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Incidence of asthma onset by age group, in different birth cohorts. The incidence rate ratios were statistically different between the latter birth cohorts and the earlier birth cohorts in age group 26–30, 31–35 and 36–40, respectively, through the Poisson regression model. Asthmatics in the birth cohort born 1978 to1969 did not reach the ages from 36 to 40 years when the study was undergoing. **P <0.001; *P <0.01; #P <0.05.
