Characteristics of the study population stratified by biogeographic region.
| All cohorts | Euro-Siberian | Mediterranean | p-value (difference)# | |
| Participants¶ | 2472 | 1108 | 1364 | |
| Wheezing (1st year) | 833/2326 (36) | 445/1044 (43) | 388/1282 (30) | <0.001 |
| Wheezing (4 years) | 449/1975 (23) | 207/802 (26) | 242/1173 (21) | <0.01 |
| Bronchitis (1st year) | 410/2320 (18) | 137/1042 (13) | 273/1278 (21) | <0.001 |
| Bronchitis (4 years) | 369/1967 (19) | 122/795 (15) | 247/1172 (21) | <0.01 |
| Asthma (4 years) | 64/1968 (3) | 42/796 (5) | 22/1172 (2) | <0.001 |
| Allergic rhinitis (4 years) | 59/1966 (3) | 29/794 (4) | 30/1172 (3) | 0.21 |
| Female infant | 1169/2408 (49) | 528/1082 (49) | 641/1326 (48) | 0.86 |
| Maternal education | <0.001 | |||
| None/primary | 545/2409 (23) | 171/1106 (16) | 374/1303 (29) | |
| Secondary | 1007/2409 (42) | 442/1106 (40) | 565/1303 (43) | |
| University | 857/2409 (36) | 493/1106 (45) | 364/1303 (28) | |
| Maternal smoking in pregnancy | 414/2350 (18) | 157/1040 (15) | 257/1310 (20) | <0.01 |
| Passive smoking (4 years) | 884/1978 (45) | 293/808 (36) | 591/1170 (51) | <0.001 |
| Season of birth | 0.30 | |||
| Winter | 608/2410 (25) | 292/1083 (27) | 316/1327 (24) | |
| Spring | 627/2410 (26) | 275/1083 (25) | 352/1327 (27) | |
| Summer | 619/2410 (26) | 278/1083 (26) | 341/1327 (26) | |
| Autumn | 556/2410 (23) | 238/1083 (22) | 318/1327 (24) | |
| Breastfeeding | 1959/2299 (85) | 803/990 (81) | 1156/1309 (88) | <0.001 |
| Maternal allergy | 638/2466 (26) | 263/1107 (24) | 375/1359 (28) | 0.03 |
| Maternal BMI | 23 (21–25) | 22 (21–25) | 23 (21–26) | 0.04 |
| Pets at birth | 734/2249 (33) | 245/1043 (24) | 489/1206 (41) | <0.001 |
| Physical activity (4 years) h·week−1 | 12 (5–16) | 14 (10–17) | 9 (2–15) | <0.001 |
| Neighbourhood SES+ (>median) | 1337/2299 (58) | 475/1095 (43) | 862/1204 (72) | <0.001 |
| NO2 (1st year) µg·m−3 | 27 (20–35) | 20 (15–25) | 34 (29–42) | <0.001 |
| Change of residency (birth to 4 years) | 424/2472 (17) | 138/1108 (13) | 286/1365 (21) | <0.001 |
Data are presented as n, n/n (%) or median (interquartile range) unless otherwise stated. BMI: body mass index. #:Chi-squared test and Wilcoxon–Mann–Whitney test (depending on the variable scale). ¶:Number based on exposure data. +:Neighbourhood socioeconomic status (SES) was determined according to the urban vulnerability index (where, at national level according to 21 indicators of socioeconomic vulnerability, 0=less vulnerable; 1=vulnerable).