Particulate matter, suspended particulate matter, atmospheric dust, particulate matter, total suspended particulate (TSP), are terms that commonly identify the group of substances suspended in the air (fibers, carbon particles, metals, silica, polluting liquid or solid).
Particulate matter is the pollutant that is now considered the greatest impact in urban areas, and is composed of all those solid and liquid particles dispersed in the atmosphere, with a diameter ranging from a few nanometers to 500 microns and above (ie billionths of a meter to half a millimeter).
In any combustion plant (from the boilers to incinerators until the engines of cars and trucks) a rise in temperature (below, however, an upper limit) improves the efficiency of combustion and should therefore decrease the total quantity of materials partially unburnt (therefore particulate).
The total amount of suspended dust is typically measured in a quantitative way (weight / volume). In the absence of environmental pollutants, dust content in the air reaches different concentrations (mg/m3) in different environments, it is generally low in high mountain areas, and increase as they move from the countryside to cities, industrial areas.
It uses a formal identification of the size, the Particulate Matter, abbreviated as PM, followed by the maximum aerodynamic diameter of the particles.
• coarse particulate – sedimenting particulate dimensions greater than 10 uM, not able to penetrate into the respiratory tract overcoming the larynx, if not in small part. 
• PM10 – particulate size from particles less than 10 microns (uM) (ie less than a hundredth of a millimeter), it is an inhalable powder, or able to penetrate into the upper respiratory tract (nose and larynx). Particles between about 5 and 2.5 uM are deposited before the bronchioles. 
• PM2, 5 – fine particles with diameter less than 2.5 microns (a quarter of a hundredth of a millimeter), is a thoracic dust, that is able to penetrate deeply into the lungs, especially during breathing from the mouth.
For even smaller (ultrafine particles, