A German study, published in the journal Thorax, has found that dampness - long associated with wheezing and coughing - is also a risk factor for asthma, partly because it helps the growth of the house dust mites which irritate asthmatics' airways.

Previous studies have always been contradictory, but the new research is extensive, involving 5,000 Munich school children over a three-year period.

The researchers, led by Dr Thomas Nicolai, of Munich University, studied children when they were 10 and then again when they were 13.

Some 234 had asthma at age 10. Around a third still had symptoms at age 13.

Children with more severe asthma at age 10 and those with allergic triggers for the condition were most likely to still have symptoms when they were 13.

However, night-time wheezing and shortness of breath was strongly linked to dampness in the home.

Wheezing

Teenage asthmatics living in damp homes were three times as likely to suffer night-time wheezing and other breathing problems as asthmatics whose homes were not damp.

One of the reasons for the problem could be a link between dampness and house dust mite levels.

Many British school children are now dependent on inhalers

Damp houses were more likely to have high levels of dust mites.

Dampness is thought to aid the growth of dust mites and mould.

Other studies have suggested the number of mould spores in damp homes could be a factor in respiratory problems, but the researchers say the evidence on dust mites is stronger.

However, when levels of dust mites were taken out of the equation, dampness still proved to have a significant impact on asthma.

Huge increase

The number of people in Britain who are diagnosed with asthma has been climbing fast since the 1970s.

In 1973, 4% of the population were diagnosed as asthmatic. In 1996, the number was 21%, 750,000 of whom are children.