Author(s)

Wenshuo Zheng

Corresponding Author

Wenshuo Zheng

Abstract

Trihalomethanes (THMs), including chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform, are prevalent disinfection by-products (DBPs) formed in drinking water through the reaction of chlorine-based disinfectants with natural organic matter. Although essential for microbial control, chlorination leads to unintended human exposure to potentially carcinogenic THMs, raising significant public health concerns. This study performs a quantitative cancer risk assessment of THMs based on publicly accessible toxicity data and risk assessment methodologies from the U.S. Environmental Protection Agency (EPA). By evaluating individual and total THM exposure scenarios, this research quantifies lifetime cancer risks at current regulatory limits and compares them to acceptable risk thresholds (10⁻⁶ to 10⁻⁴). Results indicate that concentrations aligning with an idealized acceptable cancer risk (10⁻⁶) are substantially lower—often sub-microgram per liter—compared to existing standards such as the EPA's total THM limit of 80 µg/L. At present regulatory concentrations, lifetime cancer risks frequently approach or exceed the upper boundary of acceptable risk (10⁻⁴), particularly when considering brominated THMs (BDCM and DBCM). These findings highlight significant discrepancies between current regulatory limits and health-protective benchmarks. The discussion explores technological, economic, and policy reasons behind the gap, emphasizing the trade-offs involved in DBP regulation. Ultimately, the study recommends advancing water treatment technologies and periodically reviewing regulatory standards to better align permissible THM concentrations with health-based objectives. Continuous improvement in DBP control practices is advocated to reduce chronic cancer risks without undermining the essential role of disinfection in protecting against microbial pathogens.

Keywords

Trihalomethanes, disinfection by-products, cancer risk, drinking water safety, quantitative risk assessment