The use of anti-allergen bedding on its own is unlikely to have any significant effect on the level of asthma symptoms a person experiences, a study suggests.
Dust mites are thought to cause asthma
Researchers tested the effectiveness of allergen impermeable bed-covers in over 1,000 adults with asthma.
They found that using the covers did not reduce the level of asthma symptoms - or help people to reduce their levels of asthma medication.
Dutch scientists reported similar findings last year - but that was a much smaller study.
Special bed covers for mattresses, pillows and quilts are one of the most commonly used ways of trying to reduce exposure to dust mite allergen by adults with asthma.
Lead researcher Professor Ashley Woodcock, an expert in respiratory medicine at the North West Lung Centre in Manchester, said: "Our experience shows that around one in 10 adults with asthma have already bought anti-allergy bedcovers, yet if they are the only measure being used they are unlikely to give any clinical benefit.
"We now need to explore whether this is also the case for children and investigate if more intensive allergen avoidance measures can have a beneficial effect on symptoms.
"It is also still unclear as to whether avoiding exposure to allergens in early childhood could reduce the risk of asthma and allergies developing in the first place.
"We hope that ongoing research will provide some answers to this question soon."
Donna Covey, chief executive of the National Asthma Campaign, said: "People with asthma tell us they want to know how they can best avoid their asthma triggers.
"This new research tells us that as a single method of reducing exposure to dusts mites - bedcovers - are ineffective.
"We now need more research to tell us if a wider regime of lifestyle change can be of any benefit."
Around 50% of people with asthma report that house dust mites are a trigger for their asthma.
Although it is still unknown how far people need to go in avoiding dust mites to see any benefit, measures probably need to include:
The research is published in the New England Journal of Medicine.
- washing bed linen at a high temperature
- removing soft toys from beds
- applying anti-dust mite chemicals (acaricides) to soft furnishings
- removing carpets
- reducing humidity