Two sessions within the Particulate Matter (PM) Working Group have highlighted a critical shift in air quality monitoring: the move away from traditional mass-based measurements toward tracking the number and surface area of the smallest particles. While PM2.5 is currently well-regulated, the scientific community is increasingly concerned about submicron and ultrafine particles (UFP), which pose unique health hazards that mass-based metrics fail to capture.

Health Risks and Scientific Consensus

There is a high consensus among experts that submicron particles are a superior indicator of health impact compared to PM2.5. Key findings from the literature review shared with the group include:

Submicron particles penetrate deeper into the human body and have a stronger association with lung cancer, cardiovascular disease, and childhood pneumonia than PM2.5.

Ultrafine particles (UFPs) can bypass the alveolar-capillary barrier, distributing to major organs including the heart, brain, and liver, where they can cause severe inflammation.

Submicron particles are primarily byproducts of human activity, such as fossil fuel combustion and vehicle brake wear, making them more targetable for regulation than natural dust.

Technical and Economic Barriers

The working group identified significant challenges in widespread monitoring:

• Instrumentation: Many low-cost sensors are actually “poor” optical particle counters masquerading as mass measurements; however, they could count even if they struggle with precise sizing. An ISO 21501-4 compliance offers less uncertainty for these optical particle counters (mainly for submicron particles). When it comes to UFP the group noted the difficulty of performing continuous 24-hour monitoring in occupied indoor environments because the “instruments are noisy” and residents or occupants typically do not want them running in a real-life situation.

• Cost: Submicron sensors are relatively affordable €150–€1,500, but UFP instruments remain expensive, ranging from €5,000 to over €50,000.

Working Group Recommendations

The Particulate Matter Working Group members recommend several strategic shifts for researchers and policymakers:

1. Transition to Particle Number (PN) Metrics: The group predominantly recommends using particles per cubic centimeter (p/cm³) as the primary metric for both submicron and ultrafine particles.

2. Standardize Size Definitions:

    ◦ For submicron particles, the group suggests focusing on the PNC0.3 μm with a range from 0.3 to 0.49 μm and PNC0.5 μm with a range from 0.5 to 0.99μm as this is how it is defined by the cleanroom industry.

    ◦ For ultrafine particles, members advocate for a lower detection limit of 10 nm to align with modern automotive and atmospheric monitoring standards, but not less in the indoor environment. Keep in mind, in general, when we refer to UFP we generally talk about particles with a diameter of less than 0.1μm (100 nm). However, in some cases instruments can measure bigger size particles but they won’t add up significant more particle in the total particle number concentration.

3. Adopt Lung Deposited Surface Area (LDSA): Some members strongly recommend LDSA as a more toxicologically relevant metric that combines particle size and deposition fraction in the lungs. However, not all UFP instruments can determine the LDSA value as it requires the particle size and the total particle number. In many cases we have to calculate both the Alveolar LDSA and Total Lung LDSA.

4. Create a Roadmap for Monitoring: Instead of waiting for perfect technology, members suggest a tiered approach: use existing, affordable optical counters for number counts today while developing a roadmap for more sophisticated surface area and oxidative potential (OP) measurements in the future.

5. Collaborative Data Collection: The group invites the research community to share unpublished measurements from diverse indoor environments (schools, offices, homes) to help us and epidemiologists determine safe levels if any.

6. Clarify Nomenclature: To avoid confusion with mass-based “PM” standards, some members proposed using “Dp” (Diameter) or “Particle Number Count (PNC)” terminology to be explicit about what is being measured.

While setting specific regulatory limits remains difficult due to the lack of sufficient measurements and health data, the group concluded that consistent, repeatable metrology is the first step toward effective future regulation.

Next Steps

Please note that this information will be incorporated into the white paper soon.

Recordings

GO AQS PM Working Group – Submicron and UFP Option 1 (10 AM CET)

GO AQS PM Working Group – Submicron and UFP Option 2 (7 PM CET)