Are the chlorination byproducts in tap water dangerous?
Long-term exposure may be harmful. Trihalomethanes from chlorinated tap water are associated with higher cancer risk and genetic damage.
What's actually in it
Chlorine is added to municipal water to kill bacteria and viruses. This is an important public health measure. But chlorine reacts with organic matter in water to form trihalomethanes (THMs), including chloroform, bromodichloromethane, and dibromochloromethane. These are the disinfection byproducts that remain in tap water you drink.
THMs are classified as possible to probable human carcinogens by regulatory agencies. Long-term exposure at high levels is linked to bladder cancer, colorectal cancer, and adverse reproductive outcomes.
What the research says
A 2026 genome-wide interaction analysis examined how long-term THM exposure in drinking water interacts with genetic variation to affect cancer risk. The study found that THMs cause measurable DNA damage, with some people's genetics making them significantly more sensitive to this damage.
The study also identified specific genetic variants that modify how the body processes and eliminates THMs. People with these variants faced meaningfully higher cancer risk at the same exposure levels.
THM levels in tap water vary by location and season. Carbon-based filters (Brita-style) reduce THMs modestly. Letting tap water sit in an open container for 30-60 minutes before drinking also reduces them, as THMs partially evaporate. Reverse osmosis filters are more effective.
The research at a glance
| Study | Journal | Year |
|---|---|---|
| Genome-wide interaction analysis of long-term trihalomethane exposure in drinking water | Environmental Health Perspectives | 2026 |
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