Thousands of asthma sufferers who use anti-allergy mattress covers may be wasting their time, say Dutch researchers. A study into the clinical benefits of these measures, involving 30 people with moderate to severe asthma, found covers helped cut down house dust mites, a known cause of asthma, but did little to improve patients' condition.

The organisers of the experiment suggested this might be because the chronic illness had progressed too far, or that exposure to other allergens, viruses and pollution, outside the bedroom, outdid any usefulness of the covers.

The study, reported in the medical journal Thorax today, compared two small groups of patients in Hilversum, in the Netherlands, over a year. Both groups were on normal medication and slept in carpet-free bedrooms. But only one group used mattress covers.

Combating mites at night has traditionally been seen as the most effective anti-allergen measure, because beds are their favourite habitat.

But while mattress covers might have meant the mites triggered fewer problems in people's noses, there was found to be no effect on lower airways, the researchers said.

John Harvey, chairman of the communications committee of the British Thoracic Society, cautioned against jumping to conclusions. It was clear the long term benefits of the covers was uncertain, and studies of larger numbers of patients were needed.

The way forward might be in preventing the triggers that "switch on" the asthma gene in susceptible children, he said. This might involve keeping pets away in the later stages of pregnancy and during babies' early months, and washing sheets and clothes at much higher temperatures.