Respirable crystalline silica at crushing plants: exposure limits in Quebec, Ontario and the United States

Granite, sandstone, many metallic ores and some limestones contain quartz. Crushing them produces crystalline silica particles fine enough to reach the alveoli of the lungs: the respirable fraction. It causes silicosis and is classified as a human carcinogen. That is why dust in a crushing circuit is not just a nuisance: it is a regulated exposure, with numeric limits and an obligation to measure.

This article gathers the limits we could verify in official sources, locates the exposed positions in a crushing circuit and sets out the order in which exposure is reduced. It does not replace an occupational hygienist or a reading of the regulation that applies to your site: limits change, and each jurisdiction has its own measurement rules.

What "respirable crystalline silica" means

Silica (silicon dioxide) exists in crystalline form — quartz is by far the most common, cristobalite and tridymite are rarer — and in amorphous form. Only the crystalline form is covered by the limits discussed here. The respirable fraction is the particles small enough to pass the upper airways; it is measured with a personal sampler fitted with a size-selective separator (cyclone), worn by the worker for the shift.

Two practical consequences. First, visible dust is not the right measure: the respirable fraction cannot be seen, and a spectacular cloud may hold little respirable silica while a position that looks clean can exceed the limit. Second, the quartz content of the rock drives everything: two quarries producing the same amount of dust do not expose their workers the same way.

Exposure limits, jurisdiction by jurisdiction

Sources: CNESST crystalline silica fact sheet, Workers Health & Safety Centre — silica, OSHA — crystalline silica, general industry, MSHA — silica rule, Safety+Health — MSHA deadline postponed. Values checked in November 2026.

Beyond the number, each regulation sets obligations: assess exposure, measure it with a recognized method, put controls in place, inform and train, and, depending on the jurisdiction, provide medical surveillance. A limit met on one sample is not compliance if the programme around it is missing.

Where exposure occurs in a crushing circuit

The most exposed positions are rarely the ones you would picture from the office.

At the primary crusher. Truck dumping and the crusher opening produce the most visible clouds. An operator in a closed, filtered cab is usually protected; the people who work at the opening — unblocking, inspection, wear part changes — much less so. The emission points of a primary station are detailed in dust suppression at the primary crusher and dump hopper.

At transfer points and under conveyors. Dust escaping from chutes, and above all carryback falling along the belt return, forces manual clean-up — shovelling, sweeping, compressed air — which ranks among the most exposed jobs on a site. See the five dust sources of a transfer point.

At screens and in screen houses. A screen shakes dry, fine material; in an enclosed building without capture, concentrations rise quickly.

During maintenance. Changing jaw dies, screen media or skirt seals means working among accumulated deposits. These are short but heavy exposures that weigh a lot in an 8-hour average.

Reducing exposure: the order that matters

Regulations and occupational hygiene practice agree on the same hierarchy: treat the source before the worker.

1. Reduce at the source. Contain the emission points (hoods, shaped chutes, tight skirtboards), then suppress or capture: water spray, dry fog, fog cannons, dust collectors. The choice is made point by point — the comparison is in dust suppression technologies compared.
2. Isolate the worker. Closed, pressurized and filtered cabs for crusher and mobile equipment operators; remote control of interventions at the crusher opening, which is what an unblocking boom does.
3. Organize the work. Vacuum or wet cleaning rather than brooms or compressed air, planned maintenance, restricted access to the heaviest zones.
4. Protect as a last resort. Respirators, selected and fitted under a programme, complement the measures above; they do not replace them.

Measure before and after

A suppression system is judged by measurement, not by eye. Two levels work together: personal exposure sampling, carried out by a hygienist with the method recognized in the jurisdiction, which says whether a position complies; and continuous particulate monitors, used in operations — tuning nozzles, spotting a loose skirt seal, comparing before and after a change. Running a measurement campaign before the work and another after commissioning is the only way to document what a system changed.

Going further

Surveying emission points, choosing one technology per point, designing for a Canadian winter and measuring before and after are part of our Dust suppression systems page; a preliminary study can be run as part of our feasibility studies. For a specific site, describe your circuit and exposed positions.

Frequently asked questions

What is the crystalline silica exposure limit in the United States? OSHA sets a permissible exposure limit of 50 µg/m³ as an 8-hour time-weighted average, with an action level of 25 µg/m³, in 29 CFR 1910.1053 for general industry and 1926.1153 for construction.

Does visible dust indicate the level of silica exposure? No. The respirable fraction is invisible and its silica content depends on the rock. Only personal sampling with a size-selective separator, analysed in a laboratory, measures actual exposure.

Is a dust suppression system enough to make a site compliant? It reduces exposure at the source, which is the first measure required, but compliance also rests on assessment, measurement, work organization, training and, as a last resort, respiratory protection.