The forced expiratory volume over 0·75 second (F.E.V.0·75) was measured monthly for 3 years in 28 working ex-miners with pneumoconiosis and for one year in 17 normal laboratory workers. A significant annual cyclical variation of 0·044 litre amplitude was found in the ex-miners, with minimum and maximum values on 2 February and 3 August respectively. In the normal subjects the amplitude of the change was about 0·10 litre. An attempt was made to relate the changes in ventilatory capacity in the ex-miners to those in environmental temperature, relative and absolute humidity, and barometric pressure. Dry-bulb temperature showed a cyclical pattern in phase with that of ventilatory capacity, but there was no strong correlation between the deviations from these cyclical components, which would have been desirable to support a causal relation between change of temperature and of ventilatory capacity. There was no definite evidence of a relationship between any other of the environmental parameters and ventilatory capacity. Fourteen of the ex-miners had purulent sputum at intervals but remained at work. Immediately after these attacks the ventilatory capacity was not lower than before, but those having attacks showed a greater annual decline in their F.E.V.0·75. We think that the cyclical variation in ventilatory capacity is not directly caused by the environmental conditions measured, or by sputum purulence, but could represent an intrinsic biological rhythm which is perhaps modified by the environmental temperature.
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