A threshold limit value (TLV) is an airborne exposure guideline published by the American Conference of Governmental Industrial Hygienists (ACGIH). A TLV is the concentration of a substance to which ACGIH believes nearly all workers may be repeatedly exposed, day after day over a working lifetime, without adverse health effects. TLVs are recommendations, not law, and they are often stricter than OSHA's legal limits.
TLVs matter because they reflect more current science than many OSHA permissible exposure limits. Industrial hygienists, safety data sheet authors and safety managers use TLVs to judge whether exposures are truly acceptable, even when the legal limit is met.
What Is a Threshold Limit Value?
ACGIH is a private, nonprofit scientific association. Its TLV committees review published research on chemical substances and physical agents and set guideline values based only on health factors. They do not consider economic or technical feasibility, which is one reason TLVs differ from regulatory limits. ACGIH publishes its TLVs, along with Biological Exposure Indices (BEIs), in an annual book, and proposes changes through a Notice of Intended Changes before adopting them.
ACGIH itself states that TLVs are guidelines intended for use by industrial hygienists in making decisions about safe levels of exposure, and that they are not consensus standards or fine lines between safe and dangerous. Because individual sensitivity varies, a small percentage of workers may experience effects at or below a TLV.
Types of TLVs
- TLV-TWA: the time-weighted average concentration for a conventional 8-hour workday and 40-hour workweek.
- TLV-STEL: the short-term exposure limit, a 15-minute TWA that should not be exceeded at any time during the workday, even if the 8-hour TWA is within the TLV-TWA. Exposures above the TLV-TWA up to the STEL should last no longer than 15 minutes, occur no more than four times per day and be separated by at least 60 minutes.
- TLV-C (ceiling): a concentration that should not be exceeded during any part of the working exposure.
For substances that have a TLV-TWA but no STEL, ACGIH recommends excursion limits: exposures may exceed three times the TLV-TWA for no more than a total of 30 minutes during a workday and should never exceed five times the TLV-TWA, as long as the TLV-TWA is not exceeded.
Notations
- Skin: the substance can contribute significantly to overall exposure through the skin.
- DSEN and RSEN: potential for dermal or respiratory sensitization.
- Carcinogenicity (A1 to A5): categories ranging from confirmed human carcinogen (A1) to not suspected as a human carcinogen (A5).
ACGIH also publishes TLVs for physical agents such as noise, heat stress, vibration and radiation. Heat stress TLVs, for example, are based on wet bulb globe temperature.
TLV vs. OSHA PEL
- Legal status: an OSHA PEL is enforceable. A TLV is a guideline unless a regulation, contract or company policy adopts it.
- Basis: TLVs are based on health effects only. PELs can reflect feasibility and the rulemaking process.
- Currency: ACGIH reviews TLVs every year. Most OSHA PELs date from 1971 and were based on the 1968 TLVs.
- Stringency: for many substances the current TLV is lower than the PEL, sometimes by a wide margin.
OSHA's annotated PEL tables list current TLVs alongside PELs, NIOSH RELs and Cal/OSHA limits. OSHA's Hazard Communication Standard also requires Section 8 of each safety data sheet to list the ACGIH TLV along with the OSHA PEL.
TLVs and Biological Exposure Indices
ACGIH also publishes Biological Exposure Indices (BEIs), which are guidance values for measuring a substance or its metabolites in a worker's blood, urine or exhaled air. BEIs complement TLVs by reflecting exposure from all routes, including skin absorption and ingestion. For mixtures of substances that affect the same organ system, ACGIH recommends treating their effects as additive, by adding the ratio of each exposure to its TLV and keeping the total at or below 1.
Where to Find TLVs
- The annual TLVs and BEIs book and supporting documentation from ACGIH, available for purchase.
- Section 8 of the safety data sheet for the product.
- OSHA's annotated PEL tables, which list TLVs for comparison.
"TLV" and "Threshold Limit Values" are registered trademarks of ACGIH, and the full values and documentation are copyrighted.
How to Use TLVs Step by Step
- List your substances. Use your chemical inventory and SDSs to identify substances with TLVs.
- Find the current values. Check the latest ACGIH edition, since values change.
- Compare to the PEL. Note where the TLV is lower than the PEL and decide which limit your company will use as its internal target.
- Assess exposures. Have an industrial hygienist sample breathing-zone air and calculate TWA, STEL and ceiling results.
- Check notations. Address skin absorption and sensitization where noted.
- Control exposures. Apply the hierarchy of controls to bring exposures below your chosen limit.
- Adjust for unusual schedules. Have an industrial hygienist evaluate shifts longer than 8 hours or workweeks over 40 hours.
- Re-evaluate. Review your limits when ACGIH adopts changes or your processes change.
Common Mistakes to Avoid
- Treating the TLV as a sharp line between safe and unsafe exposure.
- Using an outdated edition of the TLVs.
- Assuming meeting the OSHA PEL means meeting the TLV.
- Applying TLVs to community or environmental exposures, which ACGIH says they are not designed for.
- Ignoring STEL and ceiling values because the 8-hour average is acceptable.
How SMS360 Helps
SMS360's Safety Data Sheets module keeps current SDSs on hand so teams can quickly look up the exposure limits in Section 8. The Risk Assessment module helps document chemical exposure hazards and controls, and Training Management helps track hazard communication training for exposed workers.
Frequently Asked Questions
Are TLVs legally enforceable?
Not on their own. TLVs are ACGIH guidelines. They become enforceable only when a regulation, contract or company policy adopts them. OSHA enforces its own PELs.
Why are TLVs often lower than OSHA PELs?
ACGIH updates TLVs every year based on current health research, while most OSHA PELs have not been updated since 1971. TLVs also consider health effects only, not cost or feasibility.
What does TLV-STEL mean?
It is a 15-minute time-weighted average exposure that should not be exceeded at any time during the workday, even if the 8-hour average is below the TLV-TWA.
How often are TLVs updated?
ACGIH publishes a new edition of the TLVs and BEIs every year, after proposed changes go through a Notice of Intended Changes period.
