Refining particulate matter classification: Addressing health risks of smaller fractions

Particulate matter (PM) is a major global health risk. Current regulations focus on PM₁₀ and PM₂.₅, while unregulated smaller fractions such as PM₁ and PM0.1 pose greater risks. These small particles penetrate deeper into the respiratory system, remain airborne longer, carry more reactive chemicals, and therefore cause more severe health effects. Existing standards also address only outdoor PM, even though people spend most of their time indoors. And indoor exposure depends strongly on filtration performance and building envelope penetration. In response, this study proposes a first step toward a novel conceptual framework for PM classification. The framework combines evidence on health effects, filtration efficiency, and building penetration. It defines ten non-overlapping size bins between 0.01 and 10 µm. These bins reflect key transitions in toxicity, indoor infiltration, and removal efficiency. The framework also derives size-specific guideline values by interpolating WHO and EU standards. Results show that particles smaller than 1 µm, especially those near 0.2–0.3 µm, present the highest health risks. They also penetrate buildings and filters most easily. The new classification therefore adds essential detail to the submicron range while remaining compatible with existing PM categories. Because particles of the same size can differ in toxicity by orders of magnitude, the framework should be interpreted within its compositional context. It represents a foundation for a future hybrid size–composition system. The proposed framework offers a more precise size structure. It can support targeted mitigation, improved indoor-air-quality management, and more consistent regulation aimed at protecting public health.