Ultrafine Particles
PM Monitoring · PM Monitoring overview
Ultrafine particles (UFPs) are airborne particles with diameters below 100 nanometres. They arise from combustion, fume, hot processes and engineered nanomaterial handling, and they behave very differently from larger PM fractions. UK workplaces increasingly require dedicated assessment of ultrafine and nanoparticle exposure where these sources are present.
What ultrafine particles are
Ultrafine particles are conventionally defined as particles with an aerodynamic or mobility diameter below 100 nm (0.1 µm). They include both unintentional ultrafine aerosols — combustion particles, hot-process condensation aerosols and welding fume — and engineered nanomaterials produced or handled deliberately as nanoscale products.
Although ultrafine particles contribute very little to PM2.5 or PM10 mass, they can dominate the airborne number concentration by orders of magnitude. Their high number-to-mass ratio means they are not well represented by mass-based PM monitoring alone; number concentration is usually a more meaningful exposure metric.
Why particle size matters at the nanoscale
Below about 100 nm, particles behave differently in air and in the respiratory tract. They diffuse rapidly rather than settle, deposit efficiently throughout the entire respiratory tract including the alveolar region, and have much greater surface area per unit mass than larger particles. Surface area and number concentration are increasingly used as exposure metrics for UFPs because mass concentration substantially underestimates the dose for such fine aerosols.
Engineered nanomaterials add a further dimension: composition, shape, coating and agglomeration state all influence biological behaviour. UFP and nanoparticle assessment therefore needs to be planned around the specific aerosol present rather than as a generic dust measurement.
Workplace sources of ultrafine particles
Common UK workplace situations producing UFPs include:
- Welding, cutting and brazing — particularly stainless steel and coated materials.
- Soldering, laser cutting and laser engraving.
- Diesel engine exhaust in workshops, vehicle bays and indoor logistics.
- Hot processes producing condensation aerosols (metalworking, plastic moulding, hot pressing).
- Cooking emissions in commercial kitchens.
- Powder handling, weighing and dispersion of engineered nanomaterials.
- 3D printing and additive manufacturing with thermoplastic, metal or resin systems.
Ultrafine monitoring versus PM and COSHH
UFP monitoring complements rather than replaces other particulate measurements. PM2.5 and PM10 describe the mass of fine and coarse fractions but are insensitive to the very large number concentrations of ultrafine particles often present in fume environments. COSHH assessment under HSE MDHS 14/4 and EH40 quantifies specific substances against their own WELs and remains the legal basis for managing exposure to hazardous substances.
Where engineered nanomaterials are used, BS EN 17058 and the HSE guidance on nanomaterials inform the planning of structured assessments. The approach is normally to combine substance-specific COSHH sampling with number-concentration monitoring (and where justified, surface-area or size-distribution measurement) so that the full exposure picture is captured.
How ultrafine particle monitoring is approached
Workplace UFP monitoring typically uses condensation particle counters (CPC) to measure total particle number concentration, often supported by optical particle counters covering larger sizes, and where size distribution is needed, scanning mobility particle sizers (SMPS) or fast-response equivalents. Surface area monitors (e.g. NSAM/diffusion charging) are used where lung-deposited surface area is the chosen metric.
Sampling strategy is task-driven. Short, well-characterised deployments around defined operations — a welding task, a kiln charging cycle, an engine start-up, a nanomaterial weighing operation — are usually more informative than long undifferentiated shift records. Background measurements at quiet periods are essential because outdoor UFP infiltration can dominate apparent indoor concentrations.
Where engineered nanomaterials are in use, a structured nanomaterial exposure assessment that combines real-time UFP monitoring with substance-specific filter sampling and electron microscopy is the established approach.
Interpreting ultrafine results cautiously
UFP data is interpreted with particular care because there is no single agreed exposure limit equivalent to a WEL for ultrafine particles in general. Comparisons are normally made between exposed and background periods, between tasks, and against benchmark levels from comparable work environments. Significance is judged in the context of the source, the substance, and the controls in place.
Instrument selection matters: different instruments respond differently to the same aerosol. Results from CPCs, optical counters and surface-area monitors are not directly interchangeable and should be reported with the instrument and configuration that produced them.
Control considerations
Controls for ultrafine particle exposure follow the COSHH hierarchy. Substitution (e.g. lower-fume welding consumables, lower-emission soldering processes, water-based alternatives where applicable) is preferred where feasible. Enclosure and high-capture-velocity local exhaust ventilation are the principal engineering controls for fume, welding and hot processes; HEPA filtration on extracted air may be required where ultrafine release is significant.
For engineered nanomaterials, glovebox or ventilated enclosure handling is usually preferred over open weighing or dispersion. RPE for ultrafine and nanoparticle work is selected with attention to filter performance against the smallest particle sizes; tight-fitting RPE requires individual face-fit testing.
When to review ultrafine particle exposure
Dedicated ultrafine particle assessment is appropriate when introducing welding, additive manufacturing, hot processing or engineered nanomaterials; when investigating fume complaints that conventional dust sampling does not explain; where diesel engine exhaust is present indoors; and as periodic verification of installed fume control performance.
Frequently asked questions
Is there a workplace exposure limit for ultrafine particles?
There is no general substance-independent WEL for ultrafine particles in HSE EH40. Specific substances within an ultrafine aerosol (for example welding fume constituents) are still subject to their own WELs, and engineered nanomaterials are managed through structured assessment under COSHH.
Why isn't PM2.5 enough for ultrafine particles?
Ultrafine particles contribute very little to PM2.5 mass but can dominate airborne number concentration. PM2.5 monitoring will therefore often miss exposures that a number-concentration or surface-area monitor would identify.
Are nanoparticles and ultrafine particles the same thing?
There is substantial overlap. Ultrafine particles refers to any particles below about 100 nm regardless of origin, while nanoparticles is usually reserved for deliberately engineered nanoscale materials. Both share the same general size-related deposition and behaviour considerations.
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