Carcinogenic chemicals called trihalomethanes (THMs), created as a byproduct of chlorinating water, can be absorbed through the skin.

People who regularly swim in chlorinated pools or take lots of showers or baths could therefore be putting themselves at risk.

Swimming in indoor pools can expose you to chemicals that may increase the risk for cancer and respiratory ailments, such as asthma. Ironically, researchers say that the danger is created, in part, by pool chemicals used as disinfectants. These chemicals react with natural organic matter and with people’s sweat, urine and skin cells to form disinfection by-products (DBPs). The greatest hazards are specifically linked to chlorine and bromine, both of which are used to rid pools of bacteria and other harmful substances. These chemicals are used in all pools, indoor and outdoor, but sunlight and air help to evaporate and dissipate the chemicals in outdoor pools.

Pool water, like tap water, is very often disinfected with chlorine to kill illness-causing microbes that lurk in the water.

Scientists have known for decades that along with the good that comes with disinfecting water with chlorine, chemicals called disinfection byproducts can also be formed when chlorine reacts with organic substances like human skin and residues from body care products.

Dr Gemma Castaño-Vinyals, of the Centre for Research in Environmental Epidemiology [CREAL] in Castilla La Mancha, Spain, said the effects negated the 'purity' benefits of drinking unchlorinated, TCM-free bottled mineral water.

She said: "People with more money and more education may think that they're reducing their risk of exposure to water contaminants by drinking bottled water.

"However, despite being apparently cleaner and taking more exercise, as a result of taking more frequent and longer baths, and using swimming pools more often, they are actually increasing their risk of THM exposure."

However, she said the additional risk of developing bladder cancer from absorbing THMs was "small".

She and colleagues were looking at exposure to THMs in 1,270 people in Spain. The results are released today (WED) by the online science publisher BioMed Central.