Assessing the effect of initial vapor-phase concentrations on inhalation risks of disinfection-by-products (DBP) in multi-use shower facilities

Inhalation during showering activities is a major pathway for exposure to volatile disinfection-by-products (DBPs). Disinfection-by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs) have been shown to significantly increase cancer risks and can also pose other health hazards. In multi-family residences and common-shower facilities located in dormitories and gymnasiums, the time-lag between showers is likely to be short and the exposure to vapor-phase DBPs may be significantly increased due to residual concentrations from earlier showering activities. Current models do not consider the impacts of the initial vapor-phase concentration on health risks to be significant. The hypothesis that non-zero initial DBP vapor-phase concentrations lead to higher exposure and health risks was evaluated here using data from the City of Corpus Christi, TX at two levels of input parameter uncertainty. The inhalation risks and hazards were found to be over 1.5 times greater for subsequent showers compared to the initial shower of the day. For non-zero initial air concentrations and triangular distribution of input parameters, the model was found to be most sensitive to the initial air concentrations, highlighting the impact of initial conditions on cumulative daily intake (CDI) and subsequently on cancer risks and hazard indices. Increasing the time-gap between showers and improving ventilation are viable solutions to contend with the increased risk. It is recommended that the effects of initial air concentrations be incorporated in future risk assessments focusing on multi-family residences in older and poor neighborhoods where single shower dwellings are more common.