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What Is Particulate Matter (PM)?

Solid particles and aerolized liquids that are present in air. PM is categorized by size, with particles small enough to be inhaled being of most concern. Inhalable PM is categorized as PM10 (between 2.5 and 10 microns in diameter), PM2.5 (between 1 and 2.5 microns), and PM1 (particles smaller than 1 micron in diameter).

Definition of Weather Terms - Full List