ACGIH Threshold Limit Values
What are ACGIH threshold limit values?
Threshold Limit Values (TLVs®) are airborne workplace exposure guidelines for chemical substances and physical agents, published by the American Conference of Governmental Industrial Hygienists (ACGIH). They are guidelines, not legal standards. ACGIH describes a TLV as the opinion of the scientific community that reviewed the supporting Documentation that exposure at or below the TLV "does not create an unreasonable risk of disease or injury" (ACGIH TLV/BEI Guidelines).
Approach: I checked the OSHA Occupational Chemical Database, the OSHA Annotated PEL Tables, the CCOHS OSH Answers chemical profiles and the New Jersey Right to Know fact sheets. I also read ACGIH's own TLV/BEI guidelines page. The TLV booklet itself is not openly published, so I didn't read it.
Key points
- Who publishes them: ACGIH publishes TLVs as guidelines for exposure to workplace chemicals [1]. They come out each year as the TLVs® and BEIs® book, with a separate Documentation volume giving the basis for each value [2][3].
- Legal status: ACGIH says "TLVs and BEIs are not standards." It calls them guidelines for industrial hygienists deciding on safe exposure levels, and only "one of multiple factors" to weigh when evaluating a workplace (ACGIH).
- Types of value:
- Related notations:
- Example, benzene: the TLV is 0.5 ppm as an 8-hour TWA and 2.5 ppm as a STEL. For comparison, the legal OSHA PEL is 1 ppm TWA and 5 ppm STEL, and the NIOSH REL is 0.1 ppm (10-hour TWA) and 1 ppm STEL [5].
- Where they carry legal weight:
- US: TLVs are not the federal standard; OSHA's enforceable limits are the PELs in 29 CFR 1910.1000. OSHA's annotated PEL tables do link each substance to its ACGIH TLV page [6].
- Canada: CCOHS notes that in many, but not all, Canadian jurisdictions the legal exposure limits are similar to the ACGIH TLVs, and that legislation varies by jurisdiction [2].
- Air levels only: TLVs cover airborne concentrations. A worker with skin contact can be overexposed even when air levels are below the limit [5].
I can look up the current TLV or the national limits for a specific substance if that would help.
How do they differ from OSHA exposure limits?
The main difference is legal force. OSHA's permissible exposure limits (PELs) are enforceable federal limits under 29 CFR 1910.1000, while ACGIH TLVs® are health-based guidelines that ACGIH says are "not intended to be used as legal standards." Most PELs also date from shortly after the OSH Act of 1970 and have not been updated. OSHA itself says many are "outdated and inadequate for ensuring protection of worker health," so the TLV is often much lower (OSHA Annotated PELs).
Approach: I searched OSHA's annotated tables of the 29 CFR 1910.1000 limits and the New Jersey Right to Know fact sheets, which list the OSHA, NIOSH and ACGIH values side by side. I also read OSHA's introduction to its annotated PEL tables.
Key differences
| OSHA PEL | ACGIH TLV | |
|---|---|---|
| Legal status | The fact sheets call the PEL "the legal airborne permissible exposure limit" [7] | Guideline only; ACGIH says TLVs are "not intended to be used as legal standards" (OSHA) |
| Basis | Set through federal rulemaking | "Based solely on health factors, there is no consideration given to economic or technical feasibility" (OSHA) |
| Currency | Most were issued shortly after the 1970 Act "and have not been updated since that time" (OSHA) | Reviewed and republished every year (see previous answer) |
| Where you find them | Tables Z-1, Z-2 and Z-3 of 29 CFR 1910.1000; some substances have their own standard instead | The annual ACGIH book. OSHA links to ACGIH's page for each substance but does not print the values, because ACGIH's permission is needed to reproduce them us_osha_annotated_pels_tables:us_osha_annotated_pels_tables__z1__003:55c900da99db:p0 |
How far apart the numbers are (8-hour TWA unless noted)
| Substance | OSHA PEL | ACGIH TLV |
|---|---|---|
| Acetone | 1,000 ppm | 500 ppm; 750 ppm STEL [7] |
| Toluene | 200 ppm; 300 ppm not to be exceeded in any 15-minute period; 500 ppm 10-minute peak | 20 ppm [8] |
| Hydrogen sulfide | 20 ppm ceiling (not to be exceeded at any time); 50 ppm 10-minute peak | 1 ppm; 5 ppm STEL [9] |
| Benzene | 1 ppm; 5 ppm STEL (set by its own standard, 1910.1028) | 0.5 ppm; 2.5 ppm STEL [5][10] |
Other practical points
- Different kinds of short-term limit: some PELs, such as those for toluene and hydrogen sulfide, use the older ceiling and "acceptable maximum peak" format from Table Z-2. The TLVs for the same substances use TWA and STEL values [11][9].
