1 General characteristics of the study population
| High-risk children | Low-risk children | |
| Subjects n | 274 | 155 |
| Sex % males | 52 | 52 |
| Age yrs | 4.06±0.22# | 4.14±0.21 |
| Height m | 1.06±0.04 | 1.06±0.04 |
| Weight kg | 18±3 | 18±2 |
| Study region % | ||
| West | 51# | 40 |
| Middle | 30# | 44 |
| North | 19 | 16 |
| Education of the mother % | ||
| Low | 21 | 23 |
| Middle | 36¶ | 46 |
| High | 42# | 31 |
| Exposure to ETS at 4 yrs % | 22 | 26 |
| Exposure to pets at 4 yrs % | 47 | 48 |
| Contact with siblings at 4 yrs % | 78# | 87 |
| Contact with other children (no siblings) at 4 yrs % | 94 | 91 |
| LRTI at 4 yrs % | 12 | 10 |
| Symptoms of cold 2 weeks before NO+ % | 45 | 43 |
| Use of inhaled corticosteroids at 4 yrs % | 9 | 7 |
Data are presented as mean±sd or %, unless otherwise stated. ETS: environmental tobacco smoke; LRTI: lower respiratory tract infections (bronchitis, pneumonia, pertussis). #: p<0.05 as compared with low-risk children; ¶: p<0.10 as compared with low-risk children; +: fever, cough, wheeze, shortness of breath, tightness of chest, coughing up phlegm.