• environmental risk factors;
  • infancy;
  • linear logistic regression analysis;
  • passive tobacco smoking;
  • wheezing

Clinical course and environmental factors were recorded in a prospective study of 276 unselected infants followed from birth to the age of 18 months. The study was performed with a questionnaire at the age of 6 and 12 months and a physical examination at 18 months. Fifty-nine (21%) of the children had ≥2 episodes of wheezing before they were 18 months old. A total of 58 (21%) of the children belonged to the lowest social class V, 182 (66%) were daily exposed to passive tobacco smoking at home and/or in daycare, 164 (59%) were breastfed ≥3 months, 192 (70%) were in daycare, 62 (22%) lived in Hats and 167 (61%) were in daily contact with furred pets at home and/or in daycare. In social class V a preponderance of children were exposed to passive tobacco smoking, a majority were living in flats and a minority were breastfed ≥ 3 months. Linear logistic regression analysis was used for the purpose of assessing the causal effect of environmental risk factors on the risk of recurrent episodes of wheezing before the age of 18 months. The study demonstrated that male sex and daily exposure to passive tobacco smoking were significant risk factors with estimated odds ratios 1.9 and 2.4, respectively. Maternal tobacco smoking seemed to be associated with the higest risk. There was a tendency – though not significant – indicating that breastfeeding ≥ 3 months had a protective effect. Low social status, daycare, daily contact with furred pets, having carpeted floor in bedroom, and living in a flat proved not to affect significantly the risk of recurrent wheezing.