Ultrafine particles (UFP) are airborne particles with a diameter of 0.1 µm (100 nm) or less, classified as one of the most significant emerging atmospheric pollutants in terms of public health in Europe, as their tiny size enables them to penetrate deep into the lungs and easily enter the bloodstream.
What are PM0.1 ultrafine particles? Definition
Ultrafine particles are generally defined as those with an aerodynamic diameter of 0.1 µm (100 nm) or less; they are also referred to as PM0.1.
The European Commission and the WHO classify UFP as part of the ‘fine fraction’ (PM2.5), but specifically distinguish particles smaller than 0.1 µm as ultrafine particles due to their unique physical and biological properties.
Although they contribute very little mass to PM2.5 or PM10, UFP typically account for more than 90 per cent of the total number of particles present in ambient air, which means that, from an exposure perspective, they dominate by ‘number’ rather than by ‘weight’.
Differences between PM10, PM2.5 and PM0.1
| Fraction | Typical diameter | Main metric | Main sources | Regulatory status |
| PM10 | ≤ 10 µm | Mass (µg/m³) | Dust resuspension, construction work, agriculture | Consolidated limit values in the EU (Directives 2008/50/EC and 2024/2881). |
| PM2.5 | ≤ 2.5 µm | Mass (µg/m³) | Combustion (traffic, industry, biomass) | Stricter limit values in Directive (EU) 2024/2881. |
| PM0.1 / UFP | ≤ 0.1 µm | Number (particles/cm³) | Road traffic, aviation, ports, industrial and domestic combustion | No specific limit values; Directive 2024/2881 requires monitoring at supersites. |
Recent scientific reviews emphasise that, due to their small size, high specific surface area and ability to adsorb metals and reactive organic compounds, UFPs may be more toxic than larger particles of the same mass.
Reports by experts from the UK and the WHO highlight that there is still no specific air quality standard for UFPs, but recommend reducing their emissions – particularly from diesel engines and combustion sources – due to their potentially significant contribution to the adverse effects associated with PM2.5.
Source and formation of ultrafine particles
UFPs are generated both primariily – emitted directly by combustion processes and industrial activity – and secondarily – formed in the atmosphere through the nucleation and condensation of precursor gases.
In European urban environments, most UFP is of anthropogenic origin, primarily from road traffic – more specifically from diesel and direct-injection petrol engines – and from ‘new particle formation’ processes involving precursor gases such as SO₂ and volatile organic compounds.
AQEG reports for the United Kingdom show that, in terms of particle numbers, PUFs originate largely from road transport but also from coal combustion in thermal power stations, maritime shipping, aviation (in the vicinity of airports) and domestic wood burning.
Urban and industrial sources of UFP
- Road traffic (diesel and GDI petrol engines), particularly on major urban roads.
- Aviation in airport surroundings, where take-offs and landings generate UFP peaks.
- Seaports and river ports, due to marine fuels containing sulphur and auxiliary engines.
- Coal-fired power stations and other combustion facilities in the energy sector.
- Biomass combustion (forest fires, agricultural burning and domestic heating with firewood).
Indoor sources (workplace and domestic environments)
- Cooking and frying in the kitchen.
- Tobacco and e-cigarettes.
- Candles, incense and decorative burning.
- Laser printers and photocopiers in offices.
- Electronic equipment with brush motors and certain household aerosols.
Epidemiological studies in Europe and Canada have linked UFPs from road traffic and aviation to increases in premature mortality, particularly in areas close to major roads, airports and industrial zones.
The study by Kwon et al. shows that, despite improvements in particulate filters and low-sulphur fuels, the condensation of semi-volatile compounds in exhaust gases can generate additional UFPs, with size distributions centred on the 10–30 nm range, particularly in modern combustion technologies.
Health effects: why is it critical to monitor UFPs?
UFPs can be deposited deep within the pulmonary alveoli, bypass respiratory defences, cross the alveolar-capillary barrier and be distributed throughout the body, contributing to respiratory, cardiovascular and possibly neurological diseases.
Both the WHO and working groups in allergology and pulmonology report that PUFs induce oxidative stress and inflammation in the airways, promote asthma exacerbations and may act as adjuvants in sensitization to allergens, increasing the allergic response.
