Evaluation of the Mass Balance Model Used by the Environmental Protection Agency for Estimating Inhalation Exposure to New Chemical Substances

Abstract

The U.S. Environmental Protection Agency (EPA) is responsible for assessing the potential for unacceptable human health and environmental risks of new chemical substances prior to commercialization. Estimates of potential inhalation exposure to workers during manufacture, processing, and use of a new chemical substance are key elements of these assessments. However, the available information with which to assess the potential for exposure is often limited for new chemicals. One approach used by EPA to develop screening level estimates of inhalation exposure to vapors in the absence of data is the use of a mass balance model to predict the airborne concentration for various activities such as drumming and sampling. The mass balance model was evaluated by comparing the exposure estimates for specific operations with monitoring data reported in selected studies from the available literature. In general the estimated exposures based on the midpoint of the range of default input values were well within one order of magnitude of the measured exposures. Selection of more conservative (i.e., protective) model input values overestimated exposures by one or more orders of magnitude. There are many simplifying assumptions inherent in the model and many variables that influence exposure that are not considered. Uncertainty analyses of the model demonstrated that values selected for the ventilation flow rate and generation rate greatly influence the estimate of exposure and should be carefully chosen. Additional research is recommended, and ultimately, model validation should be completed to further improve and refine the model.