OSHA PEL vs ACGIH TLV: what the gap means for you

This page explains what the rules and common practice call for. Every site is different, and what a survey or a filing needs at your facility is something we work out with you after we see it.

Most OSHA permissible exposure limits (PELs) date from 1971. OSHA adopted them in bulk from voluntary standards that were already in use before then, so a large part of the enforceable US limits reflects the science of the 1960s.

The voluntary limits kept moving, while the enforceable ones mostly did not.

The three limits you will see

The PEL. OSHA sets the permissible exposure limit, and it is legally enforceable. For several hundred substances it is set in OSHA's general air contaminant tables, known as Table Z. For a couple of dozen others it is set in a substance-specific standard.

The TLV. The threshold limit value comes from ACGIH, a professional body of occupational health scientists. It is not enforceable. ACGIH reviews and updates its limits continually, and they are generally regarded as reflecting current occupational health science.

The REL. The recommended exposure limit comes from NIOSH. It is not enforceable either, and it often sits between the other two.

Where the gap is widest

Manganese is the clearest case, and the one most likely to affect a fabrication or manufacturing plant.

Manganese. The OSHA PEL for manganese fume is 5 milligrams per cubic meter of air (mg/m³), and it is a ceiling limit on total particulate, set in 1971. A ceiling limit must not be exceeded at any time. The current ACGIH TLV is 0.02 mg/m³, as an 8-hour time-weighted average (TWA) of the respirable fraction, which is the fraction fine enough to reach deep into the lungs. ACGIH adopted it in 2013, down from 0.2 mg/m³ the year before.

That ratio is 250 to 1.

Those two numbers measure different things: an instantaneous ceiling on total particulate against an eight-hour average of the respirable fraction. The closer, like-for-like comparison uses ACGIH's TLV for the inhalable fraction (all the particles that can be inhaled), which is 0.1 mg/m³ against OSHA's 5 mg/m³. That is still 50-fold. NIOSH's REL sits between them, at 1 mg/m³ as a shift average with a 3 mg/m³ short-term exposure limit (STEL), and that is well below OSHA's number.

However you frame the comparison, the conclusion holds. ACGIH lowered its limit because of harm to the nervous system. The concern is a Parkinson's-like illness caused by manganese, and the research behind it is not thin.

→ Welding fume exposure monitoring

What meeting the PEL does and does not settle

The General Duty Clause is narrower here than people often assume. The clause, Section 5(a)(1) of the OSH Act, requires employers to provide work free from recognized hazards likely to cause death or serious physical harm. Where a PEL applies and you meet it, that clause is largely closed off.

OSHA's Field Operations Manual (Chapter 4) says the clause "shall normally not be used to impose a stricter requirement than that imposed by the OSHA standard."

A separate OSHA regulation adds that compliance with a specific standard counts as compliance with the General Duty Clause for the condition that standard covers.

A General Duty citation below the PEL is possible but rare. It requires evidence of actual employer knowledge that the standard was inadequate to protect workers from death or serious physical harm. It must also be reviewed before it is issued, by OSHA's Directorate of Enforcement Programs and the Office of the Solicitor. The published gap between a PEL and a current recommended limit is not, by itself, enough to support one.

So the case for looking past the PEL is not mainly about enforcement.

Lawsuits. Workers' compensation is generally the only remedy a worker has against an employer, but that does not shield everyone in the chain. In court, "we complied with a 1971 limit" is a poor answer when the health-based limit was revised in 2013 and published in a standard reference your industry uses.

It costs nothing to know. This is the practical point. The sample is the same sample and the laboratory runs the same analysis. What differs is which limit you compare the result to. Checking against both limits costs nothing extra and tells you a great deal more about where you actually stand.

What we do about it

A report has to compare your results to the PEL, because it determines your legal obligations and your monitoring frequency. Where there is a meaningful gap, ask for a comparison against the current recommended limits as well, flagged as informational and not as legal requirements.

You should know both, because confusing them is how people end up either complacent or alarmed for no reason.

Some substances have no PEL at all. The list is growing, and it includes several widely used isocyanates. For those, the recommended limit is doing all the work, and there is no "compliant" number to hit. The obligation comes from the General Duty Clause and, if respirators are in use, from OSHA's respiratory protection standard.

→ Isocyanate air sampling · → OSHA exposure monitoring frequency by substance