What Does
Threshold Limit Value – Time-Weighted Average Mean?
Threshold limit values (TLVs) are recommended exposure limits developed by the American Conference of Governmental Industrial Hygienists (ACGIH). The TLV of a chemical substance or a physical agent represents the maximum amount of it that a worker can safely be exposed to on a daily basis over the course of their working lifetime.
Time-weighted averages (TWAs) is a measure of exposure levels, averaged out over the course of a 40-hour work week or 8-hour workday.
Time-weighted average threshold limit values (or TLV – TWAs), then, are measures of the average concentration of a hazardous substance that a worker can safely be exposed to over the course of a typical work week or workday. They are the most widely used TLVs in occupational health and safety, and control methods for exposure to hazardous materials are often based on the TLV – TWAs for those substances.
TLVs are revised regularly. The ACGIH publishes updated lists of TLV recommendations on an annual basis.
Safeopedia Explains Threshold Limit Value – Time-Weighted Average
Threshold limit values for chemical products are typically measured in parts per million (ppm) for gases and in milligrams per cubic meter (mg/m³) for particulates.
TLV – TWAs and Safety Standards
The ACGIH is not a regulatory body, which means their published guidelines are not health and safety standards. As a non-profit, non-governmental organization, the ACGIH calculates TLVs solely based on the risks and potential health outcomes associated with exposure. Unlike regulatory agencies, it does not factor in additional considerations like the economic or technical feasibility of implementing recommended exposure limits in the workplace.
Despite this, TLVs are widely referenced by regulatory agencies when drafting standards. Industrial hygienists also heavily rely on them for assessing occupational risks and implementing control measures.
OSHA’s permissible exposure limits (PELs) are legally enforceable exposure limits that are based primarily on the ACGIH’s 1968 TLVs. Given that these values are out of date, OSHA’s Hazard Communication standard requires all safety data sheets (SDS) list the latest TLV alongside the PEL value.
Some states, Canadian provinces, and other jurisdictions have also passed legislation making TLVs (or selected TLVs) the legal workplace exposure limits.
TLV-TWAs and the General Duty Clause
TLVs are also relevant to the general duty clause of the OSH 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.
In cases where OSHA has not issued a PEL for a hazardous substance, employers should observe the TLV exposure guidelines to ensure compliance under the general duty clause.
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