Silica dust exposure calculator: time to the legal limit
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Today's dusty tasks | Cutting blocks: time on the task 30 min, rcs concentration 2 mg/m³, dust control None, mask No mask; Sweeping up: time on the task 60 min, rcs concentration 0.5 mg/m³, dust control None, mask No mask |
| Workplace exposure limit | 0.1 mg/m³ |
Reached the legal limit. Daily exposure 0.188 mg/m³ (8-hour TWA), 188% of the WEL
- Outside the guidance Daily silica exposure against the WEL: 0.188 mg/m³ UK-wide: HSE EH40/2005, respirable crystalline silica (8-hour TWA). Check with the source
| Result | Value |
|---|---|
| Against the legal limit | Reached the legal limit |
| Daily exposure, 8-hour TWA (inside any mask) | 0.188 mg/m³ |
| Share of the WEL | 188% |
| Daily exposure without a mask | 0.188 mg/m³ |
| Minutes on the dustiest task before the limit is reached | 24 min |
| Task | Minutes | After control | APF | Inside the mask | Minutes to the WEL alone |
|---|---|---|---|---|---|
| Cutting blocks | 30 | 2 mg/m³ | 1 | 2 mg/m³ | 24 |
| Sweeping up | 60 | 0.5 mg/m³ | 1 | 0.5 mg/m³ | 96 |
Show the working
- Cutting blocks: 2 mg/m³ ÷ APF 1 (no mask) = 2 mg/m³; minutes to the WEL on this task alone = 0.1 × 480 ÷ 2 = 24 min
- Sweeping up: 0.5 mg/m³ ÷ APF 1 (no mask) = 0.5 mg/m³; minutes to the WEL on this task alone = 0.1 × 480 ÷ 0.5 = 96 min
- Daily exposure: (2 × 30 + 0.5 × 60) ÷ 480 min = 0.188 mg/m³ 8-hour TWA
- Share of the WEL: 0.188 ÷ 0.1 × 100 = 188%
How it works
- Concentration after a dust control = measured concentration × (1 − reduction), only where a sourced reduction figure exists
- Concentration inside a mask C_eff = C ÷ APF (assigned protection factor, HSE HSG53)
- Daily exposure, 8-hour TWA = Σ(C_eff × minutes) ÷ 480 min
- Minutes before the limit on one task = WEL × 480 ÷ C_eff
Reaching the limit is a legal maximum, not a target: COSHH requires exposure to be reduced as far as reasonably practicable.
Use water suppression or on-tool extraction first. A mask is the last line of protection and a tight-fitting mask only gives its APF if the wearer has passed a face-fit test.
If your concentration was measured with water or extraction in use, it already includes that control: choose None so it isn't counted twice.
HSE CIS54 gives cutting a paving slab with a diamond blade as taking about a minute: short cuts still add up over a day.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Workplace exposure limit | 0.1 mg/m³ | Regulation or HSE figure HSE EH40/2005 Workplace exposure limits: respirable crystalline silica, 8-hour TWA (see also HSE CIS54 and HSG53 for RPE) Not yet checked against the source | hse.gov.uk (checked 19 October 2026) brief v2 S1: WEL 0.1 mg/m³ 8-h TWA (HSE EH40 / CIS54). Researcher to confirm the current EH40 table entry and the Northern Ireland (HSENI) equivalent. |
Regulations and standards
- HSE EH40/2005 Workplace exposure limits: respirable crystalline silica WEL, 8-hour TWA
- COSHH 2002 (Control of Substances Hazardous to Health Regulations)
- HSE HSG53 Respiratory protective equipment at work (assigned protection factors)
- HSE CIS54 Construction dust
Explained: HSE EH40 (workplace exposure limits).
The WEL in HSE EH40 applies in England, Wales and Scotland; Northern Ireland applies the same limits through HSENI. Researcher to confirm.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Assigned protection factors (APF) for respiratory protective equipment | HSE HSG53 Respiratory protective equipment at work (assigned protection factors) | UK-wide | 1/2 values entered Not yet checked against the source |
| Dust reduction from water suppression or on-tool extraction (fraction removed) | HSE construction dust guidance (CIS54; COSHH essentials CN sheets) | UK-wide | 0/2 values entered Not yet checked against the source |