How to Write a COSHH Assessment for Silica Dust: Complete UK Guide

What is Respirable Crystalline Silica (RCS)?

Respirable crystalline silica (RCS) is a fine dust created when materials containing silica are cut, ground, drilled, or broken. On construction sites, the most common sources are concrete, sandstone, brick, mortar, and engineered stone. The particles are so small (under 10 microns) that they penetrate deep into the lungs, where the body cannot clear them.

The health consequences are severe. Silicosis is an irreversible scarring of the lungs that progressively reduces breathing capacity. RCS is also classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC), meaning it is a confirmed cause of lung cancer. Long-term exposure is also linked to COPD and kidney disease.

The HSE estimates that around 500 construction workers in the UK die each year from diseases linked to silica dust exposure. It is not a theoretical risk. If your workers are cutting concrete or chasing walls without proper controls, they are being exposed.

UK Workplace Exposure Limit for Silica Dust

The workplace exposure limit (WEL) for respirable crystalline silica in the UK is 0.1 mg/m³ as an 8-hour time-weighted average (TWA). This is set out in the HSE’s EH40 table of workplace exposure limits.

To put that in perspective, 0.1 mg/m³ is one of the lowest WELs of any substance. For comparison, general inhalable dust has a WEL of 10 mg/m³, which is 100 times higher. The low limit reflects just how dangerous silica dust is at even small concentrations.

As a practical rule: any task that produces visible dust is almost certainly exceeding the WEL. If your workers can see dust in the air while cutting or grinding, exposure is too high and controls are inadequate.

Step-by-Step: Writing Your COSHH Assessment

A COSHH assessment for silica dust follows the same six-step structure as any COSHH assessment, but there are specific details you need to get right. Here is how to work through each step.

Step 1: Identify the Hazardous Substance

Start by recording the substance details. For silica dust generated from cutting or grinding site materials, use the following:

• Substance name: Respirable crystalline silica (quartz)

• CAS number: 14808-60-7

• GHS classifications: STOT RE 1 (causes damage to lungs through prolonged exposure), Carc. 1A (may cause cancer by inhalation)

• WEL: 0.1 mg/m³ (8-hour TWA)

If you are working with a specific product (e.g. a concrete mix or engineered stone), also reference the manufacturer’s safety data sheet (SDS) for the silica content percentage.

Step 2: Who is Exposed and How?

Identify every person who could be affected. This is not just the worker doing the cutting.

• Directly exposed workers: Operatives cutting, grinding, drilling, or breaking concrete, stone, brick, or mortar

• Bystanders: Other trades working nearby, labourers sweeping up dust, anyone in the same enclosed area

• Exposure route: Inhalation is the primary route. Skin and eye contact are secondary concerns from settled dust.

Silica dust stays airborne for a long time. Workers who enter an area after cutting has finished can still be exposed if the dust has not been suppressed or extracted.

Step 3: Assess the Risk

Consider the likelihood and severity of exposure based on the actual work being carried out. Key factors to record:

• Duration and frequency: How long does each task last? Is it a one-off or repeated daily?

• Quantity of dust: Cutting a single block creates far less dust than grinding a concrete floor for 4 hours.

• Location: Indoor work in an enclosed space is significantly higher risk than outdoor cutting with natural ventilation.

• Existing controls: Document what is already in place (water suppression, extraction, RPE) and whether it is adequate.

For silica dust, the severity is always high (irreversible lung damage, cancer). The risk level depends on how well you control the exposure.

Step 4: Control Measures (Hierarchy)

COSHH Regulation 7 requires you to prevent exposure where reasonably practicable, or adequately control it. Work through the hierarchy of controls from top to bottom.

• Elimination: Can you avoid the dust-generating task entirely? Use pre-cut or pre-formed materials. Design out the need for on-site cutting where possible.

• Substitution: Can you use a material with lower silica content? For example, some composite materials have reduced quartz levels compared to natural stone.

• Engineering controls: This is where most silica control happens in practice. Water suppression (wet cutting) reduces airborne dust by up to 90%. On-tool extraction with an H-class vacuum captures dust at the source. Enclosed cutting areas with local exhaust ventilation (LEV) contain the dust cloud.

