Scientists now believe a western lifestyle is to blame for the increase in asthma.
Asthma goes back to the time of the ancient Greeks when Hippocrates coined the term. But in those days the condition was not as common as it is now. Today asthma is on the increase around the globe. It claims more lives, causes more hospital admissions and more and more children are suffering from the wheezy illness - a doubling in the prevalence of asthma in the past 20 years. But why?
This week scientists and academics from Britain, the United States and Australia have gathered in London to tease out some answers to the puzzle. Is air pollution the culprit, as some people think? Probably not, say the experts. Air pollution is not causing new asthma, simply making it worse in patients who already have the condition.
More likely, and more worryingly, the scientists believe that the western lifestyle - broadly defined - may be to blame. It is a conclusion they have reached after studying a range of epidemological evidence.
Most of the experimental work in the developed world has been carried out in the United Kingdom, Finland, Australia, West Germany and the US. The evidence is based on two measures: longitudinal surveys through questionnaires of groups of people over a long period; and so-called objective tests of children to see if they get asthma when they take exercise or breathe in certain chemicals.
"Having data from two measures gives us the confidence that we are dealing with something real here," says Stephen Holgate, Medical Research Council clinical professor of immunopharmaco-logy at Southampton University, who chaired the symposium of international experts at the CIBA Foundation this week.
More evidence comes from countries that show rises in asthma in certain populations but not in others. Some developing countries fall into this category, as does the US, where inner city blacks experience much more asthma than people in rural and suburban areas. It therefore becomes a political hot potato - a black versus white issue. Asthma deaths, asthma admissions to hospital, and the rate of school and work absenteeism from asthma in inner city areas of New York, Washington and other places are much higher than anywhere else in America. "That's a new thing," says Holgate.
Moreover, in the developing world there has been a shift in the occurrence of asthma when a population has acquired western lifestyles. Examples are spread over the past decade and include New Guinea, where the inhabitants acquired blankets from the West and started to develop asthma, and Zimbabwe where the incidence of asthma increased two to threefold when people moved from the country to the town. In becoming town- dwellers, the Zimbabweans moved from a life in the open to living in buildings and sleeping in beds.
All this evidence points in the same direction - towards the deleterious effects of a western lifestyle. Sealing our homes, either to air condition or to heat them, decreasing air circulation and allowing allergens to accumulate in the domestic environment - house dust mites and allergens from pets such as cats and dogs - are thought to be contributing to the dramatic increase in asthma. We would probably be better off if we threw open our windows and and heated one room at a time the way the Victorians and Edwardians did.
Ann Woolcock, of the department of medicine at Sydney University in Australia and another speaker at the symposium, thinks that hygiene is also important. "The cleaner the population the more asthma," she says. "Asthma is a lifestyle disease of western, clean affluent societies." Cleanliness may be good for protection against disease but it is not necessarily good in helping small children build up their immune systems by fighting germs.
John Britten of the University of Nottingham has studied two national birth cohorts, one born in the 1950s, the other in the 1970s. After following them by means of questionnaires, he found, sure enough, an increase in the incidence of asthma among the later cohort, compared with the 1950s one.
But he found that the prevalence of other diseases - eczema, hay fever, and food allergies -had risen too. After correcting for the increase in allergy generally, the increase in asthma disappeared. In other words, the rise in asthma among children in the UK can be accounted for quite simply. These children are expressing allergy more. Asthma is one manifestation of an increase in allergy.
The difficulty is that different parts of the world have different allergens. In the UK, Australia and New Guinea, the allergens are house-dust mites. At high altitudes - in Denver, Colorado, or Calgary, in Canada, or in Davos, Switzerland - house-dust mites cannot survive because of the cold, dry climate. Yet the inhabitants of such places still get asthma, although it is less prevalent.
Are there other environmental factors that could be making children more susceptible to asthma? The simple answer is yes. One is cigarette smoking in pregnancy. That impairs lung development in the newborn and makes the immune system of the baby more sensitive to allergens at birth. One of the experts in this area is Fernando Martinez, of the University of Arizona School of Medicine. "This has to do with chemical compounds present in smoke crossing the placenta and affecting the child," he says.
Martinez is interested in the notion that maternal factors generally play a part in the development of asthma in their offspring. Younger mothers, for example, are more likely to have children who develop wheezing illnesses in early life. According to Martinez, one hypothesis is that there is competition between the young mother and the baby in the womb. A mother aged 16 or 17 may still be growing and not fully prepared to have a child biologically. Thus the child does not develop as well. "There is a very clear and unmistakable relation here, particularly early in life," he says.
More mysterious, however, are some of the associations between allergy and asthma in mothers and the development of asthma in their children in the first years of life. These associations are much stronger with mothers than fathers, says Martinez. "We believe that to understand asthma and allergies better we need to understand better the complex relationship established between the mother and the child in utero."
Another factor is diet. Woolcock says a different diet is one of the main global environmental changes. The shift to fast food - the increase in consumption of processed food, high fat and high salt, by schoolchildren and the decline in fruit and vegetables - has not helped. "Dietary factors are important but we don't know which ones," says Holgate.
Diet is linked to the role of infections. Two views are held. One is that in the old days we had nasty bacterial - as opposed to viral - infections like whooping cough and other ailments that have been eliminated by antibiotics. The theory is that the immune system was stimulated in a protective way by those bacterial infections. Therefore early this century babies and children were less susceptible to developing allergies. According to the scientists, there are special immune cells that are protective because they are stimulated by bacteria. And they believe that because we live in a cleaner world that sort of protection has disappeared.
The second view is that, if virus infections are caught at critical times in life, these then protect against infection. Research soon to be published in the British Medical Journal shows that in Guinea Bissau, Africa, children who have measles in the first year of life and survive are three times less likely to become allergic by 11-16 years of age than their siblings who do not contract measles. In other words, getting measles very early in life gives you vital protection. Scientists do not have the answer to these questions, so studies are being set up to look at the extent to which bacterial and viral infections protect the body rather than harm it.
Despite the new evidence about lifestyle and immune systems, the one trigger everyone asks about is air pollution. Research suggests that air pollution causes asthma attacks in people who have already acquired the disease. Vehicle-related pollutants such as ozone, nitrogen dioxide and small suspended particulates are particularly relevant here. But figures from such places as the Inner Hebrides do not support such a hypothesis. The Inner Hebrides, with very low traffic pollution, has as much asthma as inner-city areas. So conclusions in this area are tentative too.
Woolcock thinks it is unlikely that any new environmental risk factor such as air pollution has arisen. "In fact, air pollution probably protects you from asthma because where there's air pollution there's poor hygiene," she says.
What unites the scientists is the importance of an affluent western lifestyle in causing increased asthma. Martinez grew up and trained as a doctor in Chile where poor children had many infections but where he does not recall a single case of asthma. "It seems as if asthma is a disease of well-to-do people," he says. Martinez cites the case study of East and West Germany. When the Berlin Wall came down scientists looked at the prevalence of asthma on both sides of the wall. Germans are from common genetic stock but lived in very different conditions. There was - you guessed it - much less asthma in the East than in the West.
The reason why scientists are worrying away at the subject so much is that they want to find ways of intervening early in life to prevent asthma. They hope they will be able to suggest alterations to the environment rather than to recommend drugs. "If we felt we could put a finger on some of the major driving causes, then we could prevent asthma from occurring by giving advice to parents," says Holgate. "That's what it's all about."