Observational studies of the influence of O3 on human health are often difficult to interpret because the population is also exposed to other pollutants in the ambient air that could affect the responses observed, or to other environmental challenges that may produce comparable effects, such as environmental tobacco smoke, other pollutants in indoor air, and allergens found in indoor and outdoor air.
Grades 1 and 2 schoolchildren in Austria were studied across the summer for three successive years, and an O3 effect was detected on reducing FEV1 or lung growth.37 In the Children’s Health Study from Southern California, a new diagnosis of asthma was sought among 3,535 children with no history of asthma in twelve communities followed longitudinally.
38 Six communities had increased O3 concentration, and 265 children reported a new diagnosis of asthma over five years of follow-up. In six communities with high O3 concentration, the relative risk (RR)—the risk of an event (or of developing a disease) relative to exposure—of developing asthma in children playing three or more sports was 3.3 (95% CI 1.9–5.8) compared to children playing no sports. Sports had no effect in areas of low O3 concentrations (RR 0.8, 0.4–1.6).