Wood shops: the wood dust limit you were quoted does not exist

If someone has told you the OSHA permissible exposure limit (PEL) for wood dust is 5 milligrams per cubic meter of air (mg/m³), they are quoting a limit that a federal appeals court struck down in 1992.

OSHA has no PEL specific to wood dust

OSHA set limits specific to wood dust in its 1989 Air Contaminants rule. A federal appeals court, the Eleventh Circuit, struck that rule down in AFL-CIO v. OSHA (965 F.2d 962, 11th Cir. 1992). The decision took out the whole rule, wood dust included, and OSHA never reissued those limits.

What applies today is OSHA's catch-all category for general dust, Particulates Not Otherwise Regulated (PNOR), which sits in OSHA's general limits table, Table Z-1:

• 15 mg/m³ for total dust, as an 8-hour time-weighted average (TWA)
• 5 mg/m³ for respirable dust, as an 8-hour TWA

That is all there is, so the same limit that applies to limestone dust applies to red oak.

The health-based limits from NIOSH and ACGIH are nowhere near the OSHA figure.

The World Health Organization's International Agency for Research on Cancer (IARC) places wood dust in Group 1, carcinogenic to humans. The evidence is sinonasal adenocarcinoma, a cancer of the nose and sinuses, among hardwood workers. ACGIH classifies oak and beech as A1, confirmed human carcinogens, and birch, mahogany, teak and walnut as A2, suspected human carcinogens. Against all that, 15 mg/m³ is still the federal limit in force.

The comparison that quietly cheats

This part matters if you already have monitoring data. The OSHA total dust limit and the ACGIH TLV are not measured the same way.

OSHA's 15 mg/m³ is set for total dust, which is collected with a 37 mm closed-face cassette. ACGIH's 1 mg/m³ is set for inhalable dust, meaning all the dust a person can take in through the nose or mouth. Measuring it requires a different device, an IOM sampler or an equivalent.

For wood dust, an inhalable sampler collects much more than a closed-face cassette does from the same air, typically by a factor of roughly 1.5 to 2.5.

Suppose a closed-face total dust result comes back at 1.8 mg/m³. Describing that as "just above the ACGIH limit" is not a fair comparison, because the true inhalable value is probably higher.

When we sample against the ACGIH TLV, we use an inhalable sampler. If you already have closed-face data, we will tell you what it can and cannot support, and we will not put two results that are not comparable in the same column.

→ Dust air monitoring · OSHA PELs and ACGIH TLVs compared

Wood species changes the answer

Western red cedar is a true cause of asthma, and the agent is known: plicatic acid. Once workers become sensitized to it, they react at concentrations that never bothered them before, and removing them from the dust does not reliably clear up the condition. This is why ACGIH sets its cedar TLV at half the value it sets for all other species.

Hardwood dust and softwood dust also differ in the cancer evidence, and the strongest data is on hardwoods. A cabinet shop running oak, a millwork shop running poplar and a cedar fence plant need three different exposure assessments. A single number for "wood dust" describes none of them well.

Formaldehyde from the sheet goods

Medium-density fiberboard (MDF), particleboard and some plywood are bonded with urea-formaldehyde or phenol-formaldehyde resin, so machining them releases formaldehyde as well as dust. Routing, sanding, drilling and any hot process release more.

OSHA has a full standard for formaldehyde alone, and it sets:

• A PEL of 0.75 parts per million (ppm), as a TWA
• A short-term exposure limit (STEL) of 2 ppm
• An action level of 0.5 ppm

Behind those limits the standard has exposure monitoring, triggers for medical surveillance, training and labeling. It applies to shops that cut sheet goods, not only to plants that make them.

There is also a requirement on the product side that surprises manufacturers. EPA has formaldehyde standards for composite wood products (under Title VI of the Toxic Substances Control Act, TSCA), which put emission limits, certification and recordkeeping on panels and on finished goods that contain them. That program concerns your product, not what your workers breathe. It is enforced separately, and it is outside the four fields we work in.

→ Formaldehyde air monitoring

Isocyanates, noise and combustible dust

Three things bring isocyanates into a shop that thinks of itself as a dust shop:

• Engineered wood bonded with polymeric MDI (pMDI)
• Polyurethane reactive (PUR) hot-melt edgebanding
• Two-part polyurethane finishes

MDI is short for methylene bisphenyl isocyanate. OSHA's limit for it is a 0.02 ppm ceiling limit, a level that must not be exceeded at any time, and ACGIH's TLV is 0.005 ppm as a TWA. Sampling requires derivatizing media, which is a specially treated filter. An ordinary charcoal tube returns nothing.

Noise in a woodshop is rarely a close call. Wide-belt sanders, molders, shapers, computer-controlled (CNC) routers, dust collector fans and compressed air blow-down put most production shops above the 85 decibel (dBA) action level, and many are above 90.

Wood dust is also combustible and can explode. OSHA has no combustible dust standard. A fire safety standard, NFPA 664, covers wood processing, and OSHA enforces through the General Duty Clause and its emphasis program.

The finding that keeps coming up is a dust collector set against the building with no deflagration venting (explosion venting). Often a return-air arrangement sends filtered air, along with the fine dust still in it, straight back into the shop, which is also why respirable dust levels stay high in places that look clean. Combustible dust work is outside the four fields we work in.

→ Noise surveys · Isocyanate air sampling · Air monitoring surveys