Fine particulate matter or PM2.5 is one of the main air pollutants. All PM particles are particulate matter containing a mixture of solid and liquid elements suspended in the air, including particles of dust, metals, cement and organic matter. They are classified by particle size, measured in micrometres (thousandths of a millimetre): the smaller the particles, the more easily they can penetrate our bodies. PM2.5 are fine particles with a diameter between 0.1 and 2.5 µm. They can penetrate the respiratory system as far as the pulmonary alveoli. One component of PM2.5 is black carbon, a product of incomplete combustion, especially of diesel fuel. The main sources of PM2.5 in cities include traffic, local fuel combustion from household heating, industrial combustion, and wood and coal burning.