Social class in asthma and allergic rhinitis: a national cohort study over three decades.

The aim of this study was to assess whether the association with social class differed between allergic rhinitis and asthma and whether these associations have changed over time. The Swedish Military Service Conscription Register was linked to two other national registers for 1,247,038 male conscripts in successive cohorts born between 1952 and 1977. The percentage of asthma cases associated with allergic rhinitis was 15% in the oldest cohort and 44% in the youngest cohort. Low socio-economic status (SES) was associated with an increased risk (assessed as odds ratio) of asthma without allergic rhinitis (1.14, 95% confidence interval (CI) 1.11-1.17) but a slightly reduced risk of asthma with allergic rhinitis (0.96, 95% CI 0.93-1.00). The risk of allergic rhinitis was 0.84, 95% CI 0.82-0.85. A positive interaction between SES and year of birth occurred in all three conditions. Low SES was related to a reduced risk of asthma with allergic rhinitis in the earliest cohort (0.72, 95% CI 0.53-0.82) but a slightly increased risk in the most recent cohort (1.07, 95% CI 1.01-1.14). In conclusion, the role of social class has changed over time. The steepest increase in asthma and allergic rhinitis occurred in conscripts with a low socio-economic status.