Their large surface area enables them to carry significant quantities of polycyclic aromatic hydrocarbons, metals and other redox-active compounds that generate reactive oxygen species, thereby amplifying damage to the pulmonary epithelium and vascular endothelium.
Main described effects of ultrafine particles
- Respiratory system
- Asthma exacerbation and an increase in A&E visits due to asthma attacks.
- Inflammation of the airways, reduced lung function and increased bronchial hyperreactivity.
- Possible role in the development of chronic obstructive pulmonary disease (COPD).
- Cardiovascular system
- Alterations in heart rate variability and microvascular dysfunction.
- Promotion of atherosclerosis and ischaemic events.
- Increased levels of markers of systemic inflammation and oxidative stress.
- Other systems
- Oxidative damage to DNA and possible effects on foetal development (low birth weight).
- Emerging evidence of PUF deposition in the brain and an association with neurodegenerative diseases, although this remains uncertain.
The WHO’s 2021 reviews conclude that, although the evidence suggests increasing harmful effects as particle size decreases, the available studies on UFPs remain fewer in number and more heterogeneous than those on PM2.5, which prevents the establishment of specific limit values at this stage.
Nevertheless, reports such as that by AQEG note that UFPs “are believed to contribute to the toxicity of particulate matter in the air”, with the extent of this contribution still quantitatively “unclear”, reinforcing the need for monitoring and further research.
Why do ultrafine particles require specific monitorig?
Monitoring UFPs is key because their behaviour cannot be inferred from traditional PM2.5/PM10 measurements; they are concentrated in specific hotspots (roads, airports, ports, etc.); and the relevant metric is particle number, not mass.
European air quality networks have historically been designed to measure PM mass and regulated gases, but technical reports show that UFPs exhibit distinct spatial and temporal patterns: they decline very rapidly with increasing distance from the source, exhibit intense short-lived peaks, and their concentrations can vary by several orders of magnitude within seconds.
Furthermore, the correlation between PM2.5 mass and UFP number is weak: it has been observed that particle number concentration (PNC) correlates better with NOx and black carbon, which reinforces the view that PM2.5/PM10 are not good substitutes for assessing exposure to UFPs.
- To identify exposure hotspots near major roads, ports and airports, where the population may be exposed to levels much higher than those recorded by urban background monitoring stations.
- To assess the actual impact of abatement technologies (diesel particulate filters, low-sulphur fuels, engine improvements) on the number and size of UFPs.
- To inform epidemiological studies that distinguish the effects of UFP from those of other PM fractions.
- Design action plans and air quality roadmaps in line with the new Directive (EU) 2024/2881.
The AQEG report highlights that only three continuous UFP monitoring sites were operational in the UK (roadside, urban background and rural background), which is considered insufficient to understand emissions from aviation, shipping and biomass.
The new Directive 2024/2881 expressly acknowledges this gap and requires Member States to monitor UFP at monitoring supersites, both urban and rural, particularly in areas influenced by road, air, maritime and inland waterway transport.
Instrumentation for monitoring ultrafine particles
Monitoring ultrafine particles requires specific instrumentation capable of detecting particles in the nanometre range and reporting concentrations in terms of particle numbers per cubic centimetre, not just by mass.
Traditional mass-based PM instruments and gravimetric filters are not suitable for UFPs, as these have very low mass and are subject to forces other than gravity, such as Brownian diffusion and thermophoresis.
In advanced monitoring networks and supersites, the combination of condensation particle counters (CPCs), electrical mobility analysers (SMPS/MPSS) and miniaturised CPC solutions (MEMS, microfluidic or portable) enables the characterisation of both the total concentration and the size distribution of UFPs.
CPCs, for example, cause ultrafine particles to ‘grow’ through the condensation of a vapour (e.g. alcohol or water) to micrometre-scale sizes, and then count them individually using light-scattering optics, reporting concentrations in particles/cm³.
On the other hand, electric mobility systems (SMPS/MPSS) electrically classify particles before counting them with a CPC, yielding detailed size distributions in the approximate range of 2–1,000 nm, which are essential for distinguishing between nucleation, growth and accumulation modes.
Furthermore, significant progress has been made in miniaturised CPCs based on MEMS and microfluidic technology, capable of detecting UFPs of 3–15 nm using highly compact, low-power equipment, suitable for portable and multi-point monitoring.
