| Covariate | Change in bronchial reactivity slope¶ % (95% CI) | p-value |
| Female sex | -3.9 (-21.1–16.9) | 0.689 |
| Age | ||
| Centered at 40 yrs per yr | 0.9 (0.4–1.5) | 0.001 |
| Centered at 40 yrs2 per yr2 | 0.0 (0.0–0.1) | 0.024 |
| Centered at 40 yrs only linear term per yr | 1.5 (0.7–2.3) | <0.001 |
| ETS exposure | ||
| Never-smoker | -1.0 (-16.0–16.7) | 0.908 |
| Ever-smoker | 10.7 (-10.1–36.3) | 0.340 |
| Positive Phadiatop test at baseline | -6.3 (-15.8–4.2) | 0.230 |
| BMI | ||
| Centered at 25 kg·m−2 per unit | -0.1 (-1.6–1.4) | 0.847 |
| Change between surveys per unit | 2.7 (0.3–5.2) | 0.030 |
| Smoking | ||
| Being a smoker at baseline | 21.2 (7.5–36.7) | 0.002 |
| Being an ex-smoker at baseline | -5.1 (-18.2–10.1) | 0.487 |
| Pack-yrs in ever-smokers+ per 5 yrs | 2.1 (0.0–4.2) | 0.048 |
| Pack-yrs in smokers+ per 5 yrs | 3.6 (1.6–5.6) | <0.001 |
| Pack-yrs in ex-smokers+ per 5 yrs | -2.4 (-6.5–1.9) | 0.267 |
Bold indicates statistically significant p-values. ETS: environmental tobacco smoke; BMI: body mass index. #: n = 3,005. ¶: from the baseline value; bronchial reactivity slope is defined as percentage change in forced expiratory volume in 1 s per micromole of methacholine; estimates are expressed in percentage change from baseline slope and adjusted for all other covariates in the table plus concurrent colds at both examinations, seasonal terms, baseline lung function (forced expiratory volume in 1 s, forced vital capacity (FVC) and forced expiratory flow at 25–75% of FVC) and study area. +: pack-yrs in ever-smokers and in smokers/ex-smokers were entered into the model instead of the smoking variables.