Rising cases of asthma in European children could be partly due to indoor swimming pools, Belgian scientists said on Tuesday.

They believe exposure to chlorine bi-products, both in the air and the water, could be a factor.

"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," said Professor Alfred Bernard, of the Catholic University of Louvain in Brussels.

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.

They found that after taking into account other factors such as climate, childhood asthma and wheeze increased by 2-3 percent for every indoor swimming pool, according the research published in the journal Occupational and Environmental Medicine.

Asthma is one of the most common chronic diseases and is increasing fast. The number of cases 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.

Between 100 and 150 million people worldwide suffer from asthma, according to the World Health Organisation. About 180,000 die from it each year.

Risk factors for the illness, which can be controlled but not cured, include early exposure to allergens such as domestic mites, a family history of the illness and exposure to tobacco smoke.

Bernard and his team found that the rate of wheezing rose by 3.4 percent and asthma by 2.7 percent for every additional indoor chlorinated 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 researchers called for more study of the impact of chlorinated swimming pools and asthma risk and for better ventilation of indoor pools.