Asthma is a puzzling illness in more ways than one. It is on the increase both in the UK, where it affects up to one child in seven and one adult in twenty, and worldwide. Nobody knows the cause either of the illness itself or of this increase in prevalence. And there have been no radically new drugs.

Despite all these difficulties, doctors and scientists working on asthma do not seem to be depressed. In fact they are optimistic, even confident. In part their confidence stems from the fact that asthma treatment is one of the success stories of healthcare in the UK. There is a feeling too that we are on the threshold of understanding the causes of the disease well enough to prevent it.

The defining characterisitic of asthma is a reversible obstruction of the airways in the lungs, according to Andy Wardlaw, of the Department of Respiratory Medicine at Leicester University. The obstruction is caused by an inflammatory reaction that causes contraction of the smooth muscle controlling the diameter of airways, swelling of the membranes that line the airways and secretion of mucous.

The all too familiar wheezing sound of an asthmatic attack is caused by turbulence as air flows through narrowed airways. But wheezing alone is not a sure sign of asthma, particularly in very young children, where it may result from infection, or in the over 60s, where smoking related illnesses like bronchitis are common causes.

The best way to diagnose asthma, according to Wardlaw, is by the way airway resitance changes particularly the way it increases in response to mild irritants. The increased resistance reduces the peak expiratory flow (the maximum rate at which the patient can breathe out). Peak expiratory flows only vary by about 5 per cent in non asthma sufferers. Variation by more than 15 per cent indicates asthma. In acute severe asthma peak expiratory flow can be reduced by 50% or more says Wardlaw.

Although severe asthma attacks can be life threatening, there is no cause for panic. “Asthma treatment is a success story of the last few years”, says Michael Morgan, also of Leicester. The success has nothing to do with new drugs, but with the drafting and widespread dissemination of guidelines for treatment, by the British Thoracic Society. “Virtually every GP treats asthma in the same way and the treatments work.” says Morgan.

Asthma treatments are of two types. Bronchodilators relax the smooth muscle that constrict the airways and steroids suppress the inflammation. There have been gradual improvements in formulation and in inhaler technology, but no breakthroughs. However, the fact inhalers deliver the drugs directly to the inflamed airways means that doses, and the attendant risk of side effects, can be kept low, says Wardlaw. One of the developments in asthma treatment is an increasing emphasis on teaching patients to monitor their condition, to recognise and avoid substances that trigger their attacks, and to regulate their own treatment.

Asthma is associated with a range of allergies. One of the commonest is an allergy to the dung of house dust mites. Tiny pellets of dung coated with digestive enzymes from the mite’s gut are carried on the air and can be inhaled in large numbers, according to Fleming Carswell of the Royal Hospital for Sick Children in Bristol. The airborne pellets deliver these proteins deep into the lung. “One of the hot ideas just now is that it is exposure to these proteins at a critical age that sensitises children so they develop asthma” says Carswell.

Extremely high doses and other factors such as smoking may make it possible for people to be sensitised after the critical age, as in the case of industrial asthma, Carswell says. Over a hundred different substances used in industrial processes are now known to be potential causes asthma.

Nobody knows why the background level of asthma is increasing, but there are some clues. There are hereditary factors that predispose people to asthma, but this isn’t the cause. “We know it’s something to do with adopting a western lifestyle” says John Britton of Nottingham City Hospital. When South Pacific islanders move to New Zealand they develop a high incidence whereas in their relatives who stay behind asthma remains rare.

Although there is a temptation to blame pollution, this is unjustified. “One of the highest incidences of asthma is in New Zealand, which is one of the least polluted countries.” However, pollution shouldn’t be ignored. “Although pollution doesn’t cause asthma, it makes it worse” says Britton.

Professor Anne Tattersfield, also at Nottingham City Hospital, is optimistic about finding environmental causes for asthma “My guess is that we could know the main environmental factors in a few years…. It could be something fairly simple, something we eat, or somethin in the water.” And when we find the causes, it will be much easier to avoid the disease “You can do a lot more about the environment than you can about your genes” she says.