The rise in childhood asthma in the UK may be partly due to increased exposure to indoor swimming pools, according to research published today which shows a direct correlation between the number of swimming pools in a country and the prevalence of asthma.

The researchers, led by Alfred Bernard, professor of toxicology at the Catholic University of Louvain in Brussels, found that the incidence of childhood asthma rose in areas with indoor pools.

This may explain why 20% of children in the UK suffer from asthma, compared with less than 4% in eastern European countries such as Latvia, Romania and Poland, 3% in Russia and 5% in Greece - where there are markedly fewer indoor swimming pools. It also explains why asthma levels remain high in parts of the UK, such as Scotland, with low pollution.

"This research indicates a strong correlation," said Prof Bernard. "The data we have now suggests that this factor could be involved in the geographical variation, which is largely unexplained by other factors."

Research in Belgium has already suggested that regular pool attendance, especially by young children, increases the risk of developing asthma - especially among the 30% of children who are atopic (susceptible to allergies). Trichloramine gas - a byproduct of chlorination which produces the distinctive "chlorine smell" - is to blame. Water-insoluble, it goes straight into the lungs when inhaled and damages the tissue there.

The new research, published in the BMJ journal Occupational and Environmental Medicine, builds on the previous data. The researchers drew on the International Study of Asthma and Allergies in Childhood (ISAAC), which analysed rates of wheezing and asthma among 190,000 teenagers, aged 13-14, from 21 countries.

The figures were then set in the context of the number of indoor chlorinated swimming pools compared with population size, which ranged from one for every 50,000 people in western Europe to one for every 300,000 in eastern Europe.

The research shows that childhood asthma is particularly prevalent in countries where there are enough pools to ensure that children can swim at least once a week - two or three pools for every 100,000 people.

The study concluded that the incidence of childhood asthma rose by 2.73% for every indoor swimming pool among 100,000 of the population.

The researchers also looked at the prevalence of asthma among six- to seven-year-olds, and found that this also rose with swimming pool availability, although the increase was not as pronounced.

The findings have led Prof Bernard to conclude that children who are susceptible to allergies should not be allowed to swim in pools which smell strongly of chlorine. Outdoor chlorinated pools are safer as the gas is quickly dispersed.

"If there is a strong chlorine smell and if the child is atopic, it may increase their risk of atopic diseases," Prof Bernard warned. "This environment isn't healthy for them and I personally would be careful. I wouldn't recommend it. It's just common sense."