Age of asthma onset and vulnerability to ambient air pollution: an observational population-based study of adults from Southern Taiwan.

BACKGROUND: Late-onset asthma (onset > 12 years) is pathologically distinct from early-onset asthma. The mechanism of air pollution is not a classic allergic inflammation and could have differential effect on late-onset and early-onset asthma. However, there is little known about the association of onset-age phenotype and air pollution. In this population-based study, we aimed to determine the association of asthma severity outcomes and air pollution regarding age at onset of asthma.

METHODS: In 2004, we conducted a cross-sectional questionnaire survey about respiratory health among schoolchildren's parents randomly selected from 94 of 816 elementary and middle schools in southern Taiwan. Participants ever having typical asthma symptoms were enrolled. We used kriging method to estimate individual exposure to ambient air pollution in the preceding year before the year of asthma severity survey. Ordered logistic regression was used to determine the association of exposure and asthma severity scores. Age at asthma onset of 12 years was used as a cut-off to define early- or late-onset asthma.

RESULTS: The study surveyed 35,682 participants. Data from 23,551 participants remained satisfactory with a response rate of 66 %. Among 20,508 participants aged 26-50 years, 703 questionnaire-determined asthmatics were identified and included for analysis. Using the median of PM10 (66 μg/m(3)) as a cut-off, those exposed to higher PM10 were more likely to have higher severity scores (OR = 1.74; 95 % CI, 1.13 - 2.70) only for asthmatics with asthma onset at > 12 years.

CONCLUSIONS: In adulthood, exposure to PM10 has a greater effect on late-onset asthma than early-onset asthma and deserves greater attention among ambient air pollutants.