Swimming pools can give you cancer, because disinfectants in the water react with sunscreen, sweat, and skin to form a toxic cocktail of chemicals, a study has suggested.
The disinfectant used to keep pool water free from disease can also react with swimmers' urine and hair to cause conditions including asthma and bladder cancer.
Products like sunscreen and oil are packed with nitrogen, which reacts with the disinfectant to create toxic chemicals capable of mutating genes.
These mutations contribute to birth defects, accelerate the ageing process, cause breathing problems, and even induce cancer after long-term exposure.
A team of researchers from the University of Illinois compared samples of tap water and pool water and found using advanced DNA technology that pool water samples led to more cell damage in humans.
Lead researcher Professor of Genetics Michael Plewa said: "All sources of water possess organic matter that comes from decaying leaves, microbes and other dead life forms.
"In addition to organic matter and disinfectants, pool waters contain sweat, hair, skin, urine, and consumer products such as cosmetics and sunscreens from swimmers.
"The study compared different disinfection methods and environmental conditions and our results proved that all disinfected pool samples exhibited more DNA damage than the source tap water.
"Care should be taken in selecting disinfectants to treat recreational pool water.
"The data suggest that agents containing the chemical bromine should be avoided as disinfectants of recreational pool water.
Prof Plewa also said that carbon should be removed before disinfection when pool water is being recycled.
He added: "Swimmers can also help reduce the toxicity of pool water by showering before entering the water.
"Pool owners should also remind users about the potential harm caused by urinating in a pool.
"These simple steps can greatly reduce the production of toxic disinfection by-products."