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Levels of dust mites in children's beds are three times higher in autumn than in summer, according to research which helps explain seasonal fluctuations in asthma.

Tests on the bedding of young Sydney children have revealed high levels of the allergens in April and May and low levels in January.

Researchers from the Woolcock Institute of Medical Research at the University of Sydney say this is the most comprehensive set of observations on seasonal fluctuations of mite allergens in children's beds in Australia.

The findings, to be published in the journal Allergy next month, add a new explanation for why hospitalisations for asthma fluctuate with the seasons.

One third of Australians are allergic to these microscopic bugs but this figure rises to 70 per cent among asthmatics who typically react when the mites' excrement or "allergen" gets embedded in their lungs.

Allergy researchers tracked 500 children from birth to age 5, regularly testing for levels of mite allergen in the dust of their bedding. They also tracked humidity and temperature at the time of dust collection.

Over the seven-year study the same pattern was observed - very low levels in mid-summer and levels two to three times higher in late autumn.

Researcher Daniel Crisafulli said the findings of the Childhood Prevention Asthma Study offered valuable insight for allergic people.

"It doesn't really change the way we can help people with asthma or allergies but it really should make them aware that there are times where it may get worse," Mr Crisafulli said. "They really ought to be monitoring their symptoms all the time."

The team's next step is to further investigate the relevance of seasonal variation in symptoms of allergic diseases and in exacerbations of asthma.

House dust mites are most commonly found in furnishings, carpet and bedding, and are most difficult to control in the latter.

Studies have shown that preventative measures such as extra washing and mattress covers do little to limit mites.