Medical certification of workers for respirator use is an important activity of occupational medicine health professionals. Spirometry is a diagnostic tool to evaluate respiratory distress/insufficiency that may affect respirator use. In this study, we analyzed the pulmonary function data of 337 workers from different occupations which required medical evaluation to wear a respirator. The American Thoracic Society and National Fire Protection Association criteria were used to evaluate employees. Of 337 workers who were cleared for respiratory use on the basis of medical questionnaires for respirator compliance, 14 (4.15%) failed to pass respirator compliance on the basis of NFPA criteria and 5 (1.48%) failed to pass respirator compliance criteria on the basis of ATS criteria. We compared the use of different Spirometric equations to evaluate these criteria and we found the Crapo equation cleared more workers for respirator use as compared to the Knudson and NHANES III equations. We also measured repeated Forced Expiratory Volume in 1st Second (FEV1) and Forced Vital Capacity (FVC) and compared the results longitudinally over time. Age was the only significant factor affecting the reduction in the lung function in longitudinal analysis. Longitudinal spirometry results suggested that workers were protected while using a respirator in the workplace, but age is the significant factor in reducing their lung function. As some workers were able to qualify for respirator use based on questionnaire alone but failed respirator clearance subsequent to pulmonary function testing, it is recommended that spirometry be used to evaluate clearance for all workers who will use a respirator in the workplace. As well, using different Spirometric equations can affect the outcome on passing or failing clearance for respirator use, and this should be considered in a respiratory medical certification program.