8.38. The above condensed overview of the effects of air pollution in adults with asthma is only of relevance to the effects on worsening symptoms or airflow in patients with pre-existing asthma. There is no doubt that ozone/acid/particulate events have an effect although of slight and variable degree in adults but there is some evidence that with increasing age, airway responses to ozone attenuate. This would also be supported by the finding that adults with longer outdoor dwell times showed greater effects, and that in some studies no effect at all can be demonstrated in adults. Effect to be more in those patients who are more symptomatic or who have increased bronchial reactivity. In patients with severe asthma the ameliorating effect of treatment might be expected to mask any worsening of symptoms or peak flow induced by air pollution. Acutely, therefore, adults with asthma are less likely to show an effect of air pollution episodes than children and such an effect is likely to attenuate further with increasing age. However, those with severe disease are likely to react more although the combined effect of age and disease has not been specifically studied. What we do not know is whether related exposure to ozone/acid/particulate events or to winter SO2/NO2/particulate events will have a cumulative effect on asthmatics pushing them to the more severe end of the spectrum.
8.39 The question as to whether there is epidemiological evidence for air pollution causing an increase inn the prevalence of asthma remains open. The American Six Cities Study and its successor, The 24 Cities Study, are cross-sectional studies considering respiratory disease in a range of cities of varying pollution experience. As yet they have asthma prevalence and increased pollution, although the 24 Cities Study has yet to report on this particular aspect. There has been one longitudinal study of asthma prevalence where air pollution has been taken into consideration. The advantage of this study is that it included a population of Seventh Day Adventists who are non-smokers. The results are slightly conflicting in that males (but not females) with higher ozone exposures had a slightly increased risk of developing asthma, but the effects of passive cigarette exposure was equally important as was the presence of airflow obstruction before the age of 16 years. These findings are compatible with the hypothesis that ozone exposure can‘cause’ asthma but the finding could also be explained by shift of the distribution of bronchial reactivity towards the more severe end.
8.40. There are many questions that need to be addressed. The most pressing is to determine whether air pollution experiences, as a combination of both summer and winter types, can lead to the development of asthma in otherwise unaffected individuals. Such studies must include assessments of non-specific reactivity and take into consideration atopic status, allergen exposure, Cross-sectional studies are unlikely to be able to provide this information so longitudinal studies are needed. These should include both children and adults, perhaps focusing on younger adults in the first instance.
8.41.Further panel studies of older patients need to be undertaken to assess whether they truly do show a lesser response to summer air pollution and whether their responses to winter pollution differs from younger adults and children. In all these studies the effect of acid aerosols should be assessed in view of the recent work from Birmingham showing an effect of summer acid on symptoms and peak flow in adults with asthma. This would enable estimates to be made of the likely health load on each group in terms of hospital admission, etc., an important step before assessing the possible benefits from reducing pollution levels.