Respirable Crystalline Silica
Monitoring · Monitoring overview
Respirable crystalline silica (RCS) is the fine, lung-penetrating fraction of crystalline silica dust released whenever quartz-bearing materials are cut, ground, drilled, broken, blasted or polished. This reference page explains what RCS is, where it comes from, why it is regulated under COSHH, and how UK employers measure and manage silica exposure against the respirable crystalline silica Workplace Exposure Limit in HSE EH40.
What respirable crystalline silica actually is
Crystalline silica is silicon dioxide arranged in an ordered crystal lattice. The most common form in UK construction, stone fabrication and quarrying is quartz; cristobalite and tridymite are encountered in smaller volumes in refractories, foundry work and some thermally-treated stone. Crystalline silica is the dominant mineral phase in sandstone, granite, slate, concrete, brick, mortar, tile, kerbstone and engineered stone worktops.
The crystalline form is not itself the immediate health hazard — large grains of stone are too coarse to reach the deep lung. The hazard is the respirable fraction generated when these materials are mechanically broken: cutting, chasing, grinding, polishing, drilling, breaking, scabbling and abrasive blasting all liberate particles below roughly 10 microns aerodynamic diameter, with a peak deposition in the alveolar region for particles around 4 microns.
Respirable crystalline silica is defined for measurement purposes by the respirable convention in BS EN 481 — the size-selective fraction sampled by cyclone heads on personal pumped air sampling. That convention is the basis on which silica exposure is measured and compared with the silica WEL.
Where RCS is generated in UK workplaces
Silica dust exposure is dominated by a handful of high-energy tasks. On construction sites, RCS is generated by dry-cutting concrete blocks, kerbs and paving; chasing brick and block walls; cutting and coring reinforced concrete; tuck-pointing and mortar removal; breaking and scabbling concrete slabs; and dry sweeping debris that already contains respirable silica.
In engineered stone fabrication, RCS is released during sizing, mitring, edge profiling, grinding and polishing of high-silica worktops, where the silica content of the stone itself can sit at or above 90% by mass. Quarrying releases RCS during drilling, blasting, crushing, screening and tipping. Abrasive blasting with silica-bearing media — and any blasting of silica-bearing substrate — generates very high airborne RCS concentrations within seconds.
Less obvious tasks still matter. Sweeping, compressed-air cleaning, tipping bagged powders, demolition, tunnelling and reworking dust-contaminated surfaces all liberate respirable silica that has already been generated by an earlier process. Silica exposure assessment has to account for this re-entrainment as well as the primary source.
- Cutting, chasing, grinding and breaking concrete, brick, mortar and stone.
- Engineered stone worktop sizing, mitring, edge profiling and polishing.
- Drilling, crushing and screening in quarries and aggregate processing.
- Abrasive blasting of silica-bearing media or silica-bearing substrate.
- Dry sweeping, compressed-air cleaning and demolition of dust-contaminated surfaces.
Why RCS is controlled under COSHH
Respirable crystalline silica is a long-latency occupational health hazard. Sustained silica exposure is causally associated with silicosis, lung cancer and chronic obstructive pulmonary disease (COPD); accelerated silicosis in particular has been reported at very high engineered-stone exposure levels with relatively short latency. The disease burden is the basis on which RCS sits within the Control of Substances Hazardous to Health (COSHH) Regulations and is reviewed against the respirable crystalline silica Workplace Exposure Limit published in HSE EH40.
UK employers carry a duty to assess silica exposure, prevent it where reasonably practicable, and otherwise control it so far as is reasonably practicable using the COSHH hierarchy. That duty is operational, not symbolic: it expects substitution to be considered first, then water suppression and on-tool extraction at the cut, enclosure of dusty work, dedicated silica LEV with adequate capture velocity, and task-appropriate RPE only as a complement to engineering control or for short-duration residual peaks.
Workers who are routinely exposed to RCS are also expected to be brought into health surveillance proportionate to the exposure profile, with silica exposure data feeding back into the COSHH risk assessment cycle.
How RCS exposure is measured
The standard UK method for measuring RCS in the workplace is personal pumped air sampling on the breathing-zone lapel using a respirable cyclone (commonly the SIMPEDS / Higgins-Dewell type) operated at the manufacturer-specified flow rate. The cyclone separates the respirable fraction onto a pre-weighed filter for a representative period across the shift.
Filters are returned for gravimetric weighing to determine total respirable dust mass, then analysed by X-ray diffraction (XRD) or infrared spectroscopy (FTIR) for the crystalline silica content of that dust. Results are time-weighted to an 8-hour reference period and compared with the silica WEL in HSE EH40, against the strategy in BS EN 689 for comparison with limit values.
Static (area) sampling has a role for characterising background concentrations and for hot-spot diagnostics, but it is not a substitute for personal silica air sampling when the question is whether a worker’s silica exposure is being adequately controlled.
Silica exposure limits and what they mean operationally
Silica exposure limits exist so that silica exposure data has a defensible reference point. The respirable crystalline silica Workplace Exposure Limit in HSE EH40 is an 8-hour TWA. Measured exposures above the WEL indicate inadequate silica control; measured exposures well below the WEL are not, on their own, evidence that controls are correctly designed or maintained — sampling design and task representativeness matter as much as the headline number.
Operationally, the WEL is one of several thresholds employers track: in-house action levels are often set below the WEL to trigger review of water suppression, on-tool extraction or silica LEV before drift becomes non-compliance. The deeper guidance on silica exposure limits is covered on the dedicated silica exposure limits page.
When to commission RCS work
Silica exposure assessment is normally commissioned when a workplace introduces new silica-generating tasks, switches materials (notably to engineered stone), changes water suppression or on-tool extraction provision, redesigns workshop ventilation, encounters health surveillance findings that point to silica exposure, or simply needs current personal silica air sampling data to underpin a COSHH review.
It is also routine to commission silica monitoring before a workplace inspection, before a tender response that requires occupational hygiene data, and as part of an incident investigation where silica exposure may have been a contributing factor.
Frequently asked questions
What is the difference between silica dust and respirable crystalline silica?
Silica dust is the general airborne dust released when silica-bearing materials are worked. Respirable crystalline silica is the fine, lung-penetrating fraction of that dust — the size range sampled by cyclone heads and measured against the silica WEL. RCS is the regulatory-relevant fraction; bulk silica dust is the operational source.
Is engineered stone fabrication higher-risk than natural stone?
On average, yes. Engineered (agglomerated) stone is typically much higher in crystalline silica content than natural stone, often above 90% by mass, and the processing tasks generate high airborne RCS concentrations. Wet processing, dedicated silica LEV and rigorous task isolation are normally required to keep silica exposure below the WEL.
How long does respirable crystalline silica stay airborne?
The respirable fraction of silica dust can remain airborne for tens of minutes to hours depending on particle size, air movement and ventilation. This is why dry sweeping, compressed-air cleaning and post-task entry into a recently worked area can still represent significant silica exposure even after the source task has stopped.
Does silica exposure assessment require static or personal air sampling?
Personal pumped sampling on the operator’s breathing zone is the method used to assess silica exposure against the WEL. Static (area) sampling supports diagnostic work — characterising background, identifying hot spots or comparing two control configurations — but it does not replace personal silica air sampling for compliance.
How often should silica exposure be re-measured?
Silica exposure should be re-measured whenever the work, materials or controls change in a way that could plausibly alter exposure, and otherwise at a frequency proportionate to risk. Engineered stone, abrasive blasting and routine concrete cutting workflows typically warrant more frequent silica air sampling than low-frequency, well-controlled tasks.
Related pages