Can chlorine byproducts in tap water increase your risk of colon cancer?

What's actually in it

Most tap water in the U.S. and Europe is treated with chlorine to kill bacteria. That's a good thing. But when chlorine reacts with natural organic matter in the water, it creates chemicals called trihalomethanes (THMs). The most common one is chloroform. You're exposed every time you drink unfiltered tap water, take a shower, or fill a pot to cook.

THM levels vary depending on your water source and how much organic matter is in it. Surface water (from rivers and lakes) tends to produce more THMs than groundwater. Your local water utility is required to keep THM levels below a legal limit, but even levels within that limit may carry risk over decades of daily exposure.

What the research says

A 2026 study in Environment International looked at long-term THM exposure and colorectal cancer risk in a large Spanish case-control study. Researchers estimated each person's THM exposure over many years based on their water source and residential history.

People with higher long-term THM exposure had a greater risk of developing colorectal cancer. The study went further by analyzing genetics and found that certain gene variants made some people more vulnerable to THMs than others. Your DNA can affect how well your body detoxifies these chemicals.

THMs are absorbed quickly. You take them in through your gut when you drink water, but also through your skin and lungs during showers and baths. A hot shower in a closed bathroom can actually deliver more THMs than drinking several glasses of water.

A carbon block water filter (like those in pitcher filters or under-sink systems) removes most THMs from drinking water. For showers, a shower filter with activated carbon can help. These are simple, affordable steps that reduce a daily chemical exposure you might not have known about.