Which Occupational Exposure Limit to Use: PEL, TLV or REL?

This answer covers airborne limits for chemical substances judged against personal air samples. It does not cover biological monitoring values, skin contact on its own, flammability limits such as the lower explosive limit, or community air quality limits. Where a substance has its own standard, such as benzene under OSHA, that standard's limit and rules apply first.

Which conditions decide which exposure limit applies?

Four conditions decide which occupational exposure limit (OEL) a monitoring result is judged against: your purpose, the jurisdiction, whether the substance has its own standard, and the averaging period of the sample. Purpose comes first. A legal compliance check uses the limit a regulator enforces where the work happens, while an internal control target can use any current limit your company chooses to adopt. Jurisdiction then fixes the enforced list. Federal OSHA enforces PELs in 29 CFR 1910.1000 for general industry and adopted comparable PELs for shipyards in 29 CFR 1915.1000 and construction in 29 CFR 1926.55 (OSHA Annotated PELs, accessed 2026). California applies its own Table AC-1 at all places of employment in the state (Cal/OSHA, 8 CCR 5155(a)(1), as amended 2024). A substance-specific standard takes precedence over the general tables: the OSHA Table Z-1 entry for benzene points to 29 CFR 1910.1028 and leaves Table Z-2 only for the sectors that standard excludes (OSHA Annotated Table Z-1, accessed 2026). Finally, the averaging period must match. An OSHA Table Z-1 limit without a "C" is an 8-hour time-weighted average (TWA) for any 8-hour shift of a 40-hour week, and a limit marked "C" is a ceiling that must not be exceeded at any time (29 CFR 1910.1000(a)). A short task sample alone does not describe a full shift, so it cannot show compliance with an 8-hour TWA.

Which limit types fit which purpose?

Binding exposure limits fit legal compliance checks, and recommended exposure limits fit internal control targets and early warning. The binding limits are the OSHA PEL, the Cal/OSHA PEL inside California, the national limits an EU member state makes binding and the Great Britain workplace exposure limit (WEL). Recommended limits carry no legal force on their own. The National Institute for Occupational Safety and Health (NIOSH) recommended exposure limit (REL) is a federal agency recommendation that NIOSH sends to OSHA for use in developing enforceable standards (OSHA Annotated PELs, accessed 2026). A NIOSH REL TWA covers up to a 10-hour workday during a 40-hour workweek (NIOSH Pocket Guide, accessed 2026). The Threshold Limit Value (TLV) comes from ACGIH, which OSHA describes as a private, not-for-profit, nongovernmental corporation that is not a standards setting body, and OSHA states that TLVs "are health-based values and are not intended to be used as legal standards" (OSHA Annotated PELs, accessed 2026). ACGIH itself says the TLVs are "not developed for use as legal standards" (ACGIH TLV/BEI Policy Statement, 1988). OSHA still recommends that employers consider these alternative limits, because exposures above some of them "may be hazardous to workers, even when the exposure levels are in compliance with the relevant PELs" (OSHA Annotated PELs, accessed 2026). For an internal target, that points to the lowest current value among the binding limit, the REL and the TLV.

Which exposure limit fits your purpose and jurisdiction?

The exposure limit that fits is the enforced list of the jurisdiction where the work happens for a compliance check, and the lowest current value, binding or recommended, for an internal target. The table below decides which published limit a monitoring result is judged against:

The row that most often changes the verdict is California: a site there checks compliance against the Cal/OSHA PEL in Table AC-1, not the federal Table Z-1, because Section 5155 applies at all places of employment in the state (8 CCR 5155(a)(1), as amended 2024). Other states that run OSHA-approved State Plans can set their own limits, and OSHA notes that California has the most extensive list among them (OSHA Annotated PELs, accessed 2026), so check your state plan's list before you use the federal table. In the EU, the member state list is the one to use because, under the Chemical Agents Directive 98/24/EC, each member state sets a national binding limit "based on, but not exceeding, the Community limit value" wherever an EU binding value exists (Directive 98/24/EC, Article 3(5)). For carcinogens, mutagens and reprotoxic substances, the EU limit values are set out in Annex III of Directive 2004/37/EC (Directive 2004/37/EC, Article 16, consolidated 2024). In Great Britain, COSHH imposes requirements by reference to Table 1 of EH40/2005, which makes that table legally binding (HSE EH40/2005, 4th ed. 2020). For other countries, use the national list of the country where the work happens; this table does not cover them.

