Definitions
The main outcome was physician-diagnosed asthma. The definitions for asthma and respiratory symptoms were based on the following questions:
Physician-diagnosed asthma: “Have you been diagnosed as having asthma by a physician?”
Ever asthma : “Have you ever had asthma?”
Asthma medication use : “Do you currently use asthma medicines (on a regular basis or as needed)?”
Recurrent wheeze : “Do you usually have wheezing or whistling in your chest when breathing?”
Attacks of shortness of breath (SOB): Affirmative answer to both “Do you presently have, or have had in the last 10 years, asthma symptoms (intermittent breathlessness or attacks of shortness of breath; the symptoms may exist simultaneously with or without cough or wheezing)” and “Have you had these problems within the last year?”
Allergic rhino-conjunctivitis (ARC) “Do you have or have you had allergic nose or eye problems (hayfever)?”
Smoking habits: Smoking habits were divided into non-smokers, ex-smokers (quit smoking at least 12 months prior to the survey) and smokers (smoked during the 12 months preceding the survey)
Classification of socioeconomic status was based on main occupation and categorized into three groups; professionals, non-manual workers, and manual workers according to classifications by Statistics Sweden.