Source:McConnell R, Berhane K, Gilliland F, et al. Asthma in exercising children exposed to ozone: a cohort study.
Lancet.
2002
;
359
:
386
–391.

Between 1993 and 1998, 3,535 children between the ages of 9 and 16 with no history of asthma were recruited from schools in 12 communities in southern California and were followed for 2–5 years with annual interviews. The investigators, from the University of Southern California, the National Institute of Environmental Health Science, and the California Air Resources Board, established a network of community monitors to measure air pollutants, including ozone, PM10 (particulate matter less than 10 ∞m in aerodynamic diameter), PM2.5, NO2, and acid vapor. They calculated 4-year mean concentrations of each pollutant (1994–1997) for each community. On the basis of mean pollutant concentrations, 6 communities were categorized as high pollution communities and 6 as low pollution communities.

Of the 3,535 children, 265 developed asthma during the course of the study. The risk of developing asthma was no greater for children living in the 6 high pollution communities than for children living in the 6 low pollution communities, regardless of which pollutant was used to classify communities as high or low. In all 12 communities, there was a small increase in asthma among children playing team sports. This increased risk was largest among children who had played 3 or more team sports in the previous year (RR 1.8; 95% CI, 1.2–2.8). In communities with high ozone levels between the hours of 10 am and 6 pm, children who played 3 or more team sports had a 3.3-fold increased risk of developing asthma (95% CI, 1.9–5.8). Children living in communities with low ozone levels who played 3 or more team sports did not experience an increased risk of a new asthma diagnosis (RR .8; 95% CI, .4–1.6). The amount of time children spent outdoors was also associated with asthma in communities with high ozone (RR 1.4; 95% CI, 1.0–2.1), but not in communities with low ozone.

Playing team sports appears to increase a child’s risk of developing asthma if s/he lives in a community with high ozone levels. This is a new observation that expands on previous findings that summer ambient ozone levels are a risk factor for respiratory problems in infants and toddlers.1 

It is now fairly well accepted that the increases in new-onset asthma in the United States and other industrially-developed countries are more closely linked to exposures to indoor air pollutants than to outdoor air pollutants.2 How does this fit with the findings of the above study? One possible explanation is the following: ozone, the primary component of urban smog, is known to increase bronchial hyper-reactivity of children to house dust mite allergen. Children who participate in 3 or more team sports inhale more ozone deeper into the lungs, which could result in increased airway hyper-reactivity to indoor allergens. Studies to test this hypothesis would be of interest.

Meanwhile, what advice can clinicians provide...

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