New research into the rising number of asthma sufferers is suggesting that indoor swimming pools may be partly to blame.
Belgian scientists say they believe exposure to chlorine bi-products, both in the air and the water, could be a factor.
Asthma is one of the most common chronic diseases and the number of sufferers is rapidly increasing.
Incidents of the disease, which affects the small tubes that carry air in and out of the lungs, has risen by more than 50 percent in developed countries in the last 25 years.
According to the World Health Organization between 100 and 150 million people worldwide suffer from asthma, and about 180,000 die from it each year.
Although risk factors for the illness such as exposure to allergens and tobacco smoke, can be controlled, there is no cure.
Professor Alfred Bernard, of the Catholic University of Louvain in Brussels suggests that the prevalence of childhood asthma and the number of indoor chlorinated swimming pools in Europe are linked through associations that are geographically consistent and independent of climate, altitude and the socioeconomic status of the country.
The Belgian researchers compared the rates of asthma in 13 and 14-year-old children in 21 European countries and the number of chlorinated swimming pools per 100,000 people and after taking into account other factors such as climate, childhood asthma and wheeze, they found an increase by 2-3 percent for every indoor swimming pool.
The numbers of indoor swimming pools ranged from one for every 50,000 inhabitants in Western Europe to one for every 300,000 people in Eastern Europe.
The rate of wheezing rose by 3.39% for every additional indoor chlorinated swimming pool. Similarly, the rate of asthma rose by 2.73%.
The researchers believe the key could be exposure to chlorine, which is used to keep pools clean and these chemicals present in heavily chlorinated pools may be making the airways more irritable.
The researchers are calling for more study into the impact of chlorinated swimming pools and asthma risk and for better ventilation of indoor pools.
They say the long-term effects of chlorine by-products on children's respiratory health should be thoroughly evaluated, pools should be properly ventilated and levels of chlorine by-products regulated.
The same team published evidence three years ago that chlorine in pools can react with sweat or urine to create harmful fumes which can damage lungs.
Their latest research is published in the journal Occupational and Environmental Medicine.