Risk of adult‐onset asthma increases with the number of allergic multimorbidities and decreases with age.

Background: The aim was to study the association between allergic multimorbidity and adult‐onset asthma considering the number of allergic diseases and the age effect. Methods: We used population‐based data from Finnish national registers including 1205 adults over 30 years of age with recently diagnosed asthma (age range: 30‐93), matched for gender, age, and living region with one or two controls (n = 2050). Allergic rhinitis (AR), allergic conjunctivitis (AC), and allergic dermatitis (AD) were defined from self‐completed questionnaire. Conditional logistic regression adjusted on potential confounders (smoking, growing in countryside, childhood hospitalized infection/pneumonia, parental asthma/allergy, parental smoking, education level, professional training, number of siblings, and birth order) was applied to estimate the asthma risk associated with allergic multimorbidity. Results: A total of 1118 cases with asthma and 1772 matched controls were included [mean (SD, min‐max) 53 (11, 31‐71) years, 37% men)]. AR, AC, and AD were reported by 50.2%, 39.6%, and 33.8%, respectively, among subjects with asthma and 26.1%, 20.0%, and 23.5%, respectively, among controls. Compared to nonatopics, adult‐onset asthma increased with the number of allergic diseases; adjusted OR for asthma [95% CI] associated with 1, 2, and 3 allergic diseases was 1.95 [1.52‐2.49], 2.87 [2.19‐3.77], and 4.26 [3.07‐5.90], respectively. The association between adult‐onset asthma and ≥ 1 allergic multimorbidity decreased with increasing age (3.52 [2.51‐4.94], 2.44 [1.74‐3.42], and 1.68 [1.04‐2.71]) in subjects < 50 years, 50‐62 years, and > 62 years, respectively (p for age*≥1 allergic multimorbidity interaction, 0.002). Conclusions: Adult‐onset asthma was positively associated with the number of allergic diseases, and this association decreases with age. [ABSTRACT FROM AUTHOR]