Potential risk factors of the study infants
| Male sex | 122 (55) |
| 1 older sibling | 71 (32) |
| ≥2 older siblings | 44 (20) |
| Maternal asthma# | 22 (10) |
| Maternal atopy¶ | 85 (38) |
| Maternal smoking in pregnancy | 26 (12) |
| Low maternal education+ | 74 (34) |
| High maternal education+ | 68 (31) |
| Low paternal education+ | 42 (20) |
| High paternal education+ | 113 (53) |
| Living <150 m from a 4-m road | 162 (72) |
| Living <150 m from a 6-m road | 98 (44) |
| Living <75 m from a 4-m road | 108 (48) |
| Living <75 m from a 6-m road | 50 (22) |
Data are presented as n (%). #: defined as self-reported, doctor-diagnosed asthma; ¶: defined as self-reported, doctor-diagnosed asthma, hay fever or eczema; +: parental education was categorised into low (<4 yrs of apprenticeship), middle (≥4 yrs of apprenticeship) and high (tertiary education).