Demographic and clinical characteristics of coal miners eligible for the study and those included during the study period (Student's t test for continuous variables and χ2 test for frequencies, exact p values are given)
| Coal miners included (n=66) | Coal miners not included (n=94) | p Value | |
| Age (y, mean (SD)) | 64 (5.5) | 66 (6.9) | <0.001 |
| BMI (kg/m2, mean (SD)) | 26.1 (2.7) | 26.1 (3.7) | 0.063 |
| FEV1 (% predicted, mean (SD)) | 77.5 (22.9) | 75.2 (26.4) | 0.429 |
| Duration of work underground (y, mean (SD)) | 25.8 (9.2) | 26.0 (9.2) | 0.720 |
| Smoking (n (%)) | |||
| Non-smokers | 8/66 (12) | 15/94 (16) | 0.4951-150 |
| Smokers | 58/66 (88) | 79/94 (84) | |
↵1-150 p Value calculated with χ2 test.