The short answer
Disinfectants added to drinking water react with natural material in the water and form byproducts. EPA's national drinking water regulations list increased risk of cancer among the potential health effects of long-term exposure above the limit for trihalomethanes, haloacetic acids and bromate. EPA sets maximum contaminant levels for the regulated byproducts and weighs them against the public health benefit of disinfection.
Disinfectants react with naturally occurring material in source water to form byproducts.
EPA regulates trihalomethanes (TTHM) and haloacetic acids (HAA5), along with chlorite and bromate.
The maximum contaminant levels are 0.080 mg/L for TTHMs, 0.060 mg/L for HAA5, 1.0 mg/L for chlorite and 0.010 mg/L for bromate.
EPA lists increased risk of cancer among the effects of long-term exposure above the limit for TTHM, HAA5 and bromate.
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If you drink city tap water and have heard about chlorine byproducts and cancer, this page is for you. It explains what these byproducts are, how EPA limits them, and what you can check.
A trade-off, not an oversight
Public water systems add disinfectants for a simple reason: untreated source water carries germs such as Giardia and Cryptosporidium. EPA says these can cause stomach illness — diarrhea, vomiting, cramps, and other health risks.
The problem is chemical. Disinfectants react with natural material already in the water, These reactions form new compounds. Those new compounds are the disinfection byproducts.
They are a side effect of disinfecting the water.
What is regulated
EPA's rules cover four substances:
- trihalomethanes, grouped and regulated as TTHM, capped at 0.080 mg/L
- haloacetic acids, grouped as HAA5, capped at 0.060 mg/L
- chlorite, capped at 1.0 mg/L
- bromate, capped at 0.010 mg/L
These numbers are legal limits (maximum contaminant levels) that EPA sets for public water systems.
What the health concern is
EPA's table of national drinking water regulations lists, for each contaminant, the potential health effects of long-term exposure above the limit. For total trihalomethanes it lists liver, kidney or central nervous system problems, and increased risk of cancer. For haloacetic acids it lists increased risk of cancer. For bromate it lists increased risk of cancer. Chlorite is listed differently, against anemia and nervous system effects in infants and young children.
Read that column carefully. It describes what long-term exposure above the limit could do. It is not a description of what happens at the levels found in water that meets the standard.
A regulated limit is where the line is drawn, not a boundary between safe and poisonous.
EPA's own wording is worth noting too. It says byproducts consumed in excess of its standard over many years may increase health risks.
Why not just use less disinfectant
Because germs in source water are a real danger. EPA describes the rules as a balance: disinfection is needed because of pathogens such as Giardia and Cryptosporidium, and the rules limit the byproducts it creates. EPA's source does not say how each utility manages that balance, so ask yours.
What you can actually do
Ask your water utility for its water quality report and compare the byproduct numbers with the legal limits above.
Find out if you are on a municipal supply or a private well. The section below covers wells.
If you use a filter, check what it is actually certified to reduce. A filter certified for one contaminant says nothing about how well it handles another.
Keep the bigger picture in view: the regulated limits, the long timescale EPA describes, and the benefit of disinfected water.
One more thing about wells
EPA's public drinking water regulations apply to public water systems, not to a private household well. If you have a private well, you are responsible for having it tested. Your state or local health department can tell you how.
Common questions
Should I stop drinking tap water?
Nothing in EPA's material supports that. Disinfection exists because pathogens in untreated source water can cause illness including diarrhea, vomiting and cramps, and regulators weigh the benefits of disinfection against the risks of the byproducts.
What does a maximum contaminant level mean?
It is a legal limit that public water systems must meet. EPA's concern is described in terms of consumption in excess of the standard over many years, not a single glass of water.
Which byproducts is cancer listed against?
In EPA's table of national primary drinking water regulations, increased risk of cancer appears as a potential effect of long-term exposure above the limit for total trihalomethanes, haloacetic acids and bromate. Chlorite is listed differently, against anemia and nervous system effects in infants and young children.
Does that mean tap water gives people cancer?
That is not what a listed health effect means. EPA describes the concern in terms of consumption in excess of its standard over many years, and the limits exist so that exposure stays below that point.
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Sources last checked: 2026-10-02 what this meansLast updated: 2026-10-02Next planned review: 2027-08-11
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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