Modifying Effect of Age on the Association between Ambient Ozone and Nighttime Primary Care Visits Due to Asthma Attack

BACKGROUND: We examined the association between short-term exposure to outdoor air pollution and nighttime primary care visits due to asthma attack. We also investigated the modifying effects of age on this association. METHODS: A case-crossover study was conducted at a primary care clinic in metropolitan Tokyo. The subjects were 308 children aged 0-14 years and 95 adolescents and adults aged 15-64 years. All subjects made visits to the clinic for an asthma attack at between 7 PM and 12 AM. Data on hourly concentrations of particulate matter with a 50% cut-off aerodynamic diameter < OR =2.5 microm (PM(2.5)), ozone, and nitrogen dioxide (NO(2)) were obtained. A conditional logistic regression model was used to estimate odds ratios (ORs) of primary care visits per unit increment of each air pollutant. RESULTS: Among children, the ORs in warmer months per 10 ppb increment of the 24-hour mean concentration of ozone were 1.16 (95% confidential interval CI, 1.00-1.33) adjusted for temperature, and 1.29 (95% CI, 1.08-1.55) adjusted for PM(2.5), NO(2), and temperature. With respect to modification of the association by age, the ORs for 24-hour mean concentration of ozone--after adjustment for PM(2.5), NO(2) and temperature in warmer months--in children aged 0-1 years, 2-5 years, and 6-14 years were 1.06 (95% CI, 0.63-1.78), 1.37 (95% CI, 1.05-1.71), and 1.25 (95% CI, 0.87-1.82), respectively. There was no association between ozone and primary care visits among adults. CONCLUSIONS: An association was found between ozone and nighttime primary care visits for asthma attack in warmer months; the association was greater among preschool children.