- OSHA points to the alternatives: OSHA prints the Cal/OSHA PEL, the NIOSH REL and a link to the ACGIH TLV next to each federal PEL, as "alternate occupational exposure limits that may serve to better protect workers" (OSHA).
- States can be stricter: California's limits can be well below the federal ones. For acetone, Cal/OSHA sets 500 ppm, with 750 ppm STEL and 3,000 ppm ceiling, against the federal 1,000 ppm [12].
I can build a PEL, Cal/OSHA, NIOSH and TLV comparison for your own list of substances if that would help.
Which exposure limit should employers use?
Employers must comply with the limit that is legally binding where the work happens. In US federal OSHA jurisdiction, that is the OSHA PEL in 29 CFR 1910.1000 or, for substances such as benzene, the PEL in the substance's own standard. OSHA also says many PELs are outdated and recommends that employers consider the alternative limits (Cal/OSHA PEL, NIOSH REL, ACGIH TLV). So in practice you meet the PEL as the legal minimum and use the lower guideline as your internal control target.
Approach: I checked the OSHA Annotated PEL Tables, the OSHA Occupational Chemical Database, the New Jersey Right to Know fact sheets and the CCOHS OSH Answers profiles. I also read three OSHA pages: its introduction to the annotated PELs, a 2003 interpretation letter, and Chapter 4 of the Field Operations Manual.
1. The legal minimum
- Federal OSHA: OSHA states that "OSHA's mandatory PELs in the Z-Tables remain in effect" (OSHA Annotated PELs). The New Jersey fact sheets call the OSHA value "the legal airborne permissible exposure limit (PEL)" [5].
- Substance-specific standards come first: for benzene, the Z-1 entry says "See 1910.1028". Table Z-2 limits apply only in the operations or sectors excluded under 1910.1028(j) [10]. Those standards also impose duties beyond the number itself. For benzene, OSHA requires specific engineering controls and a regulated, marked area [13][14].
- California: Cal/OSHA sets its own PELs, and they can differ from the federal ones. For benzene, Cal/OSHA refers to Title 8, Section 5218, and its limit carries a skin notation that the federal PEL does not [3].
- Canada: the binding limit is the one set by the provincial or federal jurisdiction. CCOHS notes that in many, but not all, jurisdictions these are similar to the ACGIH TLVs, and that legislation varies [15].
2. Why the PEL alone may not be enough
- OSHA's own view: OSHA "recognizes that many of its permissible exposure limits (PELs) are outdated." It "recommends that employers consider using the alternative occupational" exposure limits printed beside each PEL (OSHA).
- General Duty Clause, Section 5(a)(1): every employer "shall furnish ... a place of employment which [is] free from recognized hazards." The Field Operations Manual says 5(a)(1) "Shall Normally Not Be Used When a Standard Applies to a Hazard." OSHA can still cite an exposure that is below the PEL but above a TLV or REL if both of these hold (OSHA FOM, Ch. 4):
- data show that exposure at that level is likely to cause death or serious physical harm; and
- the employer actually knows the PEL is inadequate.
- Substances with no PEL: OSHA may use 5(a)(1) "when a serious hazard is observed." A TLV or REL only serves to "buttress other evidence that a hazard exists"; it is not enforced "in and of [itself]" (OSHA interpretation, 24 January 2003).
- Carcinogens: for benzene, NIOSH says to "reduce exposure to lowest feasible concentration" [3]. New Jersey notes there "may be no safe level of exposure to a carcinogen" [5].
3. A working hierarchy
- Find the binding limit for the jurisdiction: federal PEL, substance-specific standard, state-plan or provincial limit.
- Compare it with the current TLV and REL. Where those are lower, use the lowest credible value as the internal target for exposure monitoring, engineering controls and respirator selection. This is the course OSHA recommends; it is not a legal requirement.
- Keep records showing why you chose that target. Once you know a PEL is inadequate, that knowledge is what can support a 5(a)(1) case.
- Air limits don't cover skin uptake. Exposure can be too high even when air levels are under the limits [5].
A correction to my earlier benzene figures: I gave the benzene TLV as 0.5 ppm TWA and 2.5 ppm STEL. That came from the New Jersey fact sheet, last revised July 2015 [5]. The CCOHS profile, adapted from the 2024 TLVs® and BEIs® book, gives a TWA of 0.02 ppm, with Skin, A1 and BEI notations [15]. Take TLVs from the current ACGIH edition, not from older secondary sheets.