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METHODS: Lifetime asthma diagnosis for adults was assessed in 1984-1992 and 1995-2011 through the California Behavioral Risk Factor Surveillance System, an annual, cross-sectional, population-based survey. Using date of survey and date of birth, we classified the 18,305 adult respondents with lifetime asthma into 7 age groups, 6 periods, and 17 cohorts. Using a hierarchical, cross-classified random effects model, birth cohort, period, and age patterns in adult lifetime asthma prevalence were analyzed, while controlling for sex, race/ethnicity, smoking status, and income.
RESULTS: Through the use of an age-period-cohort model, we were able to examine the independent effects of age, period, and cohort on increases in lifetime asthma prevalence among California adults. Among California adults, lifetime asthma prevalence appears to be influenced by an age effect that persists across all periods, such that, within any given period, lifetime asthma prevalence is highest among younger age groups and decreases with increasing age. A period effect also appears to exist for all age groups, such that lifetime asthma prevalence has increased with the passing of time. The effect of cohort was not statistically significant.
CONCLUSIONS: Our analysis of nearly three decades of historical data for adults in California supports the role of biological or diagnostic age-related factors and historical trends (i.e., period effects) as the main drivers of increased lifetime asthma prevalence among adults in California. The influence of life course factors (i.e., cohort effects) on the historical increase in lifetime asthma prevalence was not shown to be significant. These findings reinforce the need for continued research into the environmental, diagnostic, and biological drivers of asthma incidence.