For decades, the global standard for measuring indoor air quality (IAQ) has focused on PM2.5, particulate matter with an aerodynamic diameter of 2.5 micrometers or less. However, recent findings from the Particulate Matter Working Group suggest that this mass-based metric may be overlooking the most significant health risks. By synthesizing expert survey data and recent literature, we can now map the critical landscape of submicron and ultrafine particles.

Based on the analysis of the survey responses from our experts in the Particulate Matter Working Group, here is the synthesis of findings for the two categories Submicron and Ultrafine particles.

Submicron Particles (less than 1.0 μm)

• Importance and Necessity: The expert working group is evenly divided on the priority of monitoring. 47 percent of respondents believe including submicron monitoring is important based on empirical evidence, while 47 percent feel the necessity is still unclear and requires further study.
• Health Risk versus PM2.5: There is strong consensus (95 percent) that measuring sub-micron particles is a superior indicator of health impacts compared to the standard PM2.5 metric. Experts noted that these particles represent the vast majority of the total particle count and are the most likely to reach deep lung tissue or enter the bloodstream.
• Technological Readiness: 42 percent of experts state that current technology is not yet sufficiently cost-effective or accurate for wide adoption. While research-grade instruments provide high accuracy, they are often too expensive and complex for routine indoor air quality monitoring.
• Preferred Metrics: Most experts (58%) recommend reporting in particles per cubic centimeter (p/cm³). Some also suggest using micrograms or nanograms per cubic meter to maintain compatibility with existing regulatory standards.
• Commonly Used Instruments: Technologies cited include the Grimm 1.108, Alphasense OPC-R3, TSI P-Trak, and various optical particle counters.

The experts have shared a variety of research papers and literature regarding submicron particles, ranging from toxicological studies to real-world indoor environment assessments.

Barbier, E., et al. (2023). Oxidative stress and inflammation induced by air pollution-derived PM2.5-0.18 persist in the lungs of mice after cessation of their subchronic exposure. Environment International, 181, 108248.

Stratigou, E., et al. (2022). Using real time measurements to derive the indoor and outdoor contributions of submicron particle species and trace gases. Toxics, 10(4), 161.

Stratigou, E., et al. (2020). PM10, PM2.5 and PM1 in an unoccupied low-energy building: how and when can a mass balance equation approach be used? Building and Environment, 186, 107357.

Sotty, J., et al. (2019). Toxicological effects of ambient fine (PM2.5-0.18) and ultrafine (PM0.18) particles in healthy and diseased 3D organo-typic mucocilary-phenotype models. Environmental Research, 176, 108538.

Saleh, Y., et al. (2019). Exposure to atmospheric ultrafine particles induces severe lung inflammatory response and tissue remodeling in mice. International Journal of Environmental Research and Public Health, 16(7), 1210.

Chou, L. T., et al. (2025). Real-time Assessment of PM1.0-bound Trace Metal Cancer Risk through High-Resolution GED-ICP-MS and Integrated ELCR Analysis. Journal of Hazardous Materials, 138711.

Chou, L. T., et al. (2023). Particle size matters: Discrepancies in the health risks posed by traditional cigarettes and e-cigarettes in mice and humans. Journal of Hazardous Materials Letters, 4, 100088.

The Lancet Planetary Health (2025). Global research on air pollution health risks (PIIS2542-5196(25)00094-4).

Ultrafine Particles (UFP, less than 100nm)

