Water itself does not cause cancer. Pure H₂O is biologically inert and essential for every cell in your body. But the substances that can ride along in your drinking water, whether from natural geology, industrial pollution, or the treatment process itself, are a different story. Several contaminants found in tap and well water have established links to cancer, and the risk depends on what’s in your water, how much of it is there, and how long you’ve been drinking it.
Contaminants That Raise Cancer Risk
The cancer concern with drinking water is never about water molecules. It’s about what dissolves or suspends in them. The contaminants with the strongest evidence fall into a few categories: naturally occurring minerals like arsenic, chemicals created during water treatment, industrial pollutants like PFAS and hexavalent chromium, and agricultural runoff carrying nitrates. Each one acts through a different biological pathway, and each has a different threshold where risk begins to climb.
Arsenic and Lung Cancer
Arsenic occurs naturally in rock and leaches into groundwater, particularly in parts of the western United States, South Asia, and South America. It’s one of the most studied carcinogens in drinking water. A 2024 systematic review spanning 35 years of evidence found that at 50 micrograms per liter (µg/L), the relative risk of developing lung cancer was 1.67, meaning a 67% increase compared to unexposed populations. At 150 µg/L, the risk more than doubled. Even at 10 µg/L, the current legal limit set by the EPA, the data showed an 11% increase in risk, though the statistical confidence at that level was weaker.
Mortality followed a similar pattern. At 50 µg/L, people were roughly twice as likely to die from lung cancer. The dose-response curve is clear: more arsenic means more risk, and the risk doesn’t suddenly appear at some safe cutoff. It climbs gradually, which is why some researchers argue the current legal limit still isn’t low enough. Arsenic exposure has also been linked to bladder and skin cancers.
Disinfection Byproducts and Bladder Cancer
Chlorine kills dangerous bacteria in tap water, but when it reacts with organic matter (like decaying leaves in a reservoir), it creates a family of chemicals called trihalomethanes (THMs). These byproducts have been linked to bladder cancer in multiple studies, and the risk grows with decades of exposure.
In a large Spanish case-control study, people with lifetime average THM exposure above roughly 50 µg/L had about twice the odds of developing bladder cancer compared to those exposed to less than 9.4 µg/L. The dose-response trend was statistically significant, with risk climbing steadily across four exposure groups. The key word is “lifetime.” These aren’t short-term dangers. The strongest associations appear after 40 or more years of exposure, which means your cumulative history of where you’ve lived and what water you’ve drunk matters more than any single glass.
Nitrates From Agricultural Runoff
Nitrates enter water supplies mainly through fertilizer runoff and animal waste. The EPA’s legal limit is 10 mg per liter, but cancer risk may begin well below that. A Danish study tracking tens of thousands of people found that those exposed to nitrate levels above 9.3 mg/L had a 15% higher risk of colorectal cancer compared to those drinking water with less than 1.3 mg/L. More concerning, an elevated risk was detectable starting at concentrations around 4 mg/L, less than half the legal limit.
This is a particular concern in farming regions where well water is common and not subject to the same testing requirements as municipal supplies.
PFAS: The “Forever Chemicals”
PFAS are synthetic chemicals used in nonstick coatings, food packaging, and firefighting foam. They earned the nickname “forever chemicals” because they don’t break down in the environment or your body. Exposure has been linked to kidney and testicular cancers, among other health effects. In 2024, the EPA set the first-ever enforceable limits for two of the most common PFAS compounds, PFOA and PFOS, at 4 parts per trillion. That’s an extraordinarily low threshold, reflecting how seriously regulators view the risk. The health-based goal for both chemicals is actually zero, meaning no amount is considered completely safe.
PFAS contamination is widespread. The chemicals have been detected in the drinking water of communities across the United States, often near military bases or industrial sites where firefighting foam was used.
Hexavalent Chromium and Stomach Cancer
Hexavalent chromium, the chemical made famous by the Erin Brockovich case, enters water through industrial discharge and natural mineral deposits. Epidemiological evidence suggests elevated rates of stomach cancer in exposed populations, and animal studies have demonstrated that it causes tumors in the mouth and small intestine at high concentrations. The EPA regulates total chromium in drinking water but does not have a separate standard for the hexavalent form, which is the dangerous one. Several states have moved to set their own limits.
What About Fluoride, Microplastics, and Radium?
Fluoride in municipal water has been one of the most persistent cancer fears, particularly around osteosarcoma, a rare bone cancer. A well-designed case-control study found no increased risk. People who had lived in fluoridated areas and drank tap water actually had lower odds of osteosarcoma (adjusted odds ratio of 0.51) compared to those in non-fluoridated areas. Community water fluoridation, at the levels used in the U.S., is not associated with cancer.
Microplastics are increasingly found in both tap and bottled water. Some of these tiny plastic particles contain chemicals like BPA and phthalates that disrupt hormones and have shown cancer-promoting effects in lab studies. But the direct link between microplastics in drinking water and cancer in humans hasn’t been established yet. This is a watch-this-space issue rather than a proven risk.
Radium, a naturally occurring radioactive element, is found in some groundwater supplies. It’s known to cause bone cancer at high doses, based on studies of occupationally exposed workers. The EPA estimates that drinking water at the legal limit of 5 picocuries per liter carries an additional lifetime cancer risk of about 1 in 10,000.
How to Reduce Your Risk
Your first step is finding out what’s in your water. If you’re on a public water system, your utility publishes an annual Consumer Confidence Report listing detected contaminants and their levels. If you’re on a private well, you’ll need to test it yourself, since wells aren’t covered by federal regulations.
Filtration can make a meaningful difference if your water contains concerning levels of contaminants. Reverse osmosis systems are the most effective option, removing 94% or more of PFAS and performing well against arsenic, nitrates, and heavy metals. Activated carbon filters (the pitcher and faucet-mount type) removed an average of 73% of PFAS in testing at Duke University, but performance varied widely between brands and models. For serious contamination, an under-sink reverse osmosis system is the most reliable choice.
If you drink bottled water assuming it’s safer, know that it’s regulated less strictly than municipal tap water for some contaminants and may contain its own microplastic load from the packaging. The safest approach for most people is filtered tap water from a system matched to whatever contaminants are present in your local supply.