Summary table of specific instrumentation for monitoring UFPs
| Type of instrument | Principle | Typical size range | Key metric | Features relevant to UFPs |
| Reference CPC (Condensation Particle Counter) | Condensation of vapour (water, butanol, isopropanol, etc.) onto the particles, growth into micrometre-sized droplets and individual optical counting | Depending on the model, from ~1–2.5 nm to ~1,000 nm | Number of particles/cm³ | High sensitivity for UFPs; models such as the TSI 3756 Ultrafine CPC and the WCPC 3785/3786 detect particles of ~2.5–5 nm, and the CPC 3750-CEN10 is designed for UFP monitoring in accordance with EN 16976:2024. |
| SMPS / MPSS (Scanning / Mobility Particle Sizer) | Particle classification by electrical mobility and downstream CPC counting | ~2–1,000 nm | Size and number distribution (size spectra) | These enable the determination of the size spectrum of UFPs and nanoparticles, which is key to studying nucleation, growth and coagulation processes, and is to distinguishing the contributions from traffic, aviation, industry or biomass. |
| OPC (Optical Particle Counter) | Laser light scattering by each particle passing through the beam | ~100 nm – several µm (depending on design) | Approximate number and size in discrete channels | Less sensitive in the ultrafine range; useful for covering the range above 100 nm and as part of hybrid systems (e.g. MEMS-CPC + mini-OPC) in compact devices. |
| Miniaturised CPCs (MEMS, microfluidic, portable) | MEMS or microfluidic chip integrating a saturator and condenser, condensation control (usually with water) and a mini-OPC for droplet counting | Approximately ~3–15 nm (depending on design); some prototypes demonstrate detection down to 3.4 nm | Particle count per cm³ in portable or multi-point configurations | These miniaturise the CPC principle into very compact devices (e.g. 75 × 130 × 50 mm, ~200 g, ~2.7 W), with high agreement with reference instruments and the capability for portable and multi-point monitoring of UFPs. |
At Envira, we work with specialist nanoparticle manufacturers such as Palas, whose nanoparticle measurement systems enable the determination of the number concentration and size distribution of ultrafine aerosols ranging from approximately 2–4 nm to 1,000 nm. Their CPCs (such as the UF-CPC and ENVI-CPC systems) rely on condensation to enlarge the particles so that they can be counted individually, whilst the U-SMPS and Charme analysers combine electrical mobility classification with condensation counting to generate detailed size distribution spectra.
This type of instrumentation fits naturally into supersites and advanced air quality monitoring stations, where it is necessary to characterise both the total concentration of PUFs and their size distribution.
In addition to CPCs and electric mobility systems, there are solutions based on diffusion charging and electrometric measurement that enable the continuous monitoring of ultrafine particles with very little maintenance. One example is the Pegasor Airam, which combines the Pegasor PPS-G2 sensor with a continuous-flow design to measure, in real time, particle number (PN), lung-deposited surface area (LDSA), particle mass and particle size in a single system, and is specifically designed for outdoor air quality monitoring.
As it is based on ‘leakage current’ technology, charged particles flow through the sensor without accumulating, avoiding the use of working fluids and minimising maintenance requirements, making it a highly suitable tool for extensive monitoring networks and long-term applications in urban and industrial environments.
Comparison of Envira’s nanoparticle monitoring equipment
| Manufacturer | Equipment | Main measurement principle | Typical size range (approx.) | Key parameters | Typical use in networks/air quality |
| Palas | DEMC | Electric mobility classifier (part of an SMPS) | ~4–600/1,400 nm (depending on configuration) | Monodisperse size (mobility) selection for subsequent counting with a CPC | Central module in U-SMPS systems; detailed characterisation of UFP size distributions in laboratories and reference stations. |
| Palas | Charme | Faraday cup electrometer (measurement of current from charged aerosols) | ~2 nm – 10 µm | Electric current, derived number concentration (if the charge state is known) | Rapid measurement of high concentrations, calibration and verification of CPCs and SMPS systems against SI-traceable quantities. |
| Palas | U‑RANGE 2000 | U-SMPS hybrid system (DEMC + UF-CPC) + Fidas 200 optical monitor | ~4–8 nm – 40 µm | Number size distribution (UFP), PM1, PM2.5, PM10 and total mass | Supersites and advanced stations requiring simultaneous measurement of UFP (by number) and regulated PM fractions (by mass). |
| Palas | AQ Guard Smart 2000 | Diffusion charging + electrometric measurement (LDSA) | From ~10 nm | Particle number (PN), LDSA (lung-deposited surface area), mean size, environmental variables | 24/7 outdoor UFP monitoring (airports, roads, industry, smart cities) with very low maintenance and no working fluids. |
| Pegasor | Pegasor Airam | Sensor PPS‑G2 de carga por difusión y “corriente de fuga” (diffusion charging) | UFP in the nanometre range (manufacturer’s specifications) | Particle number (PN), LDSA, particle mass, size, environmental variables | Standalone outdoor air quality monitor (urban areas, traffic, airports, industrial environments), designed for large-scale networks and long-term applications. |
Monitoring supersites and Directive (EU) 2024/2881: how do they relate to UFPs?