Why do the OSHA PEL and the ACGIH TLV differ for the same chemical?

The OSHA PEL and the ACGIH TLV differ for the same chemical mainly because most PELs trace back to the 1968 TLVs, while TLVs are revised on health data alone. OSHA states that most Z-table PELs were adopted from the Walsh-Healey Public Contracts Act standards, which had been adopted from the 1968 ACGIH TLVs, and that most PELs have not been updated since shortly after the OSH Act of 1970 (OSHA Annotated PELs, accessed 2026). An update was attempted and reversed: in 1992 the 11th Circuit Court of Appeals vacated more protective PELs set in 1989 for 212 substances, moving them back to PELs established in 1971 (NIOSH Pocket Guide, accessed 2026). TLVs are based solely on health factors, with no consideration of economic or technical feasibility, and ACGIH does not believe they should become standards without that wider analysis (OSHA Annotated PELs, accessed 2026). Averaging periods also differ: the NIOSH REL TWA runs up to 10 hours against 8 hours for the OSHA PEL. The table below compares published full-shift limits for two common solvents:

For toluene, the federal PEL of 200 ppm is 20 times the Cal/OSHA PEL of 10 ppm (29 CFR 1910.1000 Table Z-2; 8 CCR 5155, as amended 2024), so an illustrative 8-hour result of 50 ppm passes the federal test and fails in California. OSHA notes that the TLVs are copyrighted and not publicly available, so read the current TLV from the SDS or the ACGIH book (OSHA Annotated PELs, accessed 2026).

Common mistakes when comparing monitoring results with exposure limits

The most common mistake when comparing monitoring results with exposure limits is comparing numbers that do not describe the same thing. A full-shift average is not a ceiling check: where instantaneous monitoring is not feasible, OSHA assesses a ceiling as a 15-minute TWA that must not be exceeded at any time in the working day (29 CFR 1910.1000(a)(1)). A second mistake is treating a result below the PEL as proof of safety, which OSHA itself warns against when it calls many PELs "outdated and inadequate for ensuring protection of worker health" (OSHA Annotated PELs, accessed 2026). A third is judging each solvent in a mixture separately. For a mixture, OSHA requires the sum of each concentration divided by its own limit to stay at or below 1; the worked example in the rule gives 0.925, which is within limits (29 CFR 1910.1000(d)(2)). A fourth is missing skin absorption: OSHA's annotated tables list air concentrations only and do not include notations for skin absorption or sensitization (OSHA Annotated PELs, accessed 2026), so check the skin designation column in Table Z-1 of 29 CFR 1910.1000 or the "S" notation in the Cal/OSHA table (8 CCR 5155(a)(1)). A fifth is reading an old copy of a list. The Cal/OSHA values printed in OSHA's annotated tables are dated 2021 (OSHA Annotated Table Z-1, accessed 2026), and Table AC-1 was amended again in 2024 (8 CCR 5155, history note 59). Check the date on any list before using it.

When should you call an industrial hygienist about exposure limits?

Call an industrial hygienist about exposure limits when the choice of limit, the sampling plan or the verdict needs professional judgment rather than a table lookup. Typical triggers are a result close enough to the limit that sampling error could change the verdict, a mixture of several substances, shifts longer than 8 hours, a substance with no published limit, and a carcinogen. ACGIH states that TLVs are "to be interpreted and applied only by a person trained in this discipline" (ACGIH TLV/BEI Policy Statement, 1988). Where engineering or administrative controls cannot keep exposure within the PELs, OSHA requires that any protective equipment or technical measures used instead be approved for each use "by a competent industrial hygienist or other technically qualified person" (29 CFR 1910.1000(e)). Carcinogens raise the bar further. In Great Britain, COSHH counts control as adequate only if any WEL is not exceeded and exposure to asthmagens, carcinogens and mutagens is reduced to as low as is reasonably practicable (HSE EH40/2005, 4th ed. 2020). Before the call, Lubinpla can organize the conditions from your own SDSs and site history and return one cited document for the hygienist to review, with your approval at each step. Bring the SDS, the sampling records and the shift pattern to the first meeting.