The results of the world's largest asthma study seem to discount the notion that environmental allergens could account for the huge variation in asthma rates across Europe.
The research, published in the September issue of the European Respiratory Journal (2001;18:598-611), identifies no single cause for the much higher rates of the disease in some countries but links paracetamol use with increased susceptibility to asthma.
The European community respiratory health survey was a cross sectional study of atopy, asthma, and other chronic respiratory disorders that was carried out in 22 countries around the world (though mostly in Europe) during the early 1990s and that involved some 140 000 participants.
Its most startling finding was the uneven geographical distribution of these respiratory symptoms. There was a fourfold difference in the prevalence of nasal allergy between the most and least affected countries, a sixfold difference in the prevalence of current asthma, and an eightfold difference in the prevalence of wheeze.
Britain, Ireland, Australia, New Zealand, and the United States had the most asthma, while low rates were found in Iceland, Norway, Spain, Germany, Italy, Algeria, India, and the former Warsaw Pact countries.
A strong genetic association was found in both asthma and atopic sensitisation. Overall, children whose mothers had asthma were at 3.2 times the average risk of developing the disease, and the odds ratio was 2.9 if the father had asthma. If both parents had the disease, the risk was seven times normal. Study of segregation models indicated that at least one major gene for asthma exists but that no single gene model can account for the disease. Having a large number of siblings seems to reduce the risk of asthma and of allergy.
Allergic sensitisation accounted for only 13% of the overall variation in bronchial responsiveness. House dust mites and cat allergens were the most frequent allergic causes of airway constriction. Occupational causes were blamed in 5-10% of cases. Use of gas stoves was associated with a mildly increased incidence of asthma in British and French women, but not in men or children in these countries, and not in other countries, where different gases may be used.
The connection between asthma and paracetamol was first proposed in a previous analysis of the respiratory health survey's data, which found that the prevalence of wheeze in 13-14 year olds increased by 0.5% per gram of drug taken per person per year. The authors of that study proposed a mechanism whereby paracetamol strips reduced glutathione from the airway, shifting the balance of oxidant and antioxidant and causing inflammation. Other researchers have postulated that an unexplained protective effect of aspirin is reduced in countries where there is a preference for paracetamol.
The paracetamol researchers noted that asthma and allergy symptoms were most prevalent in countries with per capita sales averaging over 20mg a year. Since these were mostly anglophone countries, apart from Scandinavia and France, they attempted to control for the "anglophone effect," and found that much of the association between paracetamol and asthma vanished, although there was still a clear association with eczema in adolescents.
Peter Burney, a public health scientist at King's College, London and one of the authors of this month's paper, said it was difficult to separate the anglophone effect from paracetamol use because such a large proportion of the paracetamol sold worldwide was consumed by anglophones. "What's needed is research specifically tailored to answer this question," he said.
