General dust (PNOR): when the catch-all limit applies

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's general dust limits do not apply where a specific regulated substance is in the dust.

That sentence is the whole page. OSHA's catch-all category for dust is “particulates not otherwise regulated” (PNOR), which is listed in Table Z-1, its general table of air contaminant limits. On this page we call it general dust.

The permissible exposure limits (PELs) for general dust are 15 milligrams per cubic meter of air (mg/m³) for total dust and 5 mg/m³ for respirable dust, the fine fraction that reaches deep into the lungs. These limits cover only dusts that no other standard covers.

If the dust contains crystalline silica, asbestos, lead, cadmium, beryllium or a metal listed in OSHA's limit tables, that substance's own limit applies and the general dust number tells you nothing you can act on.

Suppose someone samples a foundry sand operation as “nuisance dust” and reports 4 mg/m³ against a 5 mg/m³ limit. That result looks comfortable and means nothing, because the silica in that dust has a PEL of 50 micrograms per cubic meter (µg/m³).

Compare the two on the same scale. 5 mg/m³ is the same as 5,000 µg/m³, so the silica PEL is a hundred times lower. It also applies to a different fraction of the dust, under a different standard.

Exposure limits

General dust has no action level, no required monitoring schedule, no deadline for notifying workers, and no required medical surveillance.

What you have instead is the following:

• The General Duty Clause, OSHA's catch-all provision
• OSHA's respiratory protection standard, where respirators are in use
• Whatever your own program commits you to

Where a general dust survey is the right call

• Warehouse and bulk handling of inert powders: limestone, gypsum, talc, carbonates and mineral fillers
• Food and feed processing: flour, sugar, starch, spice, milk powder and animal feed
• Plastics and rubber compounding: polymer powder, carbon black handling and mixing rooms
• Paper converting, printing and packaging: trim dust and fiber
• Woodworking and millwork, which has a regulatory history of its own (see below)
• Bagging, sacking and tote filling anywhere
• General construction and demolition cleanup where the dust contains no regulated substance

The wood dust history

OSHA has no PEL specific to wood dust. Wood dust falls under the general dust limits.

OSHA set PELs for wood dust in 1989, as part of an update to its air contaminant limits. A federal appeals court, the Eleventh Circuit, vacated those limits in 1992, along with the rest of that update, in AFL-CIO v. OSHA, 965 F.2d 962 (11th Cir. 1992).

The vacated 5 mg/m³ figure still appears in safety data sheets and summaries, but OSHA has not been able to enforce it since 1992.

What does exist are recommended limits. NIOSH publishes a recommended exposure limit (REL), and ACGIH, the professional body behind the threshold limit values (TLVs), publishes TLVs for wood dust.

Do not compare those numbers directly. OSHA's “total dust” and ACGIH's “inhalable” dust are two different sampling conventions. They use different samplers and collect different mixes of particle sizes, so they are not two opinions about the same measurement.

Lining them up in one column and calculating a ratio does not produce a meaningful comparison, even when the conclusion happens to point the right way.

A narrower statement holds up and is still useful. The health-based limits for wood dust are well below the general dust limits OSHA enforces, the species of wood matters, and cancer of the nose and sinuses is the health effect behind the NIOSH listing.

How we sample for it

The cyclone ahead of the filter is what lets only the respirable fraction reach it.

You cannot swap one cyclone's flow rate for another's. Each cyclone has its own specified flow rate, and that flow is what makes it pass the right particle sizes for a respirable sample.

Suppose someone runs a Higgins-Dewell cyclone at 1.7 L/min because the last job used that rate. The sample is no longer respirable dust but some unknown range of sizes, and the result cannot be compared to a respirable limit.

A gravimetric analysis also has no chemistry to hide behind. The result is the difference between two weighings of the same filter, days apart, and it is often a fraction of a milligram. Two things therefore matter a great deal:

• Pre-weighed, low-ash PVC filters supplied by the lab, not filters weighed on arrival.
• Enough air through the filter that the dust collected is well above the smallest weight the lab's scale can read.

A short run on a clean process gives a number that is mostly noise.

→ What an air monitoring survey costs in 2026

When you may not need this

If you know the dust contains a regulated substance, a general dust survey is the wrong thing to buy. Sample for that substance instead.

• Dust with silica needs a cyclone and analysis by X-ray diffraction or infrared, under OSHA's silica standard.
• Dust with metals needs a mixed cellulose ester filter and metals analysis by ICP.

A gravimetric result on those materials does not answer the question of whether you meet the standard.

General dust sampling is genuinely useful in three cases:

1. Showing that a dust is in fact inert and the exposure is under control.
2. Giving you a defensible basis for respirator selection under OSHA's respiratory protection standard.
3. Measuring a housekeeping or ventilation problem that people are arguing about.

In the third case, the gravimetric result is often the quickest way to end the argument.

If we walk your process and find that the real question is silica or metals, we will tell you that before we quote you for dust.

→ Respirable crystalline silica air monitoring for general industry