Air pollution and children’s respiratory health : Luftföroreningar och barns luftvägshälsa

Summary In children, air pollution may cause, contribute to or aggravate respiratory diseases, which are a major cause of their ill-health. Childhood exposure to traffic-related air pollution is therefore of great concern, and has been discussed as a key aetiological factor. This review summarises and evaluates the findings of recent epidemiological studies of how short- and long-term exposure to traffic-related air pollution affects respiratory health and allergic sensitisation in children. In adults, air pollution is also known to contribute to cancer and heart disease. The same may, in principle, be true of children too, but these effects have scarcely been studied; and given the rarity of these diseases in childhood, air pollution is probably a minor contributory factor, at most, in terms of public health. Moreover, there are no reports on possible effects, in the form of adult cancer and heart disease, of childhood exposure to air pollution. Since respiratory tract symptoms are particularly prevalent in young children, elevated risks due to exposure to traffic-related air pollution have major implications for public health. Effects of air pollution may be divided into short-term and long-term effects, depending on the time lag from exposure until they make themselves felt. Exposure to NO2, PM10 and to some extent PM2,5 has been associated with symptoms of the lower respiratory tract in asthmatic children and with their hospital admission for asthma, often on the same or the following day. The overall estimated associations with hospital admissions for asthma were 1.3 % and 2 % rises in such admissions for every 10 µg/m³ increase in NO2 and PM10 respectively. No threshold levels (i.e. ‘safe’ levels below which there is no risk) can be identified from the existing data. The areas with the best air quality in which a significant association with admissions was found had average levels of 53 µg/m³ NO2 and 52 µg/m³ PM10. The symptom studies were more heterogeneous and no overall estimate can be derived. However, the areas with the best air quality where significant effects on asthmatic children’s symptoms were found had average levels of 16 µg/m³ NO2and 57 µg/m³ PM10. Based on a few studies only, asthmatic children thus appear to be at substantially higher risk than non-asthmatic children of having lower respiratory tract symptoms caused by exposure to air pollution. Owing to the variety of methods and definitions in the studies reviewed, no combined analysis of the long-term effects of ambient air pollution on respiratory symptoms or disease was feasible. However, there is evidence that exposure early in life to traffic-related air pollution, including gases (CO, O3, NOx) and particles (PM2.5, PM10), contributes to the onset of respiratory airway diseases, such as asthma and rhinitis, during childhood. This applies especially to children living close to major roads. The lowest long-term levels found to be associated with long-term health effects are 17–20 μg/m3 of NO2 (in Dresden and Munich, Germany and Stockholm, Sweden). There is strong evidence that exposure to traffic-related air pollution in childhood adversely affects children’s age-dependent improvement in lung function, at least up to adolescence, an effect that probably — although this remains to be studied — persists into adulthood, thus exerting an impact on adult respiratory health as well. The diversity of the studies reviewed makes formal quantitative comparisons of the findings difficult, and rules out any generalisation on how far air pollution affects lung function in children. The cleanest environments where lung function has been shown to be related to air pollution were in Austria and Sweden, where mean levels of NO2/NOx were 18–23 μg/m3. One study of lung function in children up to late adolescence showed a 3 % rise per µg/m³ of NO2 or PM10 in the risk of having a clinically serious deficit in lung function after eight years’ exposure. Exposure to NO2 (or NOx) has been shown, in some studies, to be associated with sensitisation to common allergens in children. The pooled estimate gives a 7 % rise in the risk of sensitisation to outdoor allergens for children living in areas with 10 µg/m³ higher NO2 levels. In addition, the health effects of air pollution may be especially pronounced in individuals who are genetically susceptible, as well as in those exposed to other environmental factors. However, the association between air pollution and sensitisation needs to be studied further before it can serve as the basis for preventive action. Children are judged to be more susceptible than adults to the adverse effects of air pollution. Their developing airway and lung function distinguishes them from adults and, since reduced lung growth may be seen as a permanent impairment, children may indeed be seen as a susceptible group. Whether they are especially susceptible when it comes to other respiratory effects is less clear. In addition, children cannot choose or modify their environment to the same extent as adults. Urban air pollution is caused by a mix of numerous components. Some are correlated, thereby confounding explanations of health effects, if any, in specific studies. Moreover, differences in observed effects may reflect random variation. Error rates in exposure assessments vary from one component and location to another. Accordingly, little emphasis should be laid on the fact that air pollutants derived from the same source seem to be variously related to certain health effects in different studies. Considering the components of air pollution as representing a complex mix is often more meaningful. Selective measures to improve air quality by removing specific components may achieve considerably smaller health gains than the ‘single component’ risks observed in epidemiological studies might lead one to expect.