Asthma is the most common chronic disease in childhood, and the prevalence has risen in industrialized countries. Many factors have been implicated in this increase, including air pollution. While competitive athletes have a high prevalence of asthma and bronchial hyper-reactivity, no studies have systematically examined the risk in children who participate in outdoor sports. McConnell and colleagues monitored the development of asthma in children who engaged in outdoor team sports in California communities in which the concentrations of ambient ozone and other air pollutants were known.
The authors selected 12 communities in southern California with different levels of air pollution as assessed by ozone, particulate matter, and nitrogen dioxide concentrations. In each of the communities, 300 children were invited to participate in the study in 1993, and an additional 175 children were recruited in 1996. Baseline information included questions about symptoms suggestive of asthma as well as any confirmed diagnosis of the condition. The data also concerned information about participation in sports, including the type of sport, frequency of participation, and location of the activity.
Other data collected included measures of general health, exposure to tobacco smoke, family history of asthma, and family income. Stations that monitored air pollution were established in each of the communities to record levels of common pollutants every hour. From these data, annual and four-year mean concentrations of each of the principal pollutants were calculated. Because the year-to-year variations were small in individual communities, four-year mean was used to identify six “high-pollution” and six “low-pollution” communities.
Of the 3,535 children who participated in the study, 265 developed asthma during the four-to five-year follow-up period. Ozone was the most significant pollutant associated with the development of asthma. The relative risk of developing asthma in communities with high ozone levels was at least three times higher in children playing three or more outdoor sports than in children not participating in outdoor sports. In low-ozone communities, playing outdoor sports had little effect on the development of asthma. Overall, the risk was similar in boys and girls; however, of those who played three or more outdoor sports in high-ozone communities, the risk was somewhat greater in girls (relative risk: 4.7 in girls compared with 2.5 in boys). There was no interaction between other variables that are often associated with asthma and the study variables of participation in outdoor sports and ozone levels.
The authors conclude that in communities with high ozone levels, participation in strenuous outdoor exercise is associated with the development of childhood asthma. This effect does not occur when ozone levels are low. They suggest that this result may be caused by a combination of factors.