Particulate matter is binned by aerodynamic diameter. The bins matter because deposition in the respiratory tract is size-dependent. PM10 (anything up to 10 µm) is captured mostly in the nose and upper airway. PM2.5 reaches the small bronchi and alveoli. PM1 and the ultrafine fraction below 0.1 µm cross from alveoli into the bloodstream within minutes. Same sensor, four bins, very different health profiles.
Sources have characteristic size distributions. Pollen, dust-mite debris, and pet dander dominate the coarse fraction (PM10 with little PM2.5). Cooking, candles, fireplaces, and wildfire smoke peak in the PM2.5 range with a strong PM1 tail. Diesel exhaust, 3D-printer plumes, and gas-burner UFP push primarily into the ultrafine bin. When you see PM10 rise without PM2.5, the source is coarse and probably indoor. When you see PM2.5 rise without PM10, the source is combustion and may be outdoor.
The mass-vs-count distinction matters too. PM mass concentration (µg/m³) is what regulators use; PM number concentration is what biology often responds to. A million PM10 particles by mass is dominated by a few hundred large ones; a million PM1 particles by mass involves a billion small ones. Ultrafines are dominant by number even when negligible by mass; this is why a single 3D printer can affect health more than its mass concentration suggests.
The interpretation layer reads the cascade together. The ratio of fine to coarse, the ratio of PM1 to PM2.5, the presence or absence of UFP-implied signal in the PM1 trace: each is diagnostic. PM10, PM2.5, and ultrafines each have their own articles; this one exists to put them in one place when the question is comparative.
