News · Longevity & Aging
Chlorination byproducts in Swedish tap water tracked with kidney cancer in men, in 58,664 adults over 25 years
Chlorine keeps drinking water safe from germs, but it also creates chemicals called trihalomethanes. A Swedish cohort found a link with kidney cancer in men at levels below current limits, and none with liver cancer.
- An observational study of 58,664 Swedish adults on public water supplies, followed for 25 years.
- Their exposure to trihalomethanes was estimated from where they lived and local water testing records.
- Men with the highest exposure had about 54% higher risk of kidney cancer than men with unchlorinated water.
- There was no link with kidney cancer in women, or with liver cancer in anyone.
- The numbers of cancers were small, and the authors say the finding needs confirming.
Adding chlorine to drinking water is one of the great public health advances of the last century, and nothing in this story changes that. But chlorine does not only kill germs. It reacts with natural material in the water to form byproducts, the most common being trihalomethanes, and animal studies have long pointed to the liver and kidneys as places they might cause harm.
A study in Environment International followed 58,664 Swedish adults for 25 years and linked their homes to decades of water testing records. Among men, higher exposure went with more kidney cancer. There was no link with liver cancer, and none in women.
What trihalomethanes are
MedlinePlus notes that water treatment may include adding fluoride to prevent cavities and chlorine to kill germs. It is that second step that creates trihalomethanes, when chlorine meets leaves, soil and other organic matter dissolved in the water.
The authors set out the tension plainly: chlorination of drinking water effectively prevents waterborne infections but also results in the formation of potentially carcinogenic disinfection by-products, including trihalomethanes. Evidence on liver and kidney cancer in people, they note, has been limited.
How the Swedish tap water study measured exposure
The participants came from two large Swedish cohorts, and everyone included was supplied by public drinking water. To estimate exposure, individual residential histories were linked to a national database of drinking-water monitoring results, giving each person a long-term estimate based on where they had lived.
Cancers were identified through Sweden’s national cancer register over 25 years, which recorded 183 liver cancer cases and 347 kidney cancer cases. Those are small numbers for a study of this size, because both cancers are fairly uncommon.
What the water records showed for kidney and liver cancer
For liver cancer, there was nothing: residential drinking water THM exposure was not associated with liver cancer.
For kidney cancer, the picture split by sex. Among men, risk rose steadily with exposure, and men in the highest exposure group had about 54% higher risk than men whose water was not chlorinated at all. No association was observed among women. The authors describe the evidence as novel for kidney cancer and note it appeared at exposure levels below current regulatory limits for drinking water.
Why the kidney cancer finding matters
Most people in wealthy countries drink chlorinated water every day, so even a small increase in risk would matter across a population. The finding also fits the animal evidence pointing at the kidney, which gives it more weight than a random signal would have.
The fact that it appeared below current limits is what makes it relevant to policy. A signal under those limits, if confirmed, would be a reason to look at them again, or at treatment methods that produce fewer byproducts.
What a Swedish water study cannot settle
The authors are cautious, and with reason. The kidney cancer estimate rests on a limited number of cases, and its lower confidence limit sits right at no effect, which means it could still be chance. They say plainly that the association warrants confirmation in future independent studies.
Exposure was estimated from where people lived, not from how much tap water they actually drank, showered in or boiled. Men and women may differ in those habits, which could help explain the split by sex, but the study could not test that.
What this changes for your tap water
It is not a reason to stop drinking tap water, and certainly not a reason to question chlorination, which prevents serious infections. The levels in this study were below legal limits, and the possible risk is small and unconfirmed.
For anyone who wants to lower their exposure anyway, activated carbon filters reduce trihalomethanes, and local water quality reports list measured levels. For water utilities and regulators, the study is a signal to watch: another reason to keep byproduct levels as low as practical while keeping water safe.
People also ask
What did the study find?
Over 25 years, 183 liver cancer cases and 347 kidney cancer cases were recorded. Trihalomethane exposure was not associated with liver cancer. For kidney cancer among men, risk rose with exposure, with a hazard ratio of 1.54 (95% CI 1.00 to 2.38) comparing 15 micrograms per liter or more with no chlorination. No association was observed among women.
What are trihalomethanes?
A group of chemicals, including chloroform, that form when chlorine used to disinfect water reacts with natural organic matter in it. They are among the most common byproducts of water treatment.
Is chlorinated water unsafe?
No. Chlorination prevents serious waterborne infections, and its benefits are large and certain. The levels in this study were below current regulatory limits, and the possible risk found is small and unconfirmed.
Why only men?
The study does not explain it. It could reflect differences in how much tap water men and women drink, biology, or chance, given the small number of cases.
Can I reduce trihalomethanes at home?
Activated carbon filters, such as many pitcher and tap filters, can reduce them. Your water utility's annual quality report lists measured levels. This is general information rather than medical advice.
Were the levels high?
No. The highest exposure group, 15 micrograms per liter or more, is well below the limits set in many countries, which is part of what makes the finding worth following up.