Water itself is not carcinogenic, but dozens of contaminants that ride along in drinking water have been linked to increased cancer risk. The list includes naturally occurring substances like arsenic, industrial chemicals like trichloroethylene, byproducts of the very disinfection process that keeps water safe, and newer concerns like PFAS. The strength of the evidence varies widely from one contaminant to the next, and the risks are generally tied to years or decades of exposure rather than a single glass. Still, a county-level analysis of Arizona drinking water found statistically significant associations between several common water contaminants and cancer rates, with the strongest correlation between chromium and bladder cancer.
Arsenic in Groundwater
Of all the substances that contaminate drinking water, arsenic has the most robust body of evidence linking it to cancer. It occurs naturally in rock formations and dissolves into groundwater, reaching especially high concentrations in parts of South Asia, Latin America, and the western United States. The International Agency for Research on Cancer classifies it as a Group 1 carcinogen, meaning there is sufficient evidence it causes cancer in humans. Long-term exposure through drinking water is associated with cancers of the skin, lung, bladder, and kidney.
Arsenic does its damage through several overlapping mechanisms. It generates reactive oxygen species that damage DNA, while simultaneously interfering with DNA repair processes and disrupting how genes are expressed through epigenetic changes. It also displaces zinc in certain proteins, altering their function in ways that promote tumor development.1PubMed Central. Arsenic and cancer: Evidence and mechanisms Among heavy metals found in drinking water, arsenic consistently carries the highest estimated lifetime cancer risk. An analysis of drinking water across Fars Province, Iran, found that arsenic’s average incremental lifetime cancer risk was roughly fifty times higher than that of cadmium or chromium in the same water supplies.
There is some scientific debate over whether arsenic’s cancer-causing effects follow a threshold model, meaning low enough doses might not pose any meaningful risk. Some toxicologists argue that because arsenic’s mode of action involves disrupting proteins in target cells, a minimum concentration is needed before normal cellular function breaks down.2PubMed. Dose-response for assessing the cancer risk of inorganic arsenic in drinking water: the scientific basis for use of a threshold approach Current U.S. drinking water standards set the maximum contaminant level at 10 parts per billion, but millions of private well users never test their water, meaning real-world exposures can be far higher than what regulated systems allow.
Hexavalent Chromium and Stomach Cancer
Hexavalent chromium, the chemical that became publicly famous through the Erin Brockovich case, enters water supplies from both industrial discharge and natural mineral deposits. Animal studies have shown it induces tumors in the digestive tract when consumed in drinking water, and limited epidemiological evidence in humans points toward elevated risks for stomach cancer.3PubMed Central. Chromium in drinking water: sources, metabolism, and cancer risks A study of Chinese populations exposed to hexavalent chromium in their drinking water found that stomach cancer death rates were roughly 70 to 80 percent higher in contaminated areas compared to unexposed regions and province-wide rates.4Epidemiology. Cancer Mortality in a Chinese Population Exposed to Hexavalent Chromium in Drinking Water
Chromium’s regulatory status in the U.S. is complicated. Federal standards cover total chromium, not hexavalent chromium specifically, and the current limit was set before much of the cancer evidence emerged. California has moved toward setting a separate standard for hexavalent chromium, though the process has been slow and politically contentious.
Disinfection Byproducts and the Chlorination Paradox
Chlorinating water to kill disease-causing microorganisms is one of the great public health achievements of the modern era. The paradox is that chlorine reacts with naturally occurring organic matter in source water to create its own family of potentially harmful chemicals, known as disinfection byproducts. The most studied of these are trihalomethanes and haloacetic acids.5Journal of Cleaner Production. Chlorination disinfection by-products in municipal drinking water – A review
The connection between these byproducts and bladder cancer has been investigated for decades, and the evidence is genuinely mixed. A large Canadian case-control study found that people exposed to chlorinated surface water for 35 years or more had about 1.4 times the risk of bladder cancer compared to those exposed for less than a decade. At higher concentrations of trihalomethanes, the risk climbed to about 1.6 times as high. The researchers estimated that chlorination byproducts might account for roughly 14 to 16 percent of bladder cancer cases in the studied population.6PubMed. Case-control study of bladder cancer and chlorination by-products in treated water (Ontario, Canada)
But a large Scandinavian cohort study reached a different conclusion, finding no overall association between trihalomethane levels in drinking water and bladder cancer risk, even among long-term residents. The researchers noted that their results applied to the trihalomethane concentrations typical of chlorinated water in most European countries.7PubMed. Chlorination by-products in drinking water and risk of bladder cancer – A population-based cohort study One likely reason for the discrepancy is that European water systems often produce lower trihalomethane levels than some North American systems, either because of different source water quality or different treatment approaches. The upshot is that risk from disinfection byproducts probably depends heavily on concentration and duration of exposure, and removing chlorination altogether is not the answer since waterborne infectious disease would do far more damage.
