Lung function measurements: Lung function measurements were performed using an SMI spirometrics volume dispslacement spirometer (Sensormedics Inc, California) for both baseline across-shift and follow-up evaluations. Techniques followed the standards set by the American Thoracic Society . During the 1989/90 period, lung function was measured on-site in the barn before entering the work environment, every 2 h during the work shift, and at the end of the work shift. The difference between the first measurement of the day and the last measurement of the day was cosidered the across-shift change.

During the follow-up in 1994/95, lung function measurement was conducted at a time during the work day at the subject’s home or at a clinic location near the swine confinement facility. Lung function indexes measured during both shift change and follow-up were FEV1, FVC, FEV1/FVCx100, and maximal midexpiratory flow rate (FEF25-75). Predicted values were obtained using the regression equation of Crapo et al.

Smoking habits: Based on smoking habits reported in 1989/90 and at follow-up in 1994/95, subjects were allocated into the categories of ‘nonsmoker’, ‘current smoker’ and ‘exsmoker’. Subjects were assigned a nonsmoker status if they denied smoking at both time points. Subjects who reported smoking in 1994/95 were assigned to the current smoker category. Those who reported smoking in 1989/90 and denied smoking in 1994/95, those who were exsmokers at both times and those who were exsmokers only in 1994/95 were assigned to the exsmoker category.