Several common water disinfection methods produce byproducts that are classified as known, probable, or possible carcinogens. Chlorination is the biggest concern, generating the highest concentrations of cancer-linked compounds, but ozonation and chloramination also create their own problematic byproducts. No disinfection method used in drinking water today is completely free of risk, though the levels vary dramatically.
How Disinfection Creates Carcinogens
When a disinfectant like chlorine reacts with naturally occurring organic matter in water (decomposed leaves, soil particles, algae), it forms new chemical compounds called disinfection byproducts, or DBPs. These aren’t added intentionally. They’re the unintended chemical consequences of making water safe to drink. More than 700 DBPs have been identified across different disinfection methods, and a growing number have been flagged for cancer risk by the International Agency for Research on Cancer (IARC).
Chlorination: The Most Studied Risks
Chlorine is the most widely used water disinfectant in the world, and it produces the two most abundant classes of cancer-linked byproducts: trihalomethanes (THMs) and haloacetic acids (HAAs). Both form when chlorine reacts with organic matter, and both are present in virtually all chlorinated tap water.
IARC classifies several of these compounds as Group 2B, meaning “possibly carcinogenic to humans.” The list includes chloroform, bromodichloromethane, dichloroacetic acid, trichloroacetic acid, bromochloroacetic acid, dibromoacetic acid, and dibromoacetonitrile. One chlorination byproduct, chloral hydrate, sits a tier higher at Group 2A: “probably carcinogenic to humans,” based on mechanistic evidence showing how it damages cells.
The strongest epidemiological link is between THMs and bladder cancer. A large international meta-analysis found that men exposed to THM levels above 50 micrograms per liter had a 51% increased risk of bladder cancer compared to those exposed to levels below 5 micrograms per liter. Even at more moderate levels above 25 micrograms per liter, bladder cancer risk rose by 35%. The relationship appears to be linear, meaning risk climbs steadily as exposure increases rather than jumping at a single threshold.
Colorectal cancer has also been linked to long-term chlorinated water consumption. A case-control study in Ethiopia found that people drinking chlorinated surface water had 2.6 times the odds of developing colorectal cancer compared to those on non-chlorinated supplies. A Canadian study found a 49% higher risk of colorectal cancer among people who drank chlorinated surface water for 30 years or more versus those on groundwater.
Chloramination: Fewer THMs, Different Problems
Many water utilities have switched from chlorine to chloramines (a combination of chlorine and ammonia) specifically to reduce THM formation. It works for that purpose. But chloramination creates its own signature carcinogen: NDMA, or N-nitrosodimethylamine, classified by IARC as a probable human carcinogen (Group 2A).
In one study comparing two water systems, NDMA was detected in 98% of all samples. The chloraminated system consistently produced higher NDMA concentrations (0.39 to 1.32 nanograms per liter) than the chlorinated system (0.20 to 0.54 nanograms per liter). So the tradeoff is real: chloramination lowers THMs but raises NDMA. Neither method eliminates cancer-linked byproducts entirely.
Ozonation and Bromate
Ozone is often considered a cleaner alternative to chlorine, and it does avoid THM and HAA formation. The catch is bromate. When water contains naturally occurring bromide (common in many source waters), ozone reacts with it through a complex chain of oxidation steps to form bromate, classified as a possible human carcinogen (Group 2B). The formation pathway involves both ozone itself and hydroxyl radicals produced during ozone breakdown, making it difficult to fully prevent.
Bromate is regulated with a strict maximum contaminant level of 10 micrograms per liter in drinking water, set by both the EPA and international agencies. While there is evidence that bromate may cause cancer through a non-genotoxic mechanism (meaning it promotes tumor growth rather than directly damaging DNA), regulators treat it as a genotoxic carcinogen for safety purposes.
UV Disinfection: Mostly Clean, Not Perfect
Ultraviolet light disinfection was initially considered free of byproduct concerns because it doesn’t involve adding chemicals to water. That assumption has been partly revised. Researchers have found that UV processes can generate mutagenic compounds, specifically nitrogen-containing aromatic chemicals formed when UV breaks down nitrate in the presence of natural organic matter. However, these byproducts can be removed through filtration, and the consensus remains that UV-based processes can be safely applied in water treatment. UV disinfection produces far fewer concerning byproducts than any chemical method.
Exposure Beyond Drinking
Swallowing tap water isn’t the only way DBPs enter your body. Volatile compounds like THMs evaporate from hot water during showers and baths, where you inhale them as steam. They also pass through skin on contact. The Agency for Toxic Substances and Disease Registry has developed specific models to calculate cancer risk from these inhalation and dermal exposures during everyday activities like showering and hand washing.
Swimming pools present a concentrated exposure scenario. A study of 49 adults found that after just 40 minutes of swimming in a chlorinated pool, brominated THM levels in exhaled breath were elevated, and blood markers showed signs of genetic damage, including increased micronuclei counts in immune cells and changes in gene expression patterns associated with bladder cancer risk.
U.S. Regulatory Limits
The EPA sets maximum contaminant levels for the most common byproducts in public water systems. Total trihalomethanes are capped at 80 micrograms per liter, haloacetic acids (a group of five) at 60 micrograms per liter, bromate at 10 micrograms per liter, and chlorite (from chlorine dioxide disinfection) at 1,000 micrograms per liter. These limits are based on balancing cancer risk against the very real danger of waterborne disease from under-disinfected water. No NDMA standard currently exists at the federal level, though several states have set their own guidelines.
Reducing Your Exposure at Home
Activated carbon filters are effective at removing the major classes of carcinogenic byproducts from tap water. Testing of common pitcher and faucet-mount filters found that four brands (PUR, Brita, Culligan, and Espring) removed 91% to 98% of THMs. For haloacetic acids, PUR, Brita, and Espring removed 80% to 89%. Even lower-performing carbon filters removed at least 40% of DBPs overall. Performance varies by brand and model, so checking the specific filter’s rated capacity matters. Filters lose effectiveness as they approach their replacement date.
For shower and bath exposure, you can reduce inhalation of volatile THMs by ventilating your bathroom, shortening hot showers, or using a showerhead filter rated for chlorine and THM removal. Letting tap water sit in an open pitcher for a few hours also allows volatile THMs to off-gas before drinking, though this does nothing for non-volatile compounds like haloacetic acids.