Respirable Dust
Core Dust · Core Dust overview
Respirable dust is the fraction of airborne particulate fine enough to reach the deep, gas-exchange region of the lung. In UK workplaces it is the fraction most often associated with chronic respiratory disease, and the fraction against which many of the Workplace Exposure Limits in HSE EH40 are set.
What respirable dust is
Respirable dust refers to the size fraction of airborne solid particulate that can penetrate beyond the ciliated airways and deposit in the alveolar (gas-exchange) region of the lung. Internationally it is defined by the respirable convention in ISO 7708 and BS EN 481, which describes a size-selective sampling curve with a 50% cut at approximately 4 µm aerodynamic diameter.
Because the cut is a curve rather than a sharp threshold, respirable samplers do not collect a single particle size. They collect a size-weighted sample that approximates what would actually reach the deep lung under normal breathing. For UK occupational hygiene, this is the fraction that matters for substances with chronic lung effects — including respirable crystalline silica, hardwood dust, certain pharmaceutical actives and respirable cement dust.
Why the respirable fraction matters
The alveolar region of the lung lacks the clearance mechanisms that protect the upper airways. Particles that deposit there can remain for long periods, and for some substances the biological response — inflammation, fibrosis, accumulation of insoluble material — is dose-dependent over years. This is why HSE EH40 expresses Workplace Exposure Limits for several substances specifically against the respirable fraction (for example respirable crystalline silica at 0.1 mg/m³ as an 8-hour TWA).
Measuring total or inhalable dust alone can substantially under- or over-estimate the dose to the deep lung. For chronic-effect substances, the respirable fraction is the correct measurand and the correct comparison point against the relevant WEL.
Common workplace sources
Respirable dust is generated wherever solid material is reduced in size — cutting, grinding, sanding, drilling, blasting, milling, mixing dry powders, or breaking down bagged material. It is also released when settled dust is re-suspended by sweeping, compressed-air cleaning, vehicle movement or vigorous activity.
- Stone cutting, kerb dressing and concrete drilling — respirable crystalline silica.
- Hardwood and MDF machining — respirable wood dust.
- Cement mixing, dry-bag handling and dry sweeping — respirable cement and silica.
- Pharmaceutical weighing, milling and tabletting — respirable API exposure.
- Foundry shake-out, fettling and refractory work — mineral and silica dust.
Respirable vs inhalable dust
Respirable and inhalable dust are not alternatives — they describe different size fractions of the same airborne cloud. The inhalable fraction (median around 100 µm) describes everything small enough to be drawn into the nose or mouth during breathing. The respirable fraction is a subset of the inhalable: it is the much smaller proportion of those particles that can reach the alveoli.
For substances acting in the upper airways or systemically once deposited and dissolved (for example flour dust and many soluble materials), the inhalable fraction is the appropriate measurand. For substances with deep-lung effects, the respirable fraction is required. Some substances — wood dust is the most familiar UK example — have WELs set against the inhalable fraction, even though respirable wood dust is the more biologically significant subset.
How respirable dust monitoring is approached
Respirable dust is most commonly measured by personal gravimetric sampling. A constant-flow sampling pump worn on the worker's belt draws air through a size-selective sampler — typically a cyclone (e.g. SIMPETER, GS-3 or BGI cyclone at the appropriate flow rate) — onto a pre-weighed filter. The cyclone removes particles too large to be respirable, so the filter collects only the respirable fraction.
The filter is conditioned and weighed in a controlled laboratory environment before and after sampling; the gravimetric difference divided by the sampled air volume gives the airborne concentration in mg/m³. The reference UK method for gravimetric sampling is HSE MDHS 14/4. Sampling duration aims to be representative of the working shift, and is interpreted against the 8-hour time-weighted average WEL in EH40.
Real-time optical particle monitors can be used alongside gravimetric sampling to identify which tasks contribute most to exposure. They are not a substitute for gravimetric mass measurement against a WEL, but they are valuable for prioritising control improvements.
What results help employers understand
A respirable dust monitoring report is most useful when it answers three questions: what was the 8-hour TWA exposure for each similar exposure group, how does it compare to the relevant EH40 WEL, and which specific tasks drove the peaks. Results should be interpreted under BS EN 689, which sets out a structured strategy for deciding whether exposure is reliably below the limit value or whether further monitoring or control is required.
Single results in isolation are rarely sufficient. A defensible position usually requires repeat measurements across representative shifts and conditions, particularly where exposure approaches the WEL.
Control considerations
Where respirable dust exposure is not adequately controlled, the COSHH hierarchy applies. Elimination and substitution sit at the top — for example replacing dry sweeping with HEPA-filtered vacuuming, or specifying lower-silica materials. Enclosure and on-tool extraction are the principal engineering controls for cutting, grinding and drilling tasks. Respiratory protective equipment is the last line, not the first, and is selected only after engineering and procedural controls are exhausted.
When to request a respirable dust assessment
A respirable dust assessment is appropriate when a COSHH assessment cannot reasonably conclude that exposure is adequately controlled — for example following a process or material change, after workers report respiratory symptoms, where controls cannot be relied upon, or to verify that existing controls remain effective over time. Periodic monitoring is also expected where prior results have approached a WEL.
Frequently asked questions
What is the 50% cut point for respirable dust?
The respirable convention in ISO 7708 and BS EN 481 defines a size-selective curve with a 50% collection efficiency at approximately 4 µm aerodynamic diameter. Respirable samplers approximate this curve so the collected sample reflects what would reach the alveolar region of the lung.
Which UK Workplace Exposure Limits are set against the respirable fraction?
EH40 sets several limits specifically for the respirable fraction, including respirable crystalline silica (0.1 mg/m³ 8-hour TWA) and respirable dust as a general nuisance limit (4 mg/m³ 8-hour TWA). The relevant fraction is always stated explicitly in the EH40 entry for the substance.
Can a real-time monitor replace gravimetric sampling?
No. Real-time optical instruments are useful for mapping task contribution and identifying peaks, but they measure light scattering, not collected mass. Comparison with a Workplace Exposure Limit requires gravimetric sampling against a size-selective convention under MDHS 14/4 or equivalent.
How long should respirable dust sampling run?
Sampling should be representative of the working shift. For comparison with an 8-hour TWA WEL, sampling usually covers a full shift or a defined working period that includes the high-exposure tasks. BS EN 689 sets out how to structure sampling and interpret results.
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