Particulate matter (PM2.5 and PM10)

Particulate matter is airborne solid and liquid particles, ranging from dust and pollen down to combustion soot, classified by size because size determines how deeply they penetrate the respiratory system. PM10 refers to particles 10 micrometers or smaller in diameter, PM2.5 to the finer fraction 2.5 micrometers or smaller, which can reach deep into the lungs and bloodstream and carries the strongest evidence of health harm.

Measurement comes from reference-grade gravimetric monitors, which physically weigh collected particles and are treated as the accuracy benchmark, and increasingly from low-cost optical sensors that estimate concentration from light scattering; the latter are cheaper and denser but drift and read differently in humid or smoky conditions, which is why sensor calibration against a reference monitor is standard practice before trusting a low-cost network's readings. PM2.5 and PM10 are two of the pollutants combined into a broader air quality index alongside ozone, nitrogen dioxide and other gases.

Particulate matter data drives public health alerts, wildfire smoke tracking, and industrial and transportation emissions regulation, since PM2.5 exposure is linked to cardiovascular and respiratory disease even at levels below older regulatory thresholds. A common pitfall is comparing readings across sensor types or brands without accounting for calibration differences, and treating a single monitoring station's reading as representative of an entire city when particulate concentrations can vary sharply by street and by hour, especially near traffic or fire sources.