Nitrates From Agricultural Runoff
Nitrate is one of the most widespread contaminants in drinking water, reaching elevated levels wherever fertilizer and animal waste seep into groundwater. The concern is not nitrate itself but what happens after you swallow it: bacteria in the digestive tract convert nitrate into nitrite, which can react with amino acids to form N-nitroso compounds. These compounds are potent carcinogens in animal studies, and increasingly in human research as well.8PubMed. Nitrate in drinking water and colorectal cancer risk: A nationwide population-based cohort study
A Spanish case-control study found that people with the highest nitrate intake from drinking water had about 1.5 times the risk of colorectal cancer compared to those with the lowest intake. The association was stronger among men and among people who also ate a lot of red meat, which itself contributes to N-nitroso compound formation in the gut.9PubMed. Colorectal cancer risk and nitrate exposure through drinking water and diet This interaction between dietary habits and water quality is worth paying attention to, because it means two people drinking the same water can face different levels of risk depending on what else they eat.
The current U.S. drinking water standard for nitrate is 10 milligrams per liter, a threshold originally set to prevent infant methemoglobinemia (blue baby syndrome). Some researchers argue this standard is not protective enough against cancer, since several studies have found increased risk at concentrations well below the legal limit.
PFAS and Cancer
Per- and polyfluoroalkyl substances, widely called “forever chemicals” because they barely break down in the environment, have become one of the most urgent drinking water concerns of the past decade. PFAS contamination in water supplies has been linked to several cancer types. A national U.S. study found that counties with PFAS detections in their water had elevated rates of cancers in the digestive system, endocrine system, oral cavity, and respiratory system, with incidence rate increases ranging from about 2 to 33 percent depending on the specific PFAS compound and cancer site.10Journal of Exposure Science & Environmental Epidemiology. Associations between per-and polyfluoroalkyl substances (PFAS) and county-level cancer incidence between 2016 and 2021 and incident cancer burden attributable to PFAS in drinking water in the United States
The evidence is strongest for kidney cancer. A study measuring PFOA directly in people’s blood found that those with the highest levels had more than twice the risk of kidney cancer compared to those with the lowest levels.11PubMed Central. Serum Concentrations of Per- and Polyfluoroalkyl Substances and Risk of Renal Cell Carcinoma A Swedish cohort study of people whose drinking water was heavily contaminated with PFAS reinforced this finding, showing an elevated risk of kidney cancer that was highest among those living in the contaminated zone during the period of peak exposure.12PubMed. Cancer incidence in a Swedish cohort with high exposure to perfluoroalkyl substances in drinking water A meta-analysis estimated that for every 10 nanograms per milliliter increase in blood PFOA concentration, kidney cancer risk rose by about 16 percent. The same analysis found a smaller but consistent association with testicular cancer as well.13PubMed. Critical review on PFOA, kidney cancer, and testicular cancer
In 2024, the U.S. EPA finalized the first-ever national drinking water standards for several individual PFAS compounds, setting limits in the low parts per trillion. Compliance will require significant upgrades to water treatment infrastructure, and many utilities are still in the process of testing their supplies.
Industrial Solvents in Groundwater
Trichloroethylene (TCE) and tetrachloroethylene (PCE) are industrial degreasers that became pervasive groundwater contaminants during decades of careless disposal. Both have been found at military bases, manufacturing sites, and dry-cleaning operations, and they migrate easily through soil into aquifers.
The evidence linking TCE to cancer centers on the kidney, liver, and lymphatic system. Studies of occupationally exposed workers consistently found modestly elevated risks on the order of 1.5 to 2.0 times higher than unexposed populations for cancers at those sites.14PubMed Central. Trichloroethylene cancer epidemiology: a consideration of select issues Among the most rigorously assessed cohorts, kidney cancer stood out with a relative risk of about 1.7, and liver cancer at about 1.9.15PubMed Central. Trichloroethylene and cancer: epidemiologic evidence
PCE contamination of drinking water made headlines in Woburn, Massachusetts, and on Cape Cod. A study of Massachusetts communities found elevated leukemia risk among residents exposed to PCE-contaminated water, with the risk climbing steeply among those with the highest exposure levels.16PubMed. Cancer risk and tetrachloroethylene-contaminated drinking water in Massachusetts Camp Lejeune in North Carolina is the most high-profile case of solvent-contaminated military drinking water; decades of contamination there led to federal legislation providing health coverage to exposed veterans and family members.
