The lung function of 23 underground coal workers and eight surface workers at a Scottish colliery was measured immediately before and after a work shift. Changes in lung function were assessed in relation to the measured respirable dust exposure and the time of day in which the shift was worked. Large, and statistically significant, decrements of lung function during the shift were found in night-shift workers compared with workers on other shifts. Measurements derived from the forced expiratory manoeuvre, particularly FEV1, Vmax50, and Vmax25, after three vital capacity breaths of an 80% He/20% O2 mixture, showed large reductions in night-shift men, smaller reductions in afternoon-shift men, and small increases or decreases in morning-shift underground and surface workers. Within-shift changes for other tests, such as closing volume, N2 index, and volume of isoflow, did not differ significantly between shifts. No significant relationship was found between dust exposure and functional changes during a shift for any test. Lung function changes in a control group of 25 female workers not exposed to dust (hospital nurses) did not show the same large variations between day and night shifts. Examination of a further control group of 16 office workers did not show any difference in diurnal changes between smokers and non-smokers. It is concluded that these coal miners, even on permanent shift patterns, had widely different changes in their lung function cycle depending on which shift they were working. These changes did not appear to be related to dust exposure or cigarette smoking and were not consistent with other biological adaptations known to result from regular night-shift working.
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