Background Studies of asthma in migrant populations illustrate the effects of environmental changes.

Objective In this register study we investigated the importance of exposure to a western lifestyle in different phases of development in Swedish residents with an origin in regions in the world where asthma usually is less prevalent.

Methods The study population comprised 24 252 international adoptees, 47 986 foreign-born and 40 971 Swedish-born with foreign-born parents and 1 770 092 Swedish-born residents with Swedish-born parents (age 6–25 years). Purchased prescribed inhaled corticosteroids (ICS) during 2006 were used as an indicator of asthma.

Results International adoptees and children born in Sweden by foreign-born parents had three- to fourfold higher rates of asthma medication compared with foreign-born children. The odds ratios (ORs) of asthma medication declined persistently with age at immigration. For adoptees the ORs compared with infant adoptees were 0.78 [95% confidence interval (CI) 0.71–0.85] for those adopted at 1–2 years, 0.51 (0.42–0.61) at 3–4 years and 0.35 (0.27–0.44) after 5 or more years of age. Corresponding ORs for foreign-born children with foreign-born parents immigrating at 0–4 years, at 5–9 years, at 10–14 years and at 15 years or more were 0.73 (0.63–0.86), 0.56 (CI 0.46–0.68) and 0.35 (CI 0.28–0.43), respectively. The ORs were only marginally affected by adjustment for region of birth and socio-economic indicators.

Conclusions and Clinical Relevance Age at immigration is a more important determinant of purchased ICS than population of origin. This indicates the importance of environmental factors for asthma in schoolchildren and young adults.