Spirometric parameters are generally obtained at ambient (spirometer) temperature pressure saturated (ATPS) and then converted to body temperature pressure saturated (BTPS) by multiplying each observed value by a BTPS correction factor. At ambient temperatures of 23°C or higher, the accepted practice of using a constant BTPS factor introduces a relatively small error in forced expiratory volume in one second (FEV1), but as the temperature decreases below 23°C the error in FEV1increases. A dynamic BTPS correction factor model has recently been developed to reduce this error. Analysis of across-shift spirometry data from a recent survey indicates that, with an increase in temperature over a work shift of greater than 3°C, 27.1% of 302 subjects were classified as having a ≤5% FEV1drop over the shift using the dynamic BTPS factor model, compared with 41.4% when the standard BTPS correction factor was used (P<.005). These results indicate the importance of correcting for ambient temperature differences when analyzing for shift changes in spirometric parameters.