Cyanotoxins From Algal Blooms
Harmful algal blooms, which are becoming more frequent in warming lakes and reservoirs, produce toxins that can persist through standard water treatment. Microcystins, the most common cyanotoxins, are primarily known for acute liver damage, but a growing body of evidence connects them to liver cancer promotion as well. In a mouse model, microcystin exposure at concentrations typical of recreational water significantly increased the proportion of liver tumors that progressed to carcinomas.17PubMed Central. Impact of Cyanotoxin Ingestion on Liver Cancer Development Using an At-Risk Two-Staged Model of Mouse Hepatocarcinogenesis
Population-level data point in the same direction. U.S. census tracts that received drinking water from bloom-affected surface water sources had about 14 percent higher rates of liver cancer than tracts served by unaffected sources.18PubMed. Relationship between cyanobacterial bloom impacted drinking water sources and hepatocellular carcinoma incidence rates And in China, where drinking water from ponds and ditches often contains detectable microcystin levels, researchers have found that contamination tracks geographically with regions of unusually high liver cancer incidence.19Carcinogenesis. Detection of microcystins, a blue-green algal hepatotoxin, in drinking water sampled in Haimen and Fusui, endemic areas of primary liver cancer in China, by highly sensitive immunoassay As climate change drives water temperatures higher and nutrient pollution continues, blooms are expected to become more common, making this an emerging rather than diminishing concern.
Pesticides in Drinking Water
In agricultural regions, pesticide residues routinely make their way into water supplies. A study of grain-producing municipalities in southern Brazil estimated that over a recent monitoring period, pesticide-contaminated drinking water was responsible for at least 542 attributable cancer cases across 127 municipalities. More than 80 percent of those estimated cases were linked to just two pesticides: mancozeb (a fungicide) and diuron (an herbicide). Glyphosate contamination also correlated strongly with total cancer cases and breast cancer cases in the same municipalities.20PubMed. Widespread pesticide contamination of drinking water and impact on cancer risk in Brazil
This is a problem largely concentrated in rural areas dependent on agriculture. Private wells are especially vulnerable because they are not subject to routine monitoring. The specific pesticides of concern vary by country and crop type, but the underlying issue is the same everywhere: water treatment plants were not designed to remove many of the newer pesticide formulations, and regulatory standards often lag behind the science on which compounds are actually harmful.
Radioactive Contaminants in Well Water
This is an area where the threat sounds scarier than the evidence suggests. Bedrock wells naturally contain radon, radium, and uranium at varying concentrations, and the bladder and kidneys receive a radiation dose when these isotopes are excreted in urine. A Finnish study, however, found no substantial increased risk of bladder or kidney cancer from the radionuclide concentrations typically found in drilled wells.21PubMed. Well water radioactivity and risk of cancers of the urinary organs Radon in water can contribute to stomach radiation exposure, and this has been investigated as a potential stomach cancer risk factor, though the evidence there is also limited.22PubMed. Radon and other natural radionuclides in drinking water and risk of stomach cancer: a case-cohort study in Finland Overall, for most well water users, the radioactivity in their water is not a major cancer driver compared to the chemical contaminants discussed above. The bigger radon concern is usually the gas released from water into indoor air during showering, which contributes to lung cancer risk from inhalation rather than ingestion.
Microplastics and Nanoplastics
Tiny plastic particles are now detectable in tap water and bottled water alike, and understandably, people want to know whether they pose a cancer risk. The honest answer right now is that we do not know, but the biological mechanisms for concern are real. Laboratory research shows that micro- and nanoplastics can cross cell membranes, accumulate inside mitochondria and other organelles, and trigger overproduction of reactive oxygen species. This chain of events leads to DNA damage, inflammation, and disruptions in signaling pathways that control cell growth and death.23PubMed. Microplastics and nanoplastics in human toxicity: ROS-mediated mechanisms, cellular damage and systemic effects Microplastics collected from environmental water samples have been shown to provoke inflammatory responses in human cells at surprisingly low concentrations.24PubMed Central. Micro- and nano-plastics induce inflammation and cell death in human cells
What is missing is the epidemiological link. No study has yet demonstrated that the concentrations of microplastics people actually ingest through drinking water cause cancer in human populations. The lab findings are concerning enough that regulatory agencies are beginning to develop monitoring frameworks, but anyone claiming that microplastics in your water bottle are giving you cancer is ahead of where the science currently stands.
