Inhalable Dust
Core Dust · Core Dust overview
Inhalable dust is the fraction of airborne particulate that can enter the nose and mouth during normal breathing. It is the broader of the two principal occupational dust fractions and is the fraction against which many UK Workplace Exposure Limits — including the limit for wood dust — are set.
What inhalable dust is
Inhalable dust refers to the size fraction of airborne particulate that can be drawn into the nose or mouth during breathing. It is defined in ISO 7708 and BS EN 481 by an inhalability convention that extends well into the coarse-particle range, with a median around 100 µm aerodynamic diameter.
In practice, the inhalable fraction includes essentially everything most workers would describe as visible dust, plus a substantial proportion of finer material. It is sampled with heads designed to mimic the entry efficiency of the human nose and mouth — most commonly the IOM (Institute of Occupational Medicine) sampler, which is the de facto UK reference.
Why the inhalable fraction is monitored
Some substances exert their primary health effect in the upper airways — for example flour and hardwood dust, which are sensitisers and known causes of occupational asthma and nasal effects. For these substances, an inhalable measurement captures the exposure that actually matters; a respirable-only measurement would substantially underestimate the dose deposited in the upper respiratory tract.
EH40 reflects this by setting Workplace Exposure Limits against the inhalable fraction where the relevant biological target is upstream of the alveolar region. The standard 'general dust' inhalable limit is 10 mg/m³ as an 8-hour TWA, and specific substances (such as hardwood dust at 3 mg/m³ inhalable, 8-hour TWA) have their own limits.
Common sources and tasks
Inhalable dust is released by almost any dry handling, machining or finishing operation involving solid material. Because the size range is wide, both visible coarse dust and fine particulate contribute to the inhalable mass on the sampler.
- Bakeries — flour weighing, mixing and dough handling.
- Joinery and timber — sanding, sawing and shaping of hardwoods and softwoods.
- Construction — cement and plaster handling, dry sweeping and rubble clearance.
- Food and ingredient manufacturing — bulk powder transfer and bag tipping.
- Pharmaceuticals — weighing, dispensing, blending and packaging of solid actives.
Difference from respirable dust
Inhalable dust is the larger fraction; respirable dust is a much smaller subset of it. A sample collected with an IOM head and a sample collected with a respirable cyclone, run simultaneously, will produce very different numbers — both correct, each describing a different exposure question.
The choice of fraction is dictated by the substance, not the operator. If EH40 expresses a limit against the inhalable fraction, sampling must use an inhalable-convention head. Substituting a respirable result for an inhalable comparison (or vice versa) is not technically valid and is not defensible against the WEL.
Sampling and workplace assessment approach
Inhalable dust is measured by personal gravimetric sampling. An IOM sampler (or equivalent inhalable head) is worn in the breathing zone, connected to a constant-flow pump at a defined flow rate (typically 2 L/min for the IOM). The filter and internal capsule are conditioned and weighed before and after sampling under MDHS 14/4.
The sample is interpreted against the 8-hour time-weighted average WEL in EH40 using the BS EN 689 strategy — assessing whether exposure is reliably below the limit, requires further monitoring, or already indicates that controls need improvement. Where the substance is a sensitiser (notably flour and wood dust), best practice is to keep exposure as far below the WEL as reasonably practicable rather than treat the limit as a target.
Common findings
Inhalable dust monitoring frequently identifies the same recurring problems across UK industry: bag-tipping and weighing operations that release a brief but high-concentration cloud; sanding tasks performed without on-tool extraction; dry sweeping and compressed-air cleaning re-suspending settled dust; and visible dust accumulation on horizontal surfaces upstream of the breathing zone.
These findings often translate into straightforward control improvements — enclosure of weighing, on-tool extraction at sanding stations, replacement of dry sweeping with HEPA vacuuming, and changes to cleaning regimes.
Control considerations
Control of inhalable dust follows the COSHH hierarchy. Engineering controls — enclosure, local exhaust ventilation at the point of release, on-tool extraction — should take precedence over administrative measures and respiratory protective equipment. For sensitisers such as flour and hardwood dust, control must be both engineering-led and verified by monitoring; reliance on RPE alone is not an acceptable long-term position.
Frequently asked questions
What size particles are inhalable?
The inhalability convention in ISO 7708 and BS EN 481 includes particles up to around 100 µm aerodynamic diameter, with a 50% sampling efficiency at that size. In practice this captures the full range from fine respirable particles up to visible coarse dust able to enter the nose or mouth.
What is the UK WEL for general inhalable dust?
EH40 sets a general 'inhalable dust' WEL of 10 mg/m³ as an 8-hour time-weighted average. Substance-specific limits — for example hardwood dust at 3 mg/m³ inhalable 8-hour TWA — override the general limit and must be used where they apply.
Is an IOM sampler the only acceptable inhalable head?
The IOM sampler is the most widely used UK reference, but other heads that conform to the inhalability convention are acceptable. The critical requirement is that the sampling head implements the inhalable convention specified by EH40 for that substance.
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