• Administrative controls: Rotate workers to reduce individual exposure duration. Set up exclusion zones around cutting areas. Display warning signage. Monitor dust levels with personal samplers. Never dry sweep; use an H-class vacuum or damp methods.

• RPE (last resort): FFP3 disposable mask as a minimum (assigned protection factor of 20). Half-mask respirator with P3 filters for regular exposure. Powered air purifying respirator (PAPR) for prolonged or heavy exposure. All RPE must be face-fit tested under COSHH Regulation 7.

In practice, a good COSHH assessment for silica will specify a combination of engineering controls and RPE. Relying on RPE alone is not acceptable under COSHH.

Step 5: Health Surveillance

COSHH Regulation 11 requires health surveillance for workers with significant exposure to silica dust. In practice, this means anyone who regularly carries out tasks that generate RCS, even with controls in place.

Health surveillance for silica includes:

• A baseline health questionnaire and lung function test (spirometry) before the worker starts the role

• Periodic reviews, typically every 1 to 3 years depending on exposure level

• Referral to an occupational health professional if any abnormality is detected

Health surveillance records must be kept for at least 40 years. Your COSHH assessment should state that health surveillance is required and describe the programme in place.

Step 6: Review and Record

Your COSHH assessment is a living document. Review it:

• At least annually

• Whenever the work method, materials, or location change

• After any incident, near miss, or health surveillance finding

• If new information about silica risks becomes available

Record who carried out the assessment, the date, and the date of the next scheduled review. Keep the assessment accessible to all workers and supervisors on site.

Common Activities That Generate Silica Dust

If any of these tasks are happening on your site, a COSHH assessment for silica dust is required:

• Cutting concrete or paving slabs with a disc cutter or table saw

• Chasing walls for electrical cables or pipework

• Drilling into masonry, concrete, or brick

• Breaking out concrete with breakers or jackhammers

• Grinding or polishing concrete floors

• Mixing dry mortar, cement, or grout

• Demolition and refurbishment involving concrete or blockwork

• Tunnelling and earthmoving in sandstone or quartz-bearing ground

RPE Selection for Silica Dust

Choosing the right respiratory protective equipment (RPE) depends on the exposure level and task duration. Here is what the HSE guidance recommends for silica dust:

• FFP3 disposable mask: Assigned protection factor (APF) of 20. Suitable for short-duration, low-dust tasks. Must be fit tested.

• Half-mask respirator with P3 filters: APF of 20. More comfortable for regular or repeated exposure. Reusable with replaceable filters.

• Powered air purifying respirator (PAPR): APF of 40 or higher. Recommended for prolonged cutting or grinding sessions. Reduces breathing effort and heat stress.

All RPE must be face-fit tested in accordance with INDG479. A mask that does not seal properly provides almost no protection. Record all fit test results and retrain workers annually.

Remember: FFP2 masks are not adequate for silica dust. The minimum standard is FFP3.

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Related Guides

• COSHH Regulations UK: The Complete Guide

• COSHH Risk Assessment: What It Is and How to Write One

• Construction Phase Plan: Complete UK Guide (silica control is a CDM requirement)

Silica Dust Exposure by Task: What the Data Shows

Not all tasks create equal levels of silica dust. HSE research report RR878 measured real-world dust exposures across common construction activities involving concrete and masonry. The results show how quickly the workplace exposure limit (WEL) of 0.1 mg/m3 is exceeded without proper controls.

• Disc cutting concrete (dry): 10 to 50 times the WEL
• Chasing walls: 5 to 20 times the WEL
• Drilling into masonry: 2 to 10 times the WEL
• Sweeping concrete dust: 3 to 15 times the WEL
• Breaking out concrete: 5 to 30 times the WEL

The pattern is clear: virtually any dry cutting, grinding, or breaking of concrete will exceed the 0.1 mg/m3 WEL without dust suppression controls in place. Even activities that seem low-risk, like sweeping up concrete dust, can generate dangerous airborne silica concentrations. These figures are drawn from HSE research report RR878, which measured exposures across live construction sites.

