Trihalomethane Cancer Risk Assessment for Private and Shared Residences: Addressing the Differences in Inhalation Exposure

The multi-pathway cancer risk (CR) assessment of trihalomethanes (THM) involves considering exposure via ingestion, dermal contact, and inhalation. Inhalation occurs during showering due to the volatilization of THMs from chlorinated water to the air. When assessing inhalation risks, exposure models commonly assume that the initial THM concentration in the shower room is zero. However, this assumption is only valid in private shower rooms where single or infrequent showering events take place. It fails to account for continuous or successive showering events in shared showering facilities. To address this issue, we incorporated the accumulation of THM in the shower room air. We studied a community (population ≈ 20,000) comprising two types of residences with the same water supply: population A with private shower rooms, and population B with communal shower stalls. The total THM concentration in the water was 30.22 ± 14.45 µg L⁻¹. For population A, the total CR was 58.5 × 10⁻⁶, including an inhalation risk of 1.11 × 10⁻⁶. However, for population B, the accumulation of THM in the shower stall air resulted in increased inhalation risk. By the tenth showering event, the inhalation risk was 2.2 × 10⁻⁶, and the equivalent total CR was 59.64 × 10⁻⁶. We found that the CR significantly increased with increasing shower duration. Nevertheless, introducing a ventilation rate of 5 L s⁻¹ in the shower stall reduced the inhalation CR from 1.2 × 10⁻⁶ to 7.9 × 10⁻⁷.