When water is disinfected with chlorine, ozone, or chloramine, these agents react with natural organic matter in the water to create new chemicals called disinfection byproducts (DBPs). Several of these byproducts are classified as known or probable carcinogens. The main cancer-linked categories are trihalomethanes (THMs), haloacetic acids (HAAs), bromate, and nitrosamines, each tied to a different disinfection method.
Trihalomethanes From Chlorination
Trihalomethanes are the most abundant byproducts of chlorinating drinking water. When chlorine reacts with organic matter like decaying leaves or algae in the source water, it produces four main THMs: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Of these, chloroform and bromodichloromethane are both classified as Group 2B carcinogens (possibly carcinogenic to humans) by the International Agency for Research on Cancer.
A dose-response meta-analysis covering thousands of cancer cases found that people with the highest THM exposure had a 33% increased risk of bladder cancer compared to those with the lowest exposure. The risk became statistically significant at THM concentrations above 41 micrograms per liter. Colorectal cancer risk was also elevated, with a 15% increase at higher exposure levels. Notably, these elevated risks appeared at concentrations below current regulatory limits in both the U.S. and EU, suggesting existing standards may not fully protect against cancer.
The U.S. EPA sets a maximum contaminant level for total trihalomethanes at 80 parts per billion (0.080 mg/L) in public drinking water systems.
Haloacetic Acids From Chlorination
Haloacetic acids form through the same basic process as THMs: chlorine-based disinfectants reacting with organic material in water. In 2021, the National Toxicology Program added six HAAs to its Report on Carcinogens, listing all of them as “reasonably anticipated to be human carcinogens.” These six are bromochloroacetic acid, bromodichloroacetic acid, chlorodibromoacetic acid, dibromoacetic acid, dichloroacetic acid, and tribromoacetic acid.
The EPA regulates five haloacetic acids (known as HAA5) with a maximum contaminant level of 60 parts per billion. But the six HAAs listed as carcinogens don’t perfectly overlap with the five the EPA monitors, which means some cancer-linked HAAs aren’t directly regulated.
Bromate From Ozonation
Ozone is often considered a cleaner alternative to chlorine for water disinfection, but it carries its own risk. When water contains naturally occurring bromide and is treated with ozone, the oxidation process converts bromide into bromate. Bromate is classified as a probable human carcinogen and is regulated at a strict maximum of 10 micrograms per liter by multiple agencies, including the EPA and the World Health Organization.
The chemistry is complex. Both ozone itself and the hydroxyl radicals it generates as it breaks down can oxidize bromide through multiple pathways, making bromate formation difficult to prevent entirely in bromide-rich water sources.
Nitrosamines From Chloramination
Some water systems use chloramine (a combination of chlorine and ammonia) instead of free chlorine, partly because it produces fewer THMs. The tradeoff is that chloramination can generate nitrosamines, particularly N-nitrosodimethylamine (NDMA). NDMA is classified as a Group 2A carcinogen (probably carcinogenic to humans), making it one of the more serious DBP concerns.
NDMA is exceptionally potent. The EPA’s risk assessment sets the concentration associated with a one-in-a-million lifetime cancer risk at just 0.7 nanograms per liter, a level so low it’s measured in parts per trillion. This makes NDMA roughly a thousand times more potent, on a per-unit basis, than the regulated THMs. After being ingested, nitrosamines are metabolized into compounds that can directly alkylate DNA, chemically altering the genetic code in ways that initiate cancer.
How These Chemicals Damage DNA
Disinfection byproducts don’t all cause cancer through the same mechanism. Research profiling DNA damage pathways has identified two primary modes of action for halogenated byproducts like THMs and HAAs: oxidative DNA damage and base alkylation. In laboratory studies, 16 out of 20 tested DBPs triggered oxidative damage to DNA bases. This means the chemicals generate reactive molecules that directly attack and alter the building blocks of your genetic code.
Nitrosamines work differently. After your body metabolizes them, they act as alkylating agents, attaching chemical groups to DNA strands. Formaldehyde, another disinfection byproduct, creates crosslinks between DNA and proteins, physically tethering them together in ways that interfere with normal cell function. Other byproducts disrupt the cell cycle or interfere with folate metabolism, a nutrient pathway critical for accurate DNA replication.
Exposure Goes Beyond Drinking
Most people assume the risk comes from drinking tap water, but inhalation during hot showers and baths is actually the highest-risk exposure route for THMs. When heated water releases THMs as gas in an enclosed bathroom, you breathe them directly into your lungs, where they enter the bloodstream rapidly. One risk assessment found that lifetime cancer risk from inhaling THMs was roughly twice as high as the risk from drinking the same water. Skin absorption during bathing adds a smaller but still measurable contribution.
This means that even if you drink filtered water, long hot showers with unfiltered tap water still expose you to these byproducts.
Reducing Your Exposure at Home
Filtration effectiveness varies dramatically depending on the technology and the specific byproduct you’re trying to remove.
- Reverse osmosis removes about 80 to 84% of THMs, 60 to 90% of HAAs, and over 97% of NDMA in some configurations.
- Nanofiltration performs comparably, removing 74 to 99% of THMs, 80 to 100% of HAAs, and 57 to 98% of NDMA depending on the system.
- Ultrafiltration is far less effective, removing only about 20 to 53% of THMs and under 25% of HAAs.
- Activated carbon filters (the type found in pitcher filters and faucet attachments) can reduce THMs, though they’re less effective against HAAs and nitrosamines. Combined with reverse osmosis, carbon filtration pushes THM removal above 83%.
For inhalation exposure, ventilating your bathroom during showers helps. Shorter showers at lower temperatures reduce how much THM gas is released from the water. Whole-house filtration systems address all three routes of exposure: drinking, breathing, and skin contact.
The fundamental tension with disinfection byproducts is that the disinfection itself is essential. Waterborne diseases like cholera, typhoid, and dysentery killed millions before chlorination became standard. The cancer risks from DBPs, while real, are small compared to the immediate danger of untreated water. The goal isn’t to avoid disinfected water but to minimize byproduct exposure through filtration and ventilation where practical.