Dust Suppression: What Actually Works

COSHH regulation 7 requires employers to prevent exposure to hazardous substances, or where prevention is not reasonably practicable, to adequately control it. For silica dust, this means using engineering controls before relying on RPE. Here is what the evidence shows about the effectiveness of different dust suppression methods.

• On-tool water suppression reduces airborne dust by 80 to 95% (HSE INDG463). This is the single most effective control for cutting and grinding tasks.
• On-tool extraction with an H-class vacuum reduces dust by 90% or more. H-class vacuums are specifically designed for hazardous dusts including silica.
• Combining water suppression and extraction gives the best results and should be used wherever practicable.
• M-class vacuums are NOT suitable for silica. Respirable crystalline silica particles are too small for M-class filters. Only H-class vacuums (with HEPA filtration) should be used.
• Compressed air blowing is prohibited under COSHH. Using compressed air to clean off dust creates a massive airborne cloud that can expose everyone in the area. This is a common but illegal practice on construction sites.
• Clean-up must use wet sweeping or an H-class vacuum. Never dry sweep silica dust. Dry sweeping re-suspends settled dust back into the breathing zone.

Where local exhaust ventilation (LEV) systems are used, they must be thoroughly examined and tested at least every 14 months under COSHH Regulation 9. Records of these examinations must be kept for at least 5 years. If an LEV system fails its test, work must stop until it is repaired or replaced.

Face Fit Testing: A Legal Requirement

When RPE (respiratory protective equipment) is required as a control measure, COSHH Regulation 7 makes it a legal requirement that the RPE is face-fit tested for each individual wearer. An RPE mask that has not been fit tested cannot be relied upon to provide the stated level of protection.

There are two recognised methods for face fit testing:

• Qualitative fit testing (taste test): Uses a bitter or sweet solution sprayed into a hood while the wearer performs a series of exercises. If they can taste the solution, the mask does not fit.
• Quantitative fit testing (PortaCount): Uses a particle counter to measure the actual seal of the mask. This method is more precise and is required for tight-fitting full-face masks.

Facial hair prevents an effective seal between the mask and the skin. Workers must be clean shaven in the area where the mask seals to the face. Stubble, beards, and sideburns all compromise the seal. A record must be kept of each fit test, and retesting is required when a worker changes to a different type or model of RPE, or after significant weight change or dental work that could alter the face shape.

Frequently Asked Questions

What is the WEL for silica dust?

The workplace exposure limit for respirable crystalline silica (RCS) is 0.1 mg/m3 as an 8-hour time-weighted average, as published in HSE EH40. This is one of the lowest WELs, reflecting how dangerous silica dust is.

Do I need a COSHH assessment for cutting one concrete block?

Yes. There is no minimum threshold for COSHH. Any activity that could expose workers to a hazardous substance requires an assessment. Even a single cut of concrete with a dry disc cutter can generate silica dust levels 10 to 50 times the WEL.

What RPE do I need for silica dust?

Minimum FFP3 disposable mask (assigned protection factor of 20) for short duration work. For regular or prolonged exposure, use a half-mask with P3 filters (APF 20) or a powered air purifying respirator (APF 40). All RPE must be face-fit tested.

Is silica dust a carcinogen?

Yes. Respirable crystalline silica is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). It causes silicosis, lung cancer, and chronic obstructive pulmonary disease (COPD).

How often should I review my silica COSHH assessment?

Review at least annually, or sooner if work conditions change, new products are introduced, there is a change in the process or equipment, or monitoring shows control measures are not working.

Further Reading and Authority Sources

For more detailed guidance, refer to the following authority sources: HSE guidance on silica and silicosis, EH40 Workplace Exposure Limits, HSE INDG463 Control of exposure to silica dust, and the IARC Monograph on silica classifications.

You may also find these related guides helpful: COSHH Regulations UK Complete Guide, COSHH Assessment for Cement Dust, How to Write a RAMS for Working at Height, Construction Phase Plan Guide, and What is a RAMS Document?.