OSHA Permissible Exposure Limits: How Table Z and the PEL Math Work (2026 Guide)

How do OSHA permissible exposure limits actually work?

Short answer: OSHA’s exposure limits sit in three tables that behave differently — Z-1 is mostly 8-hour averages with ceilings flagged by a leading C, Z-2 adds an acceptable ceiling and a permitted peak above it, and Z-3 covers mineral dusts. Whether a measured exposure is over the limit is decided by two formulae in 1910.1000(d): one that averages a shift, and one that adds up a mixture. Both are printed in the regulation, and both are worked through below.

Section 1910.1000 is the backbone of industrial hygiene enforcement in general industry, and it is short — the rule itself runs to a few paragraphs, with the bulk of its length taken up by the tables. What it does not do is explain itself. The three tables are presented side by side without saying that they operate on different principles, and the two formulae are given with worked examples that most summaries drop.

Why this matters.
The arithmetic decides the citation. An employee can spend two hours of a shift well above the limit and still be compliant on an 8-hour average, and three chemicals each at 40 per cent of their own limit are individually fine and jointly a violation. Getting the wrong answer in the first direction means buying controls you did not owe; getting it wrong in the second means an overexposure nobody measured. Exposure is also assessed without regard to respirator use, so issuing respirators does not move the number that decides whether the standard applies.

The three tables and what each one measures

Formula one: averaging a shift

From 1910.1000(d)(1)(i), for a single substance measured at different concentrations across a shift:

E = (C₁T₁ + C₂T₂ + … CₙTₙ) ÷ 8

E is the equivalent exposure for the shift, C is the concentration during any period in which it stays constant, and T is the duration of that period in hours. The divisor is 8 regardless of how long the shift ran.

The regulation’s own worked example, for a substance with a 100 ppm 8-hour TWA limit:

81.25 ppm is under the 100 ppm limit, so the exposure is acceptable — despite two hours spent half again above the limit. That is the correct result under the rule, and it is also why a ceiling value exists for substances where a brief high excursion is the actual hazard.

Formula two: adding up a mixture

From 1910.1000(d)(2)(i), where an employee is exposed to more than one substance at the same time:

Eₜ = C₁÷L₁ + C₂÷L₂ + … Cₙ÷Lₙ
Eₜ must not exceed 1

Each contaminant contributes the fraction of its own limit that it occupies. OSHA’s worked example uses three substances that are each comfortably compliant on their own:

Push substance B to 700 ppm and the sum passes 1 while every individual reading is still well inside its own PEL. Mixed-solvent work, paint spraying and degreasing are where this bites, and it is the calculation most often skipped.

Ceiling, peak, and the fifteen-minute concession

Three limit types get conflated constantly, and the distinctions are in the text:

OSHA’s illustration of the peak column: a substance with a 10 ppm TWA, a 25 ppm ceiling and a 50 ppm peak may go above 25 ppm — but never above 50 ppm — for a maximum of 10 minutes, and the rest of the shift must be low enough that the whole-shift average still lands at 10 ppm.

Why the numbers are old, and what to design to instead

Most Table Z-1 limits were adopted wholesale in 1971 from consensus standards that were themselves older. OSHA tried to modernise the lot in 1989; the Air Contaminants rule was vacated by the Eleventh Circuit in 1992 and the 1971 values snapped back. Since then the agency has updated substances one expanded standard at a time, which is why silica, beryllium and hexavalent chromium have modern limits and several hundred other substances do not.

The practical consequence for anyone buying protective equipment: designing to the PEL is the legal floor, and designing to the current ACGIH TLV is what a competent programme does where the two diverge sharply. That choice, and its effect on assigned protection factor and cartridge selection, is worked through in ACGIH TLV vs OSHA PEL Respirator Selection and when does OSHA require a respirator.

Frequently asked questions

What is a permissible exposure limit (PEL)?

The maximum airborne concentration of a substance an employee may be exposed to, set by OSHA and enforceable as law. Most sit in the three tables of 1910.1000; about thirty substances have their own expanded standard elsewhere in Subpart Z with additional programme requirements. A PEL is a legal ceiling on exposure, not a line between safe and unsafe.

What is the difference between Table Z-1, Z-2 and Z-3?

They work differently, which is the single most common source of error. Table Z-1 lists 8-hour TWA limits, with a leading “C” marking the entries that are ceilings instead. Table Z-2 gives each substance up to three numbers — an 8-hour TWA, an acceptable ceiling, and an acceptable maximum peak above that ceiling with a stated duration. Table Z-3 covers mineral dusts and is 8-hour TWA only.

How do you calculate an 8-hour time weighted average?

Using the formula in 1910.1000(d)(1)(i): E = (C₁T₁ + C₂T₂ + … CₙTₙ) ÷ 8, where C is the concentration during a period of constant exposure and T is the length of that period in hours. The divisor is always 8, even on a longer or shorter shift — that is what makes it a time weighted average rather than a mean.

What is OSHA's worked example of a TWA calculation?

The regulation supplies one. For a substance with a 100 ppm 8-hour TWA limit, an employee exposed for two hours at 150 ppm, two hours at 75 ppm and four hours at 50 ppm gives (2×150 + 2×75 + 4×50) ÷ 8 = 81.25 ppm. Because 81.25 is below 100, the exposure is acceptable — even though the worker spent two hours above the limit.

How do you handle exposure to more than one chemical at once?

