Increase in asthma: a more toxic environment or a more susceptible population?

Serial prevalence studies have shown that childhood asthma is becoming more common. In the UK two recent studies have found that the overall prevalence of episodic wheeze and of diagnosed asthma has doubled over the last two decades. '2 In these studies the prevalence of hay fever was also shown to have increased by a factor of three to four, while that of eczema had doubled. In children in New Zealand and Australia asthma prevalence has also doubled over the same period.34 The situation in adults is less clear, and data for the UK are not available, but most evidence from other countries suggests that a similar increase is occurring. Thus, although Yunginger et al; found no evidence of an increase in the incidence of asthma in the adult population of Rochester, Minnesota during the years 196483, analysis of data from conscripts to the Finnish and Swedish defence forces reveals a sixfold increase in asthma prevalence at call-up examinations between 1966 and 1989 in Finland,6 and a 47% increase between 1971 and 1981 in Sweden.7 In Busselton, Australia8 the prevalence of diagnosed asthma in adults aged 18-55 increased from 9% to 16 3% between 1981 and 1990, and in Manitoba, Canada, physician diagnosed asthma was found to be increasing in all age groups between 1980 and 1990.9 As well as changes over time, increases in the prevalence of asthma have also followed "westernisation" of a society'0 and migration from subsistence societies to economically developed countries. " 12 It is generally agreed that the observed increase in the prevalence of asthma is not simply a consequence of improved diagnosis'3'4 or of diagnostic transfer. '5 A change in genetic susceptibility of the population to the development of asthma is unlikely in the time period over which the observed increase has occurred. A more probable explanation is that the increase is due to the effects of life in the industrialised world resulting in an increase in exposure to exogenous factors which may induce asthma, a reduction in host resistance, or a combination of both mechanisms. Among exogenous factors which have been implicated in the increase in asthma are outdoor pollutants, indoor pollutants, cigarette smoke, and allergen exposure. Factors altering host resistance have received less attention. It has been suggested recently that a similar increase in the prevalence of hay fever coincided with the industrial revolution of the 18th and 19th centuries,16 and it is not surprising that the attention of researchers should have focused on air pollution both outdoors and indoors as a potential cause of increased asthma prevalence. With regard to outdoor pollutants, there is no doubt that many of these including sulphur dioxide, ozone, sulphuric acid and oxides of nitrogen can induce bronchoconstriction in susceptible individuals in the laboratory setting. '7 There is epidemiological evidence from the USA'8 and from the UK'9 that increases in ambient photochemical oxidants may be associated with respiratory symptoms and increased bronchodilator use in asthmatic subjects. However, air pollution in general in the UK has lessened substantially since the smogs of the 1950s with reduced burning of coal in cities, and there has been a well documented fall in levels of smoke and sulphur dioxide. Ozone is primarily a rural pollutant and the limited evidence available suggests that urban levels have not altered since the early 1970s.20 There is some evidence of a rise in background levels of oxides of nitrogen in rural situations since the 1970s, but peak urban levels in cities do not appear to have risen.202' While these data over an adequate time and range of sites are sparse, there is no evidence of the significant rises in any pollutants that would have been necessary were they to have been responsible for the increase in asthma and hay fever. In support of this conclusion, no association between exposure to particulates, nitrogen dioxide, or sulphur dioxide and prevalence of asthma was found in the American six cities study,2223 nor in a more recent study24 which compared schoolchildren in the heavily polluted city of Leipzig and the cleaner city of Munich; there was significantly more hay fever and rhinitis in Munich children, while the prevalence of asthma and airway hyperreactivity did not differ significantly between the two populations. The major component of indoor air pollution is cigarette smoke and there is abundant evidence that exposure to the constituents of cigarette smoke in utero and in childhood can increase the risk of developing allergy and respiratory symptoms.25-30 Again, however, it is difficult to attribute the considerable increase in asthma to changes in parental smoking. In the UK there has been a decline in the Department of Environmental and Occupational Medicine A Seaton