Childhood exposure to chlorine byproducts in swimming pools could be partly to blame for the rise of asthma, according to a European study.

Researchers in Belgium analyzed data from almost 190,000 people aged 13 to 14, who were asked about breathing problems, hay fever and atopic eczema, an itchy swelling of the skin that may be found in people prone to allergies.

Rates of asthma rose by 2.73 per cent for every extra indoor swimming pool per 100,000 population, the team reports in Monday's online issue of the journal Occupational and Environmental Medicine.

The European findings showed an East-West divide that matches the availability of pools and asthma rates on the continent, after taking in account differences in GDP, climate and altitude, the researchers said. 

In 2003, the study's lead author, Alfred Bernard, a professor of public health at the Catholic University of Louvain, Brussels, proposed exposure to chlorine byproducts in the air and water of indoor swimming pools could increase the risk of asthma.

Chlorine reacts with ammonia in urine or sweat from swimmers. A gaseous film called trichloramine forms on the water's surface.

It's thought that when kids breathe the gas, proteins damage the protective walls inside the lungs, which may increase the risk of asthma.

Swimming pool attendance has increased in the last few decades and programs are increasingly offered to younger children, the study's authors noted.

In turn, European pools have increased water temperatures and bathing requirements, and have added recreational equipment such as slides while reducing ventilation to conserve energy.

Pools should be properly ventilated and levels of chlorine byproducts regulated, the researchers recommended.

Other suspected causes of asthma include genetic predisposition, and environmental factors, such as second-hand smoke, household dust and outdoor air pollutants.