ALERT Learn More | NASP Certification Program: The Path to Success Has Many Routes. Choose Yours

Threshold Limit Value - Time-Weighted Average (TLV-TWA)

Last updated: August 5, 2018

What Does Threshold Limit Value - Time-Weighted Average (TLV-TWA) Mean?

Threshold Limit Values (TLVs) are exposure-limit recommendations developed by the American Conference of Governmental Industrial Hygienists (ACGIH). The TLV of a chemical substance or physical agent is a measure of the maximum amount of that substance that a worker can be safely exposed to on a daily basis for a working lifetime.

Time-weighted average (TWA) TLVs are the most widely used TLVs. They provide exposure limits based on the average amount of a chemical substance that a worker is exposed to over a 40-hour workweek (eight-hour workdays).

Chemical substance TLVs are normally provided in units of parts per million (ppm) for gases and in milligrams per cubic meter (mg/m³) for particulates. The ACGIH publishes an updated list of TLV recommendations on a yearly basis.

Safeopedia Explains Threshold Limit Value - Time-Weighted Average (TLV-TWA)

TLVs are widely accepted recommendations whose value is acknowledged by OSHA and other occupational health and safety agencies. OSHA’s legally enforceable exposure limits—called permissible exposure limits (PELs)—are based primarily on the ACGIH’s 1968 TLVs. OSHA recognizes that these PELs are out-of-date. Because ACGIH regularly updates its TLV recommendations in accordance with the most recent scientific data available, OSHA’s Hazard Communication standard requires that all safety data sheets (SDS) list both the OSHA PEL and the ACGIH TLV.

ACGIH is a non-profit, non-governmental organization. It develops TLVs solely as health-based values, and it does not give consideration to the economic or technical feasibility of implementing the recommended exposure limits in the workplace. As such, the ACGIH does not believe that TLVs should be adopted as workplace standards without considering other relevant factors (e.g. technical and economic factors). However, some jurisdictions, such as certain Canadian province, have passed legislation making TLVs or selected TLVs legal workplace exposure limits.

TLVs are also relevant to the general duty clause of the OSHA Act. The Act stipulates that U.S. employers have a general duty to maintain a safe work environment, including a duty to meet recognized workplace safety standards. As a TLV is generally considered a recognized safety standard, employers may have a duty to observe TLV exposure guidelines for a chemical substance that has not been issued an associated PEL.


Share this Term

  • Facebook
  • LinkedIn
  • Twitter

Related Reading


Best PracticesStandards

Trending Articles

Go back to top