| Participants at follow-up (n = 298, 78.5%) | Subjects lost to follow-up (n = 87, 21.5%) | ||
| Sex, n, F/M | 28/270 | 10/77 | |
| Age, mean (SD) | 23.49 (7.7) | 22.80 (7.1) | |
| Smoking (S + Ex-S vs NS), n (%) | 157 (52.7) | 54 (62.1) | |
| Atopy*, n (%) | 104 (34.9) | 26 (30.0) | |
| Asthma diagnosed by a physician, n (%) | 56 (18.8) | 16 (18.4) | |
| Probable asthma†, n (%) | 22 (7.4) | 8 (10.9) | |
| Past exposure‡, n (%) | 14 (4.6) | 2 (1.7) | |
| Non-work-related respiratory symptoms, n (%) | 49 (16.4) | 18 (20.5) | |
| Non-work-related rhinoconjunctivitis symptoms, n (%) | 101 (33.9) | 33 (38.3) | |
| PC20 ⩽16 mg/ml, n (%) | 57 (19.1) | 22 (25.2) | |
| FEV1 ⩽80% predicted, n (%) | 20 (6.7) | 6 (6.9) | |
| School | |||
| A | 11 (3.7) | 5 | |
| B | 102 (34.2) | 29 | |
| C | 41 (13.8) | 24 | |
| D | 53 (17.8) | 8 | |
| E | 51 (17.1) | 7 | |
| F | 5 (1.7) | 2 | |
| G | 35 (11.7) | 12 |
*Atopy: a positive reaction to at least two skin-prick tests from a battery of 11 common aeroallergens; †probable asthma: ⩾2 symptoms suggestive of asthma and a PC20 ⩽16 mg/ml; ‡past exposure = contact ⩽3 months with isocyanates and without contact in the last 2 months (not meeting the exclusion criteria).
FEV1, forced expiratory volume in one second; PC20, provocative concentration causing a 20% decrease in FEV1; S, smoker; Ex-S, ex-smoker; NS, non-smoker.