Particulate Matter and Asthma: A Quantitative Assessment of the Current Evidence

Abstract

Numerous reports document significant worldwide increases in asthma morbidity and mortality from the late 1970s to the early 1990s. Various social and environmental factors, including exposure to indoor and outdoor pollutants and allergens, have been postulated as partial explanations of increasing asthma trends. Although air pollution concentrations have not generally increased over this period, other factors, including increases in poverty and decreases in regular medical care, may render individuals more susceptible to effects from exposures. There is a substantial literature linking exposure to several gaseous air pollutants with respiratory effects in asthmatics. Since the chemical composition and size distribution of airborne particles vary markedly with time and location, the impact of these heterogeneous mixtures on asthmatics is difficult to study in controlled exposure studies. Epidemiologic studies, however, have repeatedly demonstrated associations of particulate matter (PM) with exacerbations of asthma in ecological time-series analyses of emergency room visits and hospital admissions, as well as in panel studies examining associations with peak flow, medication use, and symptoms. In this article we briefly review asthma pathophysiology and potential pathways through which inhaled particles may affect the respiratory status of asthmatics. We also summarize the quantitative results from epidemiologic studies linking ambient PM to several measures of asthma exacerbations. Our analysis indicates that mean levels of PM occurring in urban areas of North America and Europe may be associated with increases of 2 to 5 percent for hospital admissions for asthma, from 5 to 10 percent for emergency room visits, and up to 60 percent for asthma symptoms.