Aging Pipes and What They Add to Your Water
Even if water leaves a treatment plant clean, the pipes it travels through can introduce contaminants on the way to your faucet. Asbestos-cement pipes, installed widely from the 1940s through the 1970s, are still part of many water distribution systems. As these pipes age, the cement matrix deteriorates and releases asbestos fibers into the water, particularly when the water is naturally acidic or low in minerals.25PubMed Central. Epidemiology Study of the Use of Asbestos-Cement Pipe for the Distribution of Drinking Water in Escambia County, Florida Research has confirmed that calcium leaching from hydrated cement leads to wall softening and loss of mechanical stability, which accelerates fiber release.26PubMed. Degradation of asbestos – Reinforced water supply cement pipes after a long-term operation
The cancer risk from ingested (as opposed to inhaled) asbestos is less clear-cut than from inhalation, but the scale of aging pipe infrastructure makes this a persistent concern. Utilities are encouraged to track and replace aging asbestos-cement pipes, though the cost of wholesale replacement is enormous.27AWWA Water Science. Asbestos Cement Pipes in Water Distribution Systems: Health Risks, Regulations, and Management Lead service lines are another well-known infrastructure problem. While lead is primarily associated with neurological damage rather than cancer, it illustrates the broader point that water quality at the tap can differ significantly from water quality at the plant.
Does Hard Water Protect Against Cancer?
Not everything about water’s relationship to cancer is bad news. A large study of nearly 448,000 UK Biobank participants found that people living in areas with moderately hard water (calcium carbonate concentrations between 120 and 180 milligrams per liter) had lower risks for several cancers compared to those with very soft water. The reductions were on the order of 9 to 28 percent depending on the cancer type, covering colorectal cancer, lung cancer, breast cancer, prostate cancer, and certain blood cancers.28PubMed Central. Association of Domestic Water Hardness with All-Cause and Cause-Specific Cancers: Evidence from 447,996 UK Biobank Participants A systematic review of water hardness and colorectal cancer death rates found that all included studies suggested a protective effect of higher calcium concentrations, and that greater water hardness appeared protective against mortality from both colon and rectal cancer.29ACTA BIO-MEDICA DE L’ATENEO PARMENSE. Effect of drinking water calcium, magnesium, and hardness on colorectal cancer mortality: A systematic review
This does not mean you should seek out the hardest water possible. The associations are observational, and people in different water-hardness regions differ in many other ways. But it does complicate the instinct some people have to soften their water or avoid mineral content. The calcium and magnesium in hard water might be doing something useful.
Who Faces the Greatest Exposure
Water contamination is not distributed equally across populations. In the United States, water systems serving the highest proportion of Hispanic residents exceeded 5 milligrams per liter of nitrate nearly three times as often as systems serving the lowest proportion, though the pattern did not hold for Black residents or for poverty rates overall.30PubMed Central. Environmental justice and drinking water quality: are there socioeconomic disparities in nitrate levels in U.S. drinking water? In France, rural areas with higher deprivation showed higher nitrate levels and lower concentrations of fluoride and trihalomethanes, suggesting that the profile of contamination shifts depending on both geography and socioeconomic context.31PubMed. Environmental justice issues in drinking water contaminant exposure in a European context
Private wells are a major gap in the regulatory safety net. In the U.S., the EPA’s drinking water standards apply only to public water systems, leaving the roughly 43 million Americans on private wells responsible for their own testing and treatment. Many never test. People in agricultural areas, near industrial sites, or drawing water from geologically arsenic-rich bedrock face elevated exposures that nobody is officially monitoring.
Climate Change and Future Contamination Trends
Rising temperatures are expected to make several of these problems worse. Warmer water holds more dissolved organic carbon, and when that water is chlorinated, it produces more trihalomethanes. A study of Scottish water treatment plants found strong correlations between water temperature and trihalomethane levels, projecting that a mid-range warming scenario of about 1.8°C by 2050 could increase trihalomethane formation by roughly 39 percent.32PubMed Central. Predicted Impact of Climate Change on Trihalomethanes Formation in Drinking Water Treatment Warmer surface waters also fuel cyanobacterial blooms, heavier rainfall events wash more agricultural chemicals into waterways, and drought concentrates whatever contaminants are already present. The implication is that the water quality challenges described throughout this article are more likely to intensify over the coming decades than to improve on their own.