Manganese: three limits, and why the OSHA ceiling is the weakest

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.

OSHA has no 8-hour time-weighted average (TWA) limit for manganese. The permissible exposure limit (PEL) is instead a ceiling limit of 5 milligrams per cubic meter of air (mg/m³), a level that must not be exceeded at any time. It applies to total particulate, meaning all airborne dust and fume with no particle-size selection. It is listed in Table Z-1, OSHA's general table of air contaminant limits, and has been carried forward from 1971.

The structure of the limit matters more than the number. A welding shop can run 8 hours a day at levels every current health-based limit treats as too high and never once exceed the OSHA ceiling, because the ceiling does not measure the thing that causes the harm.

That harm is neurological: manganism produces symptoms like those of Parkinson's disease, and they do not reverse. The damage is slow and cumulative, and a ceiling on a single moment is a poor tool for a hazard that builds up over time.

Manganese appears only in Table Z-1, and OSHA has no substance-specific standard for it, so there is no action level and no required monitoring schedule. That is not the same as having no obligation.

Where we find it

• Nearly all steel welding. Manganese is in the filler metal, not only the base metal, so mild steel work is not exempt. Flux-cored and stick welding generally produce more fume per hour of arc time than MIG or TIG welding (GMAW or GTAW).
• Welding inside enclosures, tanks and vessels. The same welding wire or rod, used for the same share of the shift, puts much more fume into the welder's breathing zone.
• Thermal cutting and gouging: plasma, carbon arc and oxyfuel.
• Steel foundries: melting, pouring and manganese ferroalloy additions.
• Ferroalloy production and handling: crushing, screening and bagging ferromanganese and silicomanganese.
• Grinding and abrasive finishing on manganese steel wear parts, such as crusher liners, rail frogs and dredge parts.
• Dry battery manufacturing and manganese dioxide handling.

Exposure limits

Two other organizations publish manganese limits, and both are recommendations. NIOSH sets a recommended exposure limit (REL). ACGIH is the professional body that publishes the threshold limit values (TLVs).

The gap, stated honestly

You will often see the gap quoted as 250 to 1, which sets OSHA's 5 mg/m³ against ACGIH's 0.02 mg/m³. The arithmetic is right, but the comparison needs a caveat.

Those two numbers do not measure the same thing. One is a ceiling on total particulate at any single moment, set in 1971. The other is an 8-hour TWA of the respirable fraction, the particles fine enough to reach deep into the lungs. Comparing the two directly is like comparing a speed limit to a fuel economy figure.

So take the closest match available. ACGIH also has an inhalable TLV of 0.1 mg/m³, which covers all the particles a person can breathe in through the nose or mouth. Set that against OSHA's 5 mg/m³ and the gap is still 50-fold.

The NIOSH REL sits between them, at 1 mg/m³ as a full-shift TWA. That is 5 times below the OSHA number and 10 times above the ACGIH inhalable value.

However you frame it, the conclusion holds: the federal limit that OSHA can enforce is far above what every current health-based limit treats as protective. The gap is not small, and two practical consequences follow.

It is the clearest General Duty Clause risk in a weld shop. Under the General Duty Clause, OSHA can cite a recognized hazard that has a feasible means of abatement, even where no PEL is exceeded. Here NIOSH and ACGIH both agree on a documented gap of this size, which is about as strong a record of a “recognized hazard” as exists.

It also has to be planned before the site visit. Comparing a result to the ACGIH TLVs takes more than changing the number in the report.

• The 0.02 mg/m³ TLV is a respirable value, which requires a cyclone ahead of the filter so that only the respirable fraction is collected.
• The 0.1 mg/m³ TLV is an inhalable value, which requires an inhalable sampler such as an IOM, Button or GSP head. A closed-face cassette (a closed filter holder) will not do.

Choosing which limits you want to be evaluated against is therefore also a choice of sampling equipment, and that choice is made before anyone arrives on site.

→ OSHA PELs compared with ACGIH TLVs

How we sample for it

OSHA method ID-125G allows the 25-mm cassette where space is tight. That is what makes sampling inside the welding helmet practical. A 37-mm cassette clipped inside a helmet is bulky, and welders move it, tilt it, or route the tubing in a way that changes what it collects.

The 25-mm cassette is the difference between a sample the welder tolerates for 8 hours and one you have to void. It is a permitted option, not the preferred one, and outside the helmet 37 mm is the normal choice.

Sampling inside the helmet and sampling outside it answer different questions. An outside sample shows the exposure in the workplace, while an inside sample shows what the welder actually breathes and is the right measurement when you are evaluating a powered air-purifying respirator. A report should state which one was done.

Gravimetric dust (dust by weight) is a separate sample. Manganese is collected on a 0.8-µm mixed cellulose ester filter and analyzed by ICP under method ID-125G.

Total or respirable dust by weight runs under a different OSHA method, PV2121, which uses a pre-weighed, low-ash 37-mm PVC filter. Total dust is sampled with no cyclone, while respirable dust is sampled behind a 10-mm nylon Dorr-Oliver cyclone at 1.7 L/min.

A PVC filter used for gravimetric analysis does not give a manganese result, and a cellulose ester filter analyzed for metals does not give a weight.

The same logic applies to particle size. The OSHA ceiling is for manganese in total particulate, which is collected on a closed-face cassette with no size selection. The 0.02 mg/m³ ACGIH TLV is for manganese in the respirable fraction, which requires a cyclone ahead of the filter. A cyclone removes the very material the total figure is meant to include.

To evaluate one welder against both limits, you need two sampling trains on one person, because one cassette cannot give both.

→ Welding fume air monitoring

What you do with a manganese result

There is no deadline for notifying workers of their results, because no manganese standard exists to set one. What you do get is this:

• A written record of what your workers are breathing.
• A defensible basis for respirator selection under OSHA's respiratory protection standard.
• Where the result lands between the OSHA ceiling and the health-based limits, a decision that you now get to make deliberately and not by default.

A survey planned for it can show you where you stand against all three limits. What you do about the gap is a business decision, and it should be made with the numbers in front of you, not avoided by declining to collect them.