With the mixture formula in 1910.1000(d)(2)(i): Eₜ = C₁÷L₁ + C₂÷L₂ + … Cₙ÷Lₙ, where C is each contaminant's concentration and L is its own limit. The result must not exceed 1. Three substances each at 40% of their individual PEL are individually compliant and jointly over the limit, which is exactly the trap the formula exists to catch.

What does a “C” in front of a PEL mean?

It marks a ceiling value: the exposure must not exceed that figure at any time, not merely on average. Where instantaneous monitoring is not feasible, 1910.1000(a)(1) allows the ceiling to be assessed as a 15-minute time weighted average — which is not the same thing as a STEL, and is a concession about measurement rather than a relaxation of the limit.

What is the “skin” designation in Table Z-1?

A flag that the substance can contribute materially to overall exposure by absorption through intact skin, mucous membranes or the eyes. It carries no number. Its practical meaning is that airborne sampling alone will understate the dose, and that chemical resistant glove selection and skin protection matter as much as the respirator.

Is a PEL the same as an ACGIH TLV?

No. A PEL is enforceable federal law; a TLV is a voluntary recommendation published by ACGIH and revised annually. Most Table Z-1 limits were adopted in 1971 from consensus standards that were already older, and for many substances the current TLV is several times lower. The site’s ACGIH TLV vs OSHA PEL Respirator Selection guide covers which to design a respirator programme around.

Why are OSHA's PELs so out of date?

Because changing one requires full notice-and-comment rulemaking with a substance-by-substance feasibility showing. OSHA attempted a bulk update in 1989 and the Air Contaminants rule was vacated in 1992 by the Eleventh Circuit in AFL-CIO v. OSHA, restoring the 1971 limits. The agency has since updated individual substances one standard at a time.

Can OSHA cite an employer when exposure is below the PEL?

Yes, in two ways. Where a substance has its own expanded standard, ancillary duties such as monitoring, training and medical surveillance are triggered at the action level, typically half the PEL, not at the PEL. And where a recognised hazard exists with no applicable standard, OSHA can proceed under the General Duty Clause — the route it has used for exposures compliant with an obsolete PEL but far above the current TLV.

What is an action level and how does it differ from a PEL?

An action level is a lower trigger, usually half the PEL, that switches on a standard's programme requirements before anyone is over the legal limit — periodic monitoring, medical surveillance, training. It is defined without regard to respirator use, so putting workers in respirators does not lower the measured exposure for the purpose of deciding whether the programme applies.

Do respirators count when measuring exposure against a PEL?

Not for the purpose of triggering the standard. Exposure is assessed as if the respirator were not being worn. Respirators reduce the dose the worker actually receives, and that is their point, but 1910.1000(e) makes clear that feasible engineering and administrative controls come first and respirators are what is left. Selection then runs through OSHA 29 CFR 1910.134 Respiratory Protection Standard.

Which substances have their own standard instead of a Table Z entry?

Roughly thirty, each with its own section in Subpart Z: asbestos, lead, cadmium, benzene, formaldehyde, hexavalent chromium, respirable crystalline silica, beryllium, methylene chloride and others. Those sections override the Table Z entry and add monitoring, regulated areas, medical surveillance and written programmes. Every one of them is indexed in the OSHA standards index.

Does 1910.1000 apply to construction?

No — construction has its own air contaminants rule at 1926.55, which works the same way against its own tables. The two are not identical, and a handful of substances differ. Silica is the clearest divergence: general industry uses 1910.1053 and construction uses 1926.1153 with its Table 1 control list.

What is the difference between a ceiling and a STEL?

A ceiling must never be exceeded at any moment. A short-term exposure limit is a 15-minute time weighted average that may be exceeded momentarily so long as the 15-minute average holds, usually with limits on how often and how far apart the excursions occur. Table Z-1 and Z-2 use ceilings and peaks; STELs mostly appear in the substance-specific standards.

How is a Table Z-2 peak different from a ceiling?

A Z-2 peak is a permitted, time-limited excursion above the acceptable ceiling. OSHA's own illustration: a substance with a 10 ppm TWA, 25 ppm ceiling and 50 ppm peak may go above 25 ppm — but never above 50 ppm — for at most 10 minutes, and the rest of the shift must be low enough that the 8-hour average still comes in at 10 ppm.

What units are PELs expressed in?

Parts per million (ppm) for gases and vapours, milligrams per cubic metre (mg/m³) for particulates and some vapours, and fibres per cubic centimetre for asbestos and similar fibres. Table Z-1 lists both ppm and mg/m³ for many substances; the ppm figure governs where both are given, since the mg/m³ conversion depends on temperature and pressure.

Where can I find the current PEL for a specific chemical?

Start with the safety data sheet, section 8, which lists the applicable occupational exposure limits — the site’s how to read a safety data sheet guide covers that section. Then confirm against the table itself at 1910.1000, because SDS authors sometimes list only the TLV, and check whether the substance has an expanded standard that supersedes the table entry.

Further reading on this site

• ACGIH TLV vs OSHA PEL Respirator Selection — which limit to build a respirator programme around when the two disagree.
• how to read a safety data sheet — section 8 is where the exposure limits for a specific product live.
• OSHA HazCom program requirements — the labelling and SDS duties that put those numbers in front of workers.
• OSHA carbon monoxide monitoring requirements — a single-substance worked example of the same limit structure.
• welding fume exposure limits — a mixture problem in practice, where the formula above does real work.
• best 4-gas monitor — instruments that measure against these limits in real time.
• ppm vs %LEL vs %Volume gas readings — the units a detector reports, and how they map onto a PEL expressed in ppm.
• OSHA standards index — every 29 CFR section indexed, including the expanded Subpart Z standards that supersede Table Z.