Reduction of Ingestion Exposure to Trihalomethanes Due to Volatilization

Ingestion of tap water is one of the principal exposure pathways for disinfection byproducts (DBPs). One major class of DBPs, trihalomethanes (THM), are highly volatile, and volatilization will tend to lower ingestion exposures. This study quantifies volatilization rates of the four THM species that occur while drinking tap water, specifically, losses during the preparation, storage, and serving of water. A mass transfer model based on two-resistance theory and quiescent conditions is presented, and parametrizations of all variables are provided. Volatilization rate constants are estimated in experiments representing common patterns of tap water consumption, i.e., storage of tap water in pitchers, pouring, and serving in glasses and mugs at temperatures from 4 to 100 °C. Predicted and experimental results show comparable loss rates for the four THMs. Observed volatilization rates declined exponentially, as expected, and greatly exceeded model predictions that assumed quiescent conditions in the liquid. Loss rates increased with temperature and mixing that resulted from temperature gradients and air currents. Overall, storage, pouring, and serving of tap water at temperatures below 30 °C caused minor (<20%) volatilization of THMs. Rapidly heating water to 60 or 80 °C also is not expected to result in significant volatilization. However, volatilization losses approached 75% when water was boiled even for brief periods of time and reached 90% when boiled water was poured and served. For the typical adult who drinks nearly half of their water as hot beverages, volatilization will reduce ingestion exposures of THMs by nearly a factor of 2. To account for these losses, exposure estimates for THMs and other volatile chemicals should separate the consumption of heated and unheated tap water.