Sneezy living

A third of us suffer from allergies, and the number is rising fast. Is our clean, safe world to blame?

Asthma, hay fever, skin rashes, watery eyes, sneezing and stomach pains. The symptoms of allergy come in many guises, and there is little doubt that allergies are also on the increase. Medical scientists have documented a dramatic rise in the number of allergic reactions in Britain - along with other Western countries - and they are only now beginning to understand why.

Hay fever and skin rashes are bad enough if you suffer from them, but there are even more serious, life-threatening allergies that are on the rise. The largest increases of all have been in cases of anaphylaxis - when the entire body goes into a type of toxic shock - and food allergy.

One study in 2004 found that rates of anaphylaxis rose from six cases per million in 1990-91 to 41 cases per million 10 years later - a greater than six-fold increase. Food allergies over the same period rose five-fold, from five cases per million to 28 per million.

Other forms of serious "systemic" allergies affecting the entire body, such as urticaria or hives, have also increased significantly, according to one of the authors of the study, Professor Aziz Sheikh of the University of Edinburgh. These rarer forms of allergy now account for an estimated six per cent of all GP consultations and 10 per cent of the entire NHS prescriptions budget.

Professor Sheikh says that some 40 per cent of children and 30 per cent of adults in Britain are now affected by allergic disorders. Each year the numbers of patients are increasing by about 5 per cent.

The question is, why? The reason obviously has to do with the fact that the body's immune system has evolved over millions of years to identify and attack invading viruses and microbes. But it may be that in the modern world, where many of these dangers have been eradicated, the immune system has become focused on the wrong targets.

Scientists have called this the "hygiene hypothesis" because it invokes the idea that when children are brought up in a fairly sterile environment, their immune systems can somehow become disorientated, leading to the sustained but rather futile attack on harmless substances such as pollen or household dust.

"Whilst the reasons for these dramatic increases in allergies remain as yet far from clear, evidence is converging on the possible impact of improved living standards and associated reduced risk of infection," Professor Sheikh says. "One problem is that children now live in very clean environments and do not have exposure to childhood infections. They live in houses with central heating so they are not exposed to cold, they eat clean food and are vaccinated against diseases."

An antibody called immunoglobulin E, or IgE, is released by the immune system when something enters the body to trigger an immune reaction. This is known as an allergen. IgE stimulates the immune system to produce histamines and other substances that cause the symptoms of an allergic reaction, such as sneezing and wheezing.

Some people inherit a predisposition to overproduce IgE antibodies. Others appear to acquire the trait. Knowing why this happens would help to explain the growing epidemic of allergic reactions. One focus is on what happens to the immune system at the early stages of life. There is evidence that the immune systems of young babies seem to be extra sensitive to any foreign material that could trigger an allergic reaction.

Ten or 15 years ago, some scientists were suggesting that the removal of potential allergens in the home - such as dust mites - was the answer. However, Professor Sheikh says that many studies have shown that it does not work. Instead, he and other researchers are looking at the possibility of getting the young immune system acclimatised to the introduction of potential allergens. The idea is called immune tolerance.

So in trying to understand allergies, we might have to confront some dilemmas. Cleanliness is good for preventing food poisoning. Vaccines are good for preventing dangerous infections. But they may also contribute to the rise in allergies. We may have to balance one health-protection measure against another.