Spirometry is the most common pulmonary function test in occupational health. It measures Forced Vital Capacity (FVC — total air exhaled), Forced Expiratory Volume in one second (FEV1), and the FEV1/FVC ratio. Under OSHA's respiratory protection standard (29 CFR 1910.134), spirometry is required as part of the medical evaluation for workers who wear respirators. It is also required under substance-specific standards for workers exposed to cotton dust, asbestos, and other respiratory hazards. Baseline spirometry is performed before workplace exposure begins, with periodic tests compared against the baseline to detect early signs of occupational lung disease. Testing must follow American Thoracic Society (ATS)/European Respiratory Society (ERS) quality standards.