M Class vs H Class Dust Extractors for OSHA Silica Work

M class and H class are dust categories from the European standard EN 60335-2-69. They are not OSHA designations. OSHA's construction silica standard, 29 CFR 1926.1153, does not use class letters. It states filter efficiencies and required equipment features instead.

US buyers see the letters on imported extractors and on manufacturer pages written for the UK market. The letter does not decide whether a vacuum meets Table 1. Two things on the spec sheet decide that: the stated filter efficiency and the filter-cleaning mechanism.

Last updated October 12, 2026.

Each dust class is defined by how much dust the extractor lets through. Hilti's UK blog gives these figures:

M class : Traps 99.9% of dust, so at most 0.1% passes through. Hilti lists it for dusts with exposure thresholds above 0.1 mg/m³. Its listed uses include concrete, grinding, cutting, drilling and hardwood sawdust.

H class : Captures 99.995% of hazardous dust, so at most 0.005% passes through. Hilti lists it for dusts with thresholds at or below 0.1 mg/m³. Its listed uses are carcinogenic or pathogenic particles such as lead, nickel, cobalt and cadmium.

An Australian buyer's guide from AIMS Industrial, written to the AS/NZS version of the same standard, gives L class as 99%.

OSHA's figures appear in 1926.1153:

• Table 1 dust collectors: a filter of 99% or greater efficiency, unless a task entry specifies otherwise.
• Table 1 filter cleaning: the system must have a filter-cleaning mechanism or a cyclonic pre-separator.
• Table 1 grinders: the dust collector must provide at least 25 cfm of airflow per inch of wheel diameter.
• Housekeeping under paragraph (f): HEPA-filtered vacuuming is an accepted alternative to dry sweeping. OSHA defines a HEPA filter as at least 99.97% efficient on mono-dispersed particles 0.3 micrometers in diameter.

Compared as plain numbers:

• M class (99.9%) is higher than the Table 1 baseline (99%).
• M class (99.9%) is lower than OSHA's HEPA figure (99.97%).
• H class (99.995%) is higher than OSHA's HEPA figure (99.97%).

These are comparisons of numbers, not of tests. OSHA's HEPA figure is tied to a particle size of 0.3 micrometers. The manufacturer sources quoted here do not state a particle size for the class figures. A class letter alone is therefore not evidence that a filter meets OSHA's HEPA definition.

The class figures in these sources also describe filtration only. They do not show whether a unit has a filter-cleaning mechanism. Table 1 requires that feature separately.

The CPWR equipment list. CPWR's list of equipment and control options pairs specific tools with extractors for Table 1 tasks. The list gives no M class or H class ratings. Each extractor is described by its features:

• Bosch VAC090AH: a 9-gallon dust extractor with Automatic Filter Clean.
• Hilti VC 300-17 X: includes a wet/dry HEPA filter.
• Metabo ASR 35 ACP: an all-purpose vacuum with a HEPA filter.

The filter and cleaning descriptions are taken from the makers' product descriptions.

A maker's compliance page. Guardair markets two PowerQUAD extractors as meeting 1926.1153. One holds 30 gallons and has a 2-inch inlet. The other holds 20 gallons and has a 1.5-inch inlet. Guardair states that both use HEPA filtration and its PulseAir system, which cleans the filter with compressed air without opening the unit. These are the maker's claims.

Both inlet sizes fall within the 1.5 to 2 inch hose diameter recommended in OSHA's guidance document Controlling Silica Exposures in Construction (OSHA 3362-04). The same Guardair page gives the PEL as 25 μg/m³. That figure is OSHA's action level. The PEL is 50 μg/m³. A maker's page is a claim to check against the regulation text, not a substitute for it.

A UK-market article. Hilti's UK blog uses only class letters. It frames the choice of extractor around UK HSE enforcement, citing fines of £20,000 or more. Those figures describe UK rules, not OSHA penalties.

The sources for this page do not include published class or HEPA ratings for the Festool CT 36 AC HEPA, the Makita VC4710 or the DeWalt DWV012. For those models, read the rating from each maker's own spec sheet using the checks below.

A class letter describes filtration under a European test standard. OSHA compliance under Table 1 depends on a stated efficiency and on specific equipment features. For example, AIMS Industrial states that "an L-Class shop vac is no longer compliant for silica work" under Australian rules. Its listed 99% figure is the same as Table 1's general baseline. Whether an L class shop vac is acceptable in the US therefore depends on OSHA's text, not on the Australian one.

Table 1 exempts an employer from exposure monitoring only when its controls are fully and properly implemented. If an extractor does not meet the Table 1 entry for the task, the task falls under the alternative exposure assessment method, which requires monitoring.

The standard is cited often. HazComFast's analysis of OSHA inspection records counts 632 citations of 1926.1153 in construction in FY2025. Those citations came from 271 inspections and totaled $1,219,150 in penalties.

The model assigned to each task, with its stated rating, belongs in the written exposure control plan.