Many asthma patients are allergic to the droppings of house dust mites, microscopic creatures which live in bedding, carpets, stuffed furniture, and stuffed toys.

Scientists have tried several strategies to minimise sufferers' exposure to the mites, either by killing the bugs, or by sealing them off from human contact.

Methods tried have included vacuum cleaning, heating and air filtration systems. A chemical known as Acaricides has also been developed to kill off the mites, and a tannic acid solution is used to neutralise their allergic properties.

Asthmatics have tried many measures to combat mites

However, researchers from Denmark, Sweden and Wales have found that no technique has helped to reduce symptoms in asthmatics effectively.

The scientists studied the outcome of 23 separate studies that tested the impact of both chemical and physical methods to reduce exposure to mites.

They found that 41 out of 113 patients who tested the treatments showed an improvement in their condition.

However, 38 out of 117 asthma sufferers who tested no treatment also registered an improvement in symptoms.

The researchers draw the conclusion that the treatments had no significant impact.

Writing in the British Medical Journal, they say: "Current chemical and physical methods aimed at reducing exposure to allergens from house dust mites seem to be ineffective and cannot be recommended as prophylactic treatment for asthma patients sensitive to mites."

New methods needed

Dr Michael Burr, a consultant in public health medicine at the University of Wales, said new methods were needed to tackle the problem.

He said existing methods could also be better targetted at people who were more likely to benefit.

"It is of very little value to just give blanket instructions to everybody with mite sensitive asthma to do lots of rather tedious things which will not do them very much good.

"There may be many factors causing the asthma, including allergy to cats and cigarette smoke. We need to identify those people who will really benefit from addressing the dust mite problem."

Dr Burr said methods which simply killed the mites were useless because the droppings remained behind.