Background We assessed the relationship of body mass index (BMI), smoking, drinking and solid fuel use (r; SFU), and the individual and combined effects of these factors on wheezing symptoms (WS) and on diagnosed asthma (DA).

Methods We analysed 175 000 individuals from 51 nationally representative surveys, using self-reports of WS and DA as the measures of asthma. The fixed-effects and random-effects estimates of the pooled ORs between asthma and underweight (BMI <18.5 kg/m²), obesity (BMI ≥30 kg/m²), smoking, drinking and SFU were reported.

Results The pooled risks of all individual risk factors were significantly associated with WS and DA (with the exception of current smoking with DA in women and SFU with DA in both genders). Stronger dose–response relationships were seen in women for smoking amounts and duration; BMI showed stronger quadratic relationships. The combined risks were generally larger in women than in men, with significant risks for underweight (OR=2.73) as well as obese (OR=2.00) smokers for WS (OR=2.13 and OR=1.58 for DA, respectively). The magnitude of the combined effects from low/high BMI, smoking and drinking were also consistently higher among women than among men in WS and DA. SFU among underweight smokers also had positive association with WS (men and women) and DA (women).

Conclusions BMI, smoking, drinking and SFU—in combination—are associated with double or triple the risk of development of asthma. These risk factors might help explain the wide variation in asthma burden across countries.