1991 spilled metam sod ium into the Sacramento River, and released its breakdown product, met hyl isothiocyanate (MITC), into the air. This paper describes the risk evaluation process used. Over 240 individuals reported symptoms such as eye and throat irritation, dizziness, and shortness of breath. Refe rence exposure levels (RELs) for 1 hr were developed for MITC and comp ared to exposure concentrations. Ocular irritation in cats was the mos t sensitive endpoint reported. The no observed adverse effect level (N OAEL), divided by an uncertainty factor (UF) of 100, produced an REL o f 0.5 ppb of MITC in air to prevent discomfort. An REL to prevent disa bility was estimated to be 40 ppb. An REL to prevent life-threatening injury was estimated to be 150 ppb. Measured MITC levels ranged from 0 .2-37 ppb and estimated peak levels ranged from 140-1600 ppb. The usef ulness of RELs for emergency planning is discussed.