Disinfection Byproducts: The Hidden Cancer Risk in Treated Water

Chemicals that keep water safe may cause bladder cancer. What THMs and HAAs mean.

What Are DBPs

Disinfection byproducts (DBPs) are chemicals that form when chlorine or chloramine reacts with naturally occurring organic matter in water. They are an unintended consequence of the disinfection process that keeps your water free from bacteria and viruses. Over 700 individual DBPs have been identified, but regulations cover only a handful.

The two most common regulated groups are trihalomethanes (THMs) and haloacetic acids (HAAs). THMs include chloroform, bromodichloromethane, dibromochloromethane, and bromoform. HAA5 includes five specific haloacetic acids. These form when chlorine reacts with decaying leaves, algae, and other organic material present in source water before or during treatment.

DBP levels are not static. They increase as water sits in pipes, which means homes at the end of long distribution lines often have higher levels than homes near the treatment plant. Levels also spike in summer, when warmer source water contains more organic matter and higher temperatures accelerate the chemical reactions. If your tap water has a strong chlorine smell, your DBP levels are likely elevated too.

Cancer Risk

The EPA classifies several THMs as probable or possible human carcinogens. Chloroform, the most common THM, is classified as a Group B2 probable carcinogen. Bromodichloromethane has been linked to cancer of the kidney, liver, and large intestine in animal studies.

Epidemiological studies in humans have found the strongest association between long-term DBP exposure and bladder cancer. A 2023 meta-analysis published in the International Journal of Cancer estimated that people with the highest THM exposure had a 24-44% increased risk of bladder cancer compared to those with the lowest exposure. Smaller associations have been observed with colorectal cancer and adverse reproductive outcomes, including low birth weight and miscarriage.

Exposure happens through three routes: drinking, skin absorption, and inhalation. THMs are volatile, meaning they evaporate into the air during hot showers, baths, and dishwashing. Research suggests that inhalation and skin absorption during a 10-minute shower can deliver as much DBP exposure as drinking 2 liters of the same water. This makes DBPs one of the few water contaminants where showering matters, not just drinking.

The EPA sets the MCL for total THMs (TTHM) at 80 ppb and for HAA5 at 60 ppb. Health researchers have argued these limits are too lenient. Utilities measure DBP compliance as a running annual average at specific distribution points, which can mask seasonal spikes. Your actual exposure on a hot August day may be significantly higher than the annual average suggests.

Your Levels

Your DBP levels depend on three factors: your water source, your utility's disinfection method, and your location in the distribution system. Surface water systems (rivers and lakes) typically produce more DBPs than groundwater systems because surface water contains more organic matter for chlorine to react with.

Utilities that use chloramine instead of free chlorine generally produce fewer THMs and HAAs. However, chloramine creates its own class of byproducts, including N-nitrosodimethylamine (NDMA), a potent carcinogen that is not currently regulated by the EPA. Switching disinfectants trades one set of risks for another.

Your home's position matters. Water at the far end of the distribution system has had more time for DBPs to form, so levels can be 2-3x higher than at the treatment plant. If your water routinely smells like a swimming pool or tastes noticeably of chlorine, you are likely on the higher end. Search your city on CheckMyTap to see reported THM and HAA5 levels for your water system, and compare them against both the EPA MCL and stricter health-based guidelines.

Reducing Exposure

Activated carbon filters are highly effective at removing THMs and many other DBPs. This is one contaminant category where even basic filters make a real difference. Standard carbon pitchers (Brita, PUR) reduce THMs significantly, and higher-end carbon block filters or multi-stage systems do even better.

For drinking water, any NSF 53-certified carbon filter will substantially reduce THM levels. For maximum protection including HAAs and unregulated byproducts, a reverse osmosis system removes virtually all DBPs. The Clearly Filtered pitcher is independently tested for THM and HAA removal if you prefer a no-install option.

Since DBPs are volatile, shower exposure is a legitimate concern. A whole-house carbon filter or a shower-specific filter with KDF/carbon media can reduce THMs in bath and shower water. Ventilating your bathroom during hot showers also helps by dispersing airborne THMs before you inhale them.

Free, no-cost steps: fill a pitcher with tap water and leave it uncovered in the fridge for several hours. Volatile THMs will off-gas on their own. Run the cold tap for 30 seconds before filling a glass to flush stagnant water that has been accumulating DBPs in your pipes overnight.