Spirometric parameters are generally obtained at ambient 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, using a constant BTPS factor introduces a relatively small error in FEV1, but as the temperature decreases below 23 C the error in FEV1 increases. A dynamic BTPS correction factor model has been developed to reduce this error. Analysis of a recent study indicates that when temperature differences over a work shift were greater than 3 C, a significantly lower proportion of subjects were classified as having a > 5% FEV1 drop over the shift using the dynamic BTPS factor model (94/302) as compared to the generally used BTPS correction factor (125/302). These results indicate the importance of correcting for ambient temperature differences when analyzing for shift changes.