Some are visible, such as dust, whereas others cannot be seen by the naked eye. Materials such as metals, microplastics, soil and chemicals can be in particulate matter. Particulate matter (or PM) is described in micrometres. The two main ones mentioned in reports and studies are PM10 (less than 10 micrometres) and PM2.5 (less than 2.5 micrometres). Air pollution comes from burning fossil fuels, cars, cement making and agriculture Scientists measure the rate of particulates in the air by cubic metre. Particulate matter is sent into the air by a number of processes including burning fossil fuels, driving cars and steel making.