Chlorine has become an indispensable disinfectant in water supply systems worldwide. To prevent harmful bacteria and microorganisms from being present in water sources or distribution pipes, most countries use chlorine or other approved disinfectants to treat municipal drinking water and inactivate these pathogens. However, scientific studies have suggested that long-term exposure to certain chlorination byproducts, particularly trihalomethanes (THMs), may be associated with an increased risk of certain health conditions, including bladder cancer. Evidence regarding other cancers and health conditions, such as liver cancer, stomach cancer, colon cancer, anemia, hypertension, heart disease, and arteriosclerosis, is limited or inconclusive, and these effects should not be attributed directly to chlorine in drinking water.

Studies have also shown that during showering, the body can absorb certain volatile chlorination byproducts, such as trichloromethane (chloroform), through the skin and respiratory system. Some exposure assessments have estimated that a ten-minute shower may result in chloroform exposure comparable to drinking several litres of water from the same supply, depending on the water’s chemical concentrations and individual exposure conditions. However, this is an estimate of equivalent exposure, not a measurement showing that the body absorbs the same amount as eight glasses of water. Chlorine and chlorination byproducts may also contribute to skin irritation under certain conditions, although claims that chlorine strips proteins from the skin and hair are not well established.

The addition of chlorine as a disinfectant to municipal drinking water is essential for preventing waterborne diseases caused by bacteria and other microorganisms. However, if drinking water contains elevated levels of chlorination byproducts, appropriate measures may be considered to reduce exposure without compromising disinfection. These measures may include installing a properly certified filter designed to reduce the specific contaminants of concern. For bathing, adequate bathroom ventilation and shorter showers can help reduce exposure to volatile byproducts. A shower filter or whole-house water treatment system should be selected based on verified performance against the relevant contaminants and local water quality.

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