What is particulate matter?

Particulate matter has the capacity to affect human health and to cause nuisance effects and is categorised by size and/or by chemical composition. The potential for harmful effects depends on both. In air quality assessments, particulate size ranges are commonly described as:

• TSP – refers to all (total) suspended particles in the air. In practice, the upper size range is typically 30 micrometres (μm) to 50 μm.
• PM10 – refers to all particles with equivalent aerodynamic diameters of less than 10 μm.
• PM2.5 – refers to all particles with equivalent aerodynamic diameters of less than 2.5 μm diameter. These are often referred to as ‘fine’ particles and are a subcomponent of PM10.

Deposited dust – refers to particulate matter that has settled out of the air and is measured as a dust deposition rate, which is dust settled out over a given area and time under the influence of gravity. Deposited dust can include particles of any size, but it generally comprises particles larger than 20 µm in diameter that rapidly settle out of the air near the point of emission. It is measured to assess if an emission source is causing a nuisance, such as soiling of property and materials.

Both natural and anthropogenic processes contribute to the atmospheric load of particulate matter. Coarse particles (PM2.5-10) are derived primarily from mechanical processes, resulting from the suspension of dust, soil, or other crustal materials from roads, farming, quarrying, dust storms, and so forth. Coarse particles also include sea salts, pollen, mould, spores, and other plant parts.

Fine particles, or PM2.5, are derived primarily from combustion processes, such as vehicle emissions, wood burning, gas, diesel or coal burning for power generation, hazard reduction burns and bush fires. Fine particles also consist of transformation products, including sulphate and nitrate particles, and secondary organic aerosols formed from volatile organic compound emissions.

The size of particles determines their behaviour in the respiratory system, including how far the particles can penetrate, where they deposit, and how effective the body's clearance mechanisms are in removing them. Additionally, particle size is an important parameter in determining the residence time and spatial distribution of particles in ambient air, which are key considerations in assessing exposure.