A Permissible Exposure Limit (PEL) is the legal maximum concentration of a chemical or physical agent in workplace air that employees may be exposed to, typically measured as an 8-hour Time-Weighted Average (TWA). OSHA also sets Short-Term Exposure Limits (STELs, 15-minute averages) and ceiling limits (never to be exceeded). PELs are listed in 29 CFR 1910.1000 (Tables Z-1, Z-2, Z-3). When exposures approach or exceed PELs, employers must implement engineering controls, administrative controls, and/or PPE. Many OSHA PELs have not been updated since 1971 and are less protective than ACGIH Threshold Limit Values (TLVs) or NIOSH Recommended Exposure Limits (RELs), prompting many employers to follow the more protective limits voluntarily.