Monitoring supersites are advanced stations designed to provide an integrated view of pollution in representative areas, in accordance with Directive (EU) 2024/2881, where high-quality instrumentation for multiple pollutants, including UFPs, black carbon and ammonia, is grouped together.
Directive (EU) 2024/2881 calls for an expansion of monitoring networks and, in particular, the establishment of urban and rural background supersites where the concentration and size distribution of UFPs and black carbon are measured, especially in areas influenced by air, sea, river or road transport.
According to the analysis in this directive, the emerging pollutants to be measured are:
- Ultrafine particles (UFPs): concentration and size distribution.
- Black carbon ( black carbon )
- Ammonia (NH₃) at rural supersites.
- Oxidative potential of particles, levoglucosan, and nitric acid (recommended)
The establishment of these supersites will also enable the validation of UFP models (including nucleation, condensation and coagulation processes) and support the design of air quality roadmaps, including any potential requests for a postponement of the 2030 targets, which must be justified with comprehensive information.
Conclusion
In just a few years, ultrafine particles have gone from being a virtually invisible parameter to becoming one of the key focal points of air quality in Europe, both due to their potential impact on health and the new regulatory framework driving their systematic monitoring. Directive (EU) 2024/2881 explicitly recognises UFPs as an emerging pollutant, requires their measurement at urban and rural background super-sites, and incorporates this information into the path towards the ‘zero pollution’ target for 2050.
In this context, monitoring UFPs is no longer merely a matter of advanced research, but an operational necessity for public authorities and businesses that must demonstrate due diligence in protecting public health and complying with new European obligations.
As specialists in air quality monitoring networks and campaigns, here at Envira we are well-placed to support public authorities and industry in this new landscape: by designing supersites that incorporate UFP monitoring, deploying specific stations and campaigns where they are most needed (traffic, airports, ports and industrial centres), and incorporating ultrafine particle data into strategic decision-making.
Taking the step towards UFP monitoring today means making significant progress in protecting public health and adapting early to the regulatory framework that will shape air quality in Europe over the coming decades.
Sources and references
- European Commission. Glossary: Fine particles – Ultrafine particles. Available at: https://ec.europa.eu/health/scientific_committees/opinions_layman/en/indoor-air-pollution/glossary/def/fine-particles-ultrafine-particles.htm (accessed 7 July 2026).
- UK Department for Environment, Food and Rural Affairs (Defra) – Air Quality Expert Group. Ultrafine Particles (UFP) in the UK. 2018. Available at: https://uk-air.defra.gov.uk/assets/documents/reports/cat09/1807261113_180703_UFP_Report_FINAL_for_publication.pdf (accessed 7 July 2026).
- Kwon H-S, et al. Ultrafine particles: unique physicochemical properties relevant to health. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC7156720/ (accessed 7 July 2026).
- Nel A, et al. Ultrafine particles: biological effects (AAAAI Work Group). Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC4976002/ (accessed 7 July 2026).
- Ultrafine particle. Available at: https://en.wikipedia.org/wiki/Ultrafine_particle (accessed 7 July 2026).
- CBC News. Ultrafine particles 2024. Available at: https://www.cbc.ca/newsinteractives/features/ultrafine-particles (accessed 7 July 2026).
- Yoo S-J, et al. Microelectromechanical-system-based condensation particle counter for sensitive and precise monitoring of airborne ultrafine particles (UFPs). Atmospheric Measurement Techniques, 2019. Available at: https://amt.copernicus.org/articles/12/5335/2019/ (accessed 7 July 2026).