• Importance and Necessity: More than half of the group (53 percent) considers ultrafine particle monitoring important. They emphasize that mass-based sensors (like PM2.5) often fail to detect major spikes in particle counts from indoor sources like 3D printers, laser printers, or woodsmoke.
• Health Risk versus PM2.5: 74 percent of respondents agree that ultrafine particle counts provide a more accurate health diagnostic than PM2.5. Experts highlighted that standard PM2.5 detectors often have no sensitivity for particles smaller than 300 nanometers.
• Technological Readiness: 53 percent of the group feels that technology for ultrafine particles is not yet advanced enough for mass-market use. The high cost of specialized monitors (often ranging from 5,000 to 15,000 Euros) is cited as a significant barrier.
• Preferred Metrics: The dominant recommendation (68%) is particles per cubic centimeter (p/cm³). Some experts also advocate for Lung-Deposited Surface Area (LDSA) as a more relevant metric for assessing toxicological impact.
• Commonly Used Instruments: Experts reported using TSI Condensation Particle Counters (CPCs), Scanning Mobility Particle Sizers (SMPS), nanoDUST AirPN10, and AethLabs monitors.

Based on the expert survey, the following research papers and literature were specifically shared regarding ultrafine particles (UFPs). These focus on the unique physicochemical properties, health impacts, and challenges in monitoring nanoparticles smaller than 0.1 micrometers.

Kwon, H. J., et al. (2020). Ultrafine particles: unique physicochemical properties relevant to health and disease.Experimental & Molecular Medicine, 52(3), 318–328.

Schraufnagel, D. E. (2020). The health effects of ultrafine particles. Experimental & Molecular Medicine, 52(3), 311-317.

Chang, P. K., & Hsiao, T. C. (2026). Evaluation of Spatial Distribution of Cancer Risk Induced by Traffic-Related Ultrafine Particles within an Urban Microenvironment. Journal of Hazardous Materials, 141024.

Chang, P. K., et al. (2023). Investigating the invisible threat: An exploration of air exchange rates and ultrafine particle dynamics in hospital operating rooms. Building and Environment, 245, 110870.

Chen, T. L., et al. (2024). A traffic-induced shift of ultrafine particle sources under COVID-19 soft lockdown in a subtropical urban area. Environment International, 187, 108658.

Chen, T. L., et al. (2023). Source-oriented risk and lung-deposited surface area (LDSA) of ultrafine particles in a Southeast Asia urban area. Science of the Total Environment, 870, 161733.

Chen, T. L., et al. (2023). A mobile platform for characterizing on-road tailpipe emissions and toxicity of ultrafine particles under real driving Conditions. Environmental Research, 216, 114523.

Wu, C. A., et al. (2023). Ultrafine particles in urban settings: A combined study of volatility and effective density revealed by VT-DMA-APM. Atmospheric Environment, 312, 120054.

Jung, C. R., et al. (2023). A hybrid model for estimating the number concentration of ultrafine particles based on machine learning algorithms. Environment International, 175, 1079.

Lin, T. C., et al. (2022). Deployment of a mobile platform to characterize spatial and temporal variation of on-road fine particles in an urban area. Environmental Research, 204.

Systematic Review (2023). Indoor exposure to ultrafine particles related to domestic activities: A systematic review and meta-analysis. Available via PubMed: 37690752.

Stabile, L., & Caracci, E. (2026). Indoor air quality and submicrometric particles. In New Perspectives in Indoor Air Quality, Elsevier, pp. 47-64.

Dimitroulopoulou, S., et al. (2026). Indoor air quality legislation: A focus on the current standards. In New Perspectives in Indoor Air Quality, Elsevier, pp. 7-18.

Additional Recommendations and Digital Resources

WHO Background Documents: Experts highlighted the importance of World Health Organization documents regarding nanoparticle health outcomes and cardiovascular links.

Lung Deposition Studies: Literature specifically discussing the systemic effects and lung deposition efficiency of UFPs.

CR-03/2006 (Spanish Technical Guide): Toma de muestras de aerosoles. Muestreadores de la fracción inhalable de materia particulada.

Next Steps

Moving forward, our next priority is to translate these expert insights into actionable protocols within our framework. To achieve this, we will soon convene a virtual meeting with the Particulate Matter Working Group. The primary objective of this session will be to deliberate on the analyzed findings and determine the most effective strategy for integrating submicron and ultrafine particle information into GO AQS. Stay tuned!

A special thank you to all members that participated in the survey!