Permissible exposure limit (PEL): is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent.   For chemicals, the chemical regulation is usually expressed in parts per million (ppm), or sometimes in milligrams per cubic meter (mg/m3).   Units of measure for physical agents such as noise are specific to the agent.   Permissible exposure limits are established by the Occupational Safety and Health Administration (OSHA).  A PEL is usually given as a time-weighted average (TWA), although some are short-term exposure limits (STEL).