Prevalence of atopy and urban air pollution: dirty business

A steady increase in the prevalence of allergic diseases such as asthma, rhinoconjunctivitis, and atopic eczema has been observed over the last three decades. The aetiology of these conditions is multifactorial and, in genetically susceptible individuals, depends on the fine interplay between the time and degree of allergen exposure and the presence of nonspecific adjuvant factors. Although genetic influence is important in the development of allergy, the swift rise in the prevalence of allergic diseases can be explained by changes occurring in the environment, be it in the dwellings in which we live, the food we eat or the air we breathe [1]. That environmental factors are important, is indicated by the evidence that there are considerable differences in the prevalence of atopy between populations living in urban and rural areas. In the USA, the Second National Health and Nutrition Examination Survey has shown a higher prevalence of positive skin tests in those individuals living in urban than rural areas [2]. High levels of specific immunoglobulin (Ig) E were found in 43.7% of urban and 32.6% of the rural probands, a difference which was statistically significant [3]. In Sweden, Braback and Kalvesten [4] demonstrated that atopy was significantly more prevalent in urban than rural school children. In rural Ethiopia, positive skin sensitivity was up to three times less common than in the urban population of the city of Jimma [5]. The considerable differences in the prevalence of atopy reported in several East–West comparisons support the view that specific environmental factors are crucial in the development of allergy. These studies have demonstrated a higher prevalence of hay fever and atopy in Western European children than in those living in Eastern Europe [6,7]. Recently, the large International Study of Asthma and Allergies in Childhood (ISAAC) confirmed that the prevalence of childhood atopy is much lower in Eastern European countries than in Western Europe [8]. Despite the major improvements in air quality seen in many European cities, the problem of urban air pollution remains. Levels of traditional pollutants, such as carbon black and sulphur dioxide, have significantly declined as a result of industrial restructuring, technological changes and pollution control policies, but the rapid growth in road traffic in our cities has given rise to new pollutants and new concerns. During the last four decades the global vehicular fleet has increased 10-fold leading to the generation of newer forms of air pollutants, in particular, particulate matter, ozone precursors such as oxides of nitrogen and volatile organic compounds, benzene, aldehydes, 1,3-butadiene and polyaromatic hydrocarbons [9]. Another factor that contributes to these concerns is the simple arithmetic of exposure. Estimates by the United Nations indicate that over 600 million people living in cities and towns world-wide are exposed to unhealthy and dangerous levels of motor vehicle-generated air pollutants [10]. Therefore the potential for human exposure to traffic-related pollution is huge. It appears clear from a number of epidemiological studies in several countries that individuals living in close proximity of roads with high traffic intensity suffered more respiratory symptoms and allergies than those living further away [11–13]. In Germany, Wjst e al. [14] found no association between mean daily traffic volume in school districts and lifetime prevalence of childhood asthma, but did find increased prevalence of breathlessness and reduced respiratory function with increased level of road traffic. Recently, Boezenet al. [15] demonstrated that in children with bronchial hyperresponsiveness and high levels of serum total IgE the prevalence of respiratory symptoms increased up to 139% for every 100 mg/m increase in particulate matter. Car ownership in East European countries is currently rising by 7–12% per annum [16] with a predicted increase in the levels of air pollutants from motor vehicles. The rising levels of these pollutants in cities such as Leipzig (formerly in East Germany) since the German unification in 1990 has paralleled the drastic increase in the prevalence of hay fever and childhood atopy between the 1991–92 and the 1995–96 survey [17]. Taken together these observations suggest that the increase in the prevalence of atopy may at least partially be explained by the rising levels of air pollutants from motor vehicles. Among the various pollutants emitted from motor vehicle exhausts particulate matter has generated major scientific and public concern [18]. It is generated in large amounts by road traffic, not only because of the sharp increase in the number of diesel vehicles, but also because of the huge number of particles generated from diesel engines; diesel vehicles emit up to 100 times more particles than do petrol vehicles of corresponding performance [19]. Against this background, there have been a number of experimental studies in man addressing the importance of immunological effects of road traffic pollution on the allergic response. It appears that fine particulate matter emitted from diesel exhausts acts as a mucosal adjuvant by enhancing the IgE production in response to exposure to common allergens which then contributes to the potentiation of allergic immune responses [20]. Nasal instillation of