Factors influencing the interpretation of FEV1 declines across the working shift.

An analysis of variability within individuals was conducted for FEV1, obtained from subjects employed in the cottonseed industry. Individual SDs ranged from .020 to .428 L. The within-subject SD of the preshift-postshift difference ranged from .014 to .302 L. When within-subject variability of FEV1 was analyzed in relation to symptoms, smoking history, and shift, the only statistical difference occurred in the 14 bronchitic subjects (.141 L) relative to the 179 without bronchitis (.099 L), and the three subjects with byssinosis (.192 L) relative to those without symptoms (.102 L). Changes over the working shift were significantly different for evening relative to morning and night shifts, even though there was no significant difference in smoking status, symptoms, exposure, race, age, years employed in the mill, and work area. In addition, baseline measurements were not significantly related to shift. Therefore, the effect of individual variability, symptoms, and shift should be considered if an accurate classification of reactors based on change in FEV1 across the shift is to be obtained.