Symptoms were defined as follows:
Byssinosis (all grades)=chest tightness or shortness of breath at work occurring on the first or other days of the workweek according to criteria of Schilling et al. Chronic bronchitis=sputum production occurring on most (>5) days of the week for at least 3 months a year for at least 2 consecutive years.
Chronic cough=cough without sputum for >5 days a week for at least 2 consecutive years.
Dyspnea grade 2+=having to walk slower than persons of the same age at an ordinary pace on level ground because of breathlessness.
Chest tightness at worktightness or constriction of the chest occurring anytime during the work shift and on any workday. Since the workers in these mills worked continual rotating shifts on an 8-day cycle with a 36-h rest at the end of rotation, we report chest tightness at work separately from the typical symptom periodicity which characterizes byssinosis.
Union of symptoms presence of any of any of the four symptoms noted previously.
The forced expiratory spirogram on the first day of each new 8-day cycle before and after the 8-hour work shift was recorded at both surveys. Workers were asked to refrain from smoking for at least 1 h before performing the test. Forced expiratory maneuvers were recorded for each worker on one of two 8-L water-sealed field spirometers (W.E. Collins Survey 1) under the direction of a trained technician.
The spirometric curves were read manually with the starting point defined by back extrapolation. Pulmonary function indices included FEVb and forced vital capacity. All values were corrected to conditions of body temperature and pressure saturated with water vapor. Each subject was retested on the same spirometer as used in the baseline survey. Each worker performed up to seven trials to produce three acceptable curves. Reproducibility was defined as the best two forced vital capacity values within 10 percent or 200 ml, whichever was greater. The best value of FEV i was taken from reproducible tests; otherwise, the FEV i was treated as missing.
Preshift values of FEVj and forced vital capacity were compared with the expected values derived from a population of silk yarn preparation workers studied simultaneously as a comparison group and previously published. Silk yam preparation workers rather than synthetic fiber workers were chosen as control subjects because all of the synthetic fiber mills in Shanghai were converted cotton mills. Regression equations from the silk worker population included a smoking term (pack-years) for men only, since there were only six female smokers. The models are described by the following: