A study by Germany's Federal Environment Agency (UBA) has advised parents with familial history of allergies not to take their babies to indoor swimming pools because of asthma risk.
The agency said that the potential danger lies with nitrogen trichloride, also known as trichloramine, which can be released when chlorinated water reacts with swimmers' urine, sweat or other organic matter.
"It is not possible at this stage to say definitively whether lung tissue can be damaged in the early stages (of life) and lead to asthma, because data on the effects of trichloramine are lacking," a statement said.
UBA President Jochen Flasbarth also called on swimmers to better adhere to basic hygiene rules and for swimming pool owners to do all they can to minimise the risk, for example by using the latest water-treatment technology.
"Swimming is healthy for children and adults. In order for it to remain so, everybody should shower thoroughly before swimming in order to prevent dangers to health arising from trichloramine," Flasbarth said.
A European study published in 2006 by scientists in Belgium found the incidence of asthma and wheezing among 13- and 14-year-olds can rise significantly if they use an indoor swimming pool.
Research carried out in British pools in 2003 found high levels of trichloramine-triggered proteins in the blood of young swimmers and even among parents who had sat by the poolside and not swum.
Asthma has been identified as a worsening health problem around the world, but the suspected causes for it are various.
The finger of suspicion is pointed variously at obesity, genetic predisposition, smoking, low birthweight, air pollution, household dust mites and other allergens. Strong emotions and the weather can exacerbate the condition.