To prevent harmful bacteria and microorganisms from contaminating drinking water and water distribution systems, most municipal water supplies use disinfectants such as chlorine or chloramine to inactivate disease-causing microorganisms. Disinfection is an important part of maintaining safe drinking water. However, chlorine and other disinfectants can react with naturally occurring organic matter in water and form disinfection by-products (DBPs), including trihalomethanes (THMs) and haloacetic acids (HAAs).

Some DBPs have been associated with potential health effects. Long-term exposure to certain DBPs, particularly at higher concentrations, has been studied in relation to cancer and other health outcomes. However, the evidence varies among individual compounds and studies, and the presence of chlorine in drinking water does not mean that the water is unsafe. Municipal water systems monitor and control disinfectant levels and DBPs to maintain effective disinfection while limiting these by-products.

Chlorinated water can also release small amounts of volatile disinfection by-products into indoor air, particularly during activities such as showering and bathing. People can therefore be exposed through both drinking water and inhalation. Some compounds can also be absorbed through the skin. However, the relative importance of these exposure routes varies according to the particular chemical, water concentration, temperature, duration of exposure, and individual circumstances. Claims that a ten-minute shower exposes the body to the equivalent of drinking eight glasses of water are not supported as a general scientific measure of exposure.

The most important consideration is therefore the quality of the local drinking-water supply. Consumers who are concerned about chlorine or specific contaminants should first consult their municipality’s drinking-water-quality reports. Where additional treatment is appropriate, a water filter certified for the specific contaminant of concern can be used. Activated-carbon filters can reduce chlorine and certain organic compounds, although their effectiveness depends on the filter design, certification, and proper maintenance.

Shower filters may reduce chlorine under some circumstances, but their effectiveness varies by product and water conditions. Consumers should not assume that a shower filter or whole-house purification system is necessary simply because municipal water is chlorinated. Any water-treatment device should be maintained and replaced according to its instructions to prevent it from becoming a source of contamination.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026