Attacks have fallen 32% in the past four years as better treatment and cleaner air help end the wheezing for millions

The asthma epidemic which has swept Britain for the last 20 years, condemning millions to a life of wheezing, may be over.

The asthma epidemic which has swept Britain for the last 20 years, condemning millions to a life of wheezing, may be over.

One of the most detailed surveys of the disease, based on patients who consulted their GPs with asthma, has found the number of new attacks has fallen by one-third since 1993.

The declining trend was clear for all age groups, in all areas of the country and at all seasons. It follows two decades in which the number of adult sufferers trebled and the number of children affected increased five-fold. Over three million people currently suffer from the condition.

The finding, published in the medical journal Thorax, comes from the highly respected Royal College of General Practitioners research monitoring unit in Birmingham and challenges the widely held view that asthma is still on the increase. The unit surveyed new attacks of asthma over a decade from 1988 to 1999. The figures show a rising trend from 1988 to a peak in 1993. Since then, they have fallen from an average weekly rate of 49 per 100,000 population in 1994 to 32 per 100,000 in 1999 - a drop of 35 per cent.

Dr Douglas Fleming, director of the unit, and colleagues write: "We conclude that [the] trends described here provide strong evidence that the 'epidemic' of asthma has peaked."

The fall has baffled experts. Although there are fluctuations in all diseases, the size and consistency of the drop in asthma - the number of new attacks has fallen in every quarter since 1994 - indicates a major change either in the environmental factors that trigger asthma or in the response of patients' lungs to these triggers.

The findings were given a cautious welcome by the National Asthma Campaign. Dr Martyn Partridge, chief medical adviser, said: "Most studies have shown a steady increase in the number of people with asthma over the period of the mid-1980s to the early 1990s. Clearly that was unlikely to go on increasing, for it is unlikely that the whole population would be genetically susceptible to the disease."

Dr Partridge said that in the last five years the death rate from asthma had fallen, and in the last three years so had the number of patients admitted to hospital with asthma attacks. "This may reflect better treatment but it could reflect a plateauing in the prevalence of asthma and this is what this latest study ... also suggests. If true, this is good news but it still leaves 3.4 million people suffering from the condition," he said.

Experts have been as puzzled by the rise in asthma, seen around the world between the 1970s and the early 1990s, as by its fall. Suggested reasons for the rise include the spread of central heating and wall-to-wall carpets, which provide the perfect home for the house dust mites thought to be a key cause of asthma, smoking among mothers, vaccinations and the use of antibiotics.

The strongest candidate is the "hygiene hypothesis" which suggests that asthma and other allergies may be triggered by clean modern lifestyles which protect children from bacteria and infection but also deny them the opportunity to develop natural resistance.

Dr Fleming and colleagues consider a range of explanations for the decline in asthma and find them all wanting. They reject the argument that doctors who previously diagnosed a wheeze as asthma may now be diagnosing it as bronchitis because cases of bronchitis have also fallen.

"Better treatment and management of the condition might be a factor but it is not an adequate explanation. We have seen a similar decline in all age groups and if it was the result of improved treatment you would expect to see a bigger decline in, say, children," Dr Fleming said.

Improvements in the environment, such as less pollution, may have contributed but do not explain the size of the fall. A final possibility is that asthma is following the natural course of diseases. "All diseases have an innate variability. If you go through history there is scarcely an illness that hasn't had periods of rising and falling," said Dr Fleming.