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Confounding factors complicate the interpretation of time series studies in examining the role of outdoor aeroallergens in asthma exacerbations
Despite historically low levels, outdoor environmental pollutants such as nitrogen dioxide, sulphur dioxide, and particulate matter are thought to play a role in exacerbating asthma. Much of this evidence comes from ecological “time series” studies that use sophisticated statistical methods to examine temporal associations between daily counts of asthma attacks and daily levels of air pollution at the population level. A good example of this type of study is the multi-city European study APHEA (Air Pollution and Health: an European Approach).1,2 Panel studies have also investigated temporal associations between daily outdoor air pollution levels and asthma but use the symptoms, lung function and medication use of individuals as the health status indicators. The multi-city equivalent in panel design is the PEACE study (Pollution Effects in Asthmatic Children in Europe). However, it failed to find statistically significant associations between particle measures, sulphur dioxide and nitrogen dioxide and respiratory symptoms, peak expiratory flow and medication use.3
Only a relatively small number of studies have used the time series approach to investigate the health effects of aeroallergens at the population and individual levels. Some studies of air pollution have included pollens and fungal spores as potential confounders,4–7 while others have been designed specifically to investigate the health effects of aeroallergens.8–16 The conclusions from this latter group are inconsistent—some report significant effects of pollens and spores and others …