An abbreviation for particulate matter. Often presented with a subscript, for example PM₁₀, which refers to particulates that have an aerodynamic diameter of less than 10 micrometres (10 μm). These pose respiratory health concerns as they can be inhaled and can penetrate the lower reaches of the lungs, and therefore accumulate within the respiratory system. PM_2.5 are particulates of less than 2.5 micrometers and are referred to as fine particles, which can present severe respiratory problems leading to respiratory, pulmonary, and cardiovascular diseases. See particulates.