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 degrees 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 degrees C the error in FEV1 increases. 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 degrees C, 27.1% of 302 subjects were classified as having a greater than or equal to 5% FEV1 drop over the shift using the dynamic BTPS factor model, compared with 41.4% when the standard BTPS correction factor was used (P less than .005). These results indicate the importance of correcting for ambient temperature differences when analyzing for shift changes in spirometric parameters.