Q: What are TWA, PEL and REL?

A: The Threshold Limit Value (TLV^(R) ) of a chemical substance establishes the reasonable level to which a worker may be repeatedly exposed, day after day, over a working lifetime without adverse health effects. TLV^(R) is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH)^(R) . It is however sometimes loosely used to refer to other similar concepts used in occupational health and toxicology. A list of current TLVs^(R) and biological exposure indices (BEIs) are published annually by the ACGIH^(R) .

The Canada Labour Code and OH&S regulations of most of the provinces require monitoring to ensure that no worker is exposed to a concentration of a chemical substance above the TLV established by ACGIH^(R) .

A Time Weighted Average (TWA) is a TLV^(R) based on a 8-hour workday and a 40-hour workweek. For example the TWA for carbon monoxide is 25 ppm. This means that an average of 25 ppm is considered to be the safe TLV^(R) for an 8-hour workday.

A Short Term Exposure Limit (STEL) is a TLV^(R) based on a 15 minute average.

A Ceiling is a TLV that should not be exceeded during any part of the work experience.

The Permissible Exposure Limit (PEL or OSHA PEL) is a TLV^(R) established by OSHA in the U S for exposure of an employee to a substance or physical agent, and may differ from TLVs^(R) in other jurisdictions. 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) or Ceiling Limits.

RELs or Recommended Exposure Limits are TLVs^(R) established by NIOSH for exposure of an employee to a substance or physical agent.

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