The impact of pollution on allergic disease

Abstract

Evidence suggests that allergic disease is becoming more common, particularly in industrialized societies. Two studies of schoolchildren from Aberdeen, Scotland aged 8–13 years were undertaken in 1964 and 1989 using identical questionnaires, and found that the reported prevalence of asthma had risen from 4.1% to 10.2% during this period, hay fever from 3.2% to 11.9% and eczema from 5.3% to 12%. Indication that air pollution may contribute to this increase has come from several studies. In Japan, allergic rhinoconjunctivitis was found to be more prevalent in individuals living near motorways than in cedar forests. Severe asthma also occurs more commonly than mild asthma in children living in polluted areas. Exercise-induced asthma and the use of asthma medication were twice as high in a town near two power stations compared with a non-polluted town. A recent study in Finland showed that admissions to hospital with severe asthma correlated with atmospheric levels of nitrogen dioxide. Deterioration in peak flow recordings in asthmatics and exacerbations of symptoms in hay fever sufferers correlate with ambient levels of ozone. Elucidation of the mechanisms by which exposure to air pollutants may influence the frequency of allergic disease or exacerbate symptoms has come from in vitro and in vivo experiments in animals and man. Animals exposed to ozone, sulphur dioxide, nitrogen dioxide and particles from diesel exhaust, together with allergens, show more ready development of allergic sensitization compared with those exposed to allergen alone. The dose of allergen necessary to produce a 20% fall in FEV₁ (forced expiratory volume in 1 second) in mild asthmatics is reduced by previous exposure to ozone or a mixture of nitrogen dioxide and sulphur dioxide. It has also been suggested that the allergic potency of pollen grains may be increased when they are coated in pollutants. Bronchial and nasal lavage studies have shown that air pollutants can induce an influx of inflammatory cells and proinflammatory cytokines into the respiratory tract. Studies on human epithelial cells cultured to confluence in vitro have indicated that exposure to nitrogen dioxide can decrease ciliary beat frequency which would, in theory, reduce allergen clearance and increase the risk of sensitization. Further, exposure of these cells to either ozone or nitrogen dioxide induces the release of inflammatory mediators, such as leukotriene C₄, and proinflammatory cytokines, including granulocyte-macrophage colony-stimulating factor, tumour necrosis factor-α and interleukin-8. This effect can be attenuated by the use of anti-inflammatory drugs.