In order to determine the requirements for a proper temperature correction of expired volumes, the temperature of the expired air in the bellows of a dynamic spirometer (Vitalograph) was measured in 10 and 5 subjects at 24 and 12.2°C ambient temperature, respectively. During the short period of forced expiration there was a significant temperature decrease, which showed a linear relationship to the ambient temperature. Temperature differences up to 12–15% occurred within seconds; consequently, the values obtained in bellows-type spirometers for FEVi and FVC have to be corrected to BTPS at room temperature. There was no correlation between the degree of temperature decrease and the magnitude of expiratory volume. For routine clinical work it suffices to use the room temperature as the variable.

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