Research has not shown that swimming causes cancer. Risk assessments estimate a small added cancer risk from pool disinfection byproducts, and that risk is higher for people who spend many hours in indoor pool air. Swimming is also physical activity, and the CDC says being physically active lowers your risk for at least eight types of cancer. The practical answer is to keep swimming and to keep disinfection byproducts (DBPs) in your pool as low as you can.
This article is general information, not medical advice. Talk with your doctor if you have questions about your own cancer risk.
What are pool disinfection byproducts?
Disinfection byproducts are compounds that form when chlorine reacts with organic matter that swimmers bring into the water, such as sweat, lotions, urea from pee and skin cells. Some of the best-known pool DBPs are combined chlorines, also called chloramines. The CDC’s guidance on chloramines and pool operation says chloramines irritate the skin, eyes and respiratory tract and build up most in indoor pools with poor air exchange.
For a full explainer, read What Are Disinfection Byproducts (DBPs) in Pools, and Are They Harmful? and our guide to free, total and combined chlorine.
What does research say about swimming pools and cancer?
Two lines of research are cited most often:
- Risk estimates (2018 and 2023): A study of 19 indoor pools in Budapest, Hungary (Pándics and colleagues, Journal of Water and Health, 2018) estimated that one common pool byproduct added a lifetime cancer risk of more than 1 in a million for recreational swimmers and more than 1 in 100,000 for elite swimmers and pool staff, even in pools that met national standards. Breathing pool air was the main route of exposure. A 2023 health risk assessment prepared for New Zealand’s National Public Health Service reviewed this and similar studies, noted that human data for some pool byproducts are inconclusive, and concluded that the total risk should not be considered negligible.
- Genetic damage markers (2010): In a study published in Environmental Health Perspectives (Kogevinas and colleagues), 49 healthy adults swam for 40 minutes in an indoor chlorinated pool in Barcelona. Some markers of genetic damage rose with higher levels of certain byproducts, while others did not change. The authors called the findings support for potential genotoxic effects and asked for larger studies to confirm them.
These estimates come from models and small studies, and none shows that swimming in a well-run pool gives people cancer. They are a reason to reduce byproducts, which pool owners and operators can do.
Should you stop swimming because of DBPs?
The research supports staying in the water. The National Cancer Institute’s physical activity fact sheet says there is strong evidence that higher levels of physical activity are linked to lower risk of several types of cancer, including bladder, breast and colon cancer. The 2010 Barcelona study team wrote that reducing the health risks of pool water could increase the benefits of swimming, which points to cleaner pools as the goal.
How can swimmers lower their exposure to pool DBPs?
Swimmers control much of what goes into the water. CDC recommends these habits:
- Shower or rinse off before swimming. This removes sweat, lotion and dirt that react with chlorine. Read more on pool etiquette and showering before you swim.
- Use the restroom first and never pee in the pool. Urine is a major source of byproducts. See what happens when you pee in the pool.
- Wear a bathing cap. It keeps hair and scalp oils out of the water.
- Speak up about strong chemical smells. A strong “chlorine” smell at an indoor pool usually means chloramines are building up, so tell the lifeguard or pool operator.
How can pool owners and operators reduce DBPs?
The CDC’s chloramine guidance lists the main steps: bring fresh air across the water’s surface and exhaust it outdoors, measure combined chlorine (total chlorine minus free chlorine), superchlorinate when combined chlorine is too high, and use supplemental systems that break down chloramines, such as UV or ozone.
Clear Comfort’s hydroxyl-based AOP is a supplemental system that destroys contaminants in the water so pools can run low-chlorine. Hydroxyl-based AOP still works alongside a low chlorine residual. In residential pools, the typical free chlorine range with hydroxyl-based AOP is 0.5 to 1.5 ppm, which also means less of the chlorine that forms byproducts. At aquatic facilities, Clear Comfort case studies have reported 50% to 90% reductions in chloramines and other DBPs, depending on the facility. In one 2015 YMCA case study, lab testing at the University of Colorado Boulder found 84.8% lower levels of one major group of disinfection byproducts after Clear Comfort was added (case-specific result).
To lower chlorine at home, see our guide to a low-chlorine pool that is gentle on skin and eyes, or learn how hydroxyl-based AOP technology works.
Frequently asked questions
Does swimming in a chlorinated pool cause cancer?
Research so far does not show that swimming in a chlorinated pool causes cancer. Risk assessments estimate a small added risk from pool byproducts, higher for elite swimmers and pool staff who spend many hours in indoor pool air. Physical activity such as swimming lowers the risk of several cancers.
How big is the cancer risk from pool byproducts?
A 2018 study of 19 indoor pools in Budapest estimated that one common byproduct added a lifetime cancer risk of more than 1 in a million for recreational swimmers and more than 1 in 100,000 for elite swimmers and pool staff. A 2023 New Zealand public health assessment said human data for some byproducts are inconclusive but the total risk should not be considered negligible. Both point to keeping byproducts low.
Are indoor pools worse than outdoor pools for disinfection byproducts?
Byproduct gases build up more in indoor pools. CDC says chloramines settle just above the water and buildup is worse indoors, especially in winter when pools limit fresh air, and a 2018 Budapest study found breathing pool air was the main route of exposure. Good ventilation and lower byproduct levels in the water both help.
How do you reduce disinfection byproducts in a home pool?
Ask swimmers to shower first and use the restroom, keep chlorine and pH balanced, and shock the pool when combined chlorine rises. A supplemental system such as Clear Comfort’s hydroxyl-based AOP lets a home pool run low-chlorine, with typical free chlorine of 0.5 to 1.5 ppm. Less chlorine and fewer contaminants mean fewer byproducts.
Is swimming still good for your health?
Yes. CDC says physical activity lowers the risk of at least eight types of cancer, including bladder cancer, and swimming is a low-impact way to stay active. The researchers who study pool byproducts recommend reducing byproducts, which keeps the benefits of swimming.
Ready for a low-chlorine pool at home? Find a Clear Comfort dealer near you.
