PM stands for particulate matter: solid or liquid particles suspended in air. PM10 and PM2.5 refer to particles with aerodynamic diameters of 10 and 2.5 micrometres or smaller, respectively.
Exposure to fine particulate matter is associated with cardiovascular and respiratory disease. Particle composition and sources vary; a PM reading is not a diagnosis or a measure of an individual's exposure.
Concentrations are reported in micrograms per cubic metre (µg/m³) over a stated averaging period. Always check that period beside a reading; values averaged over different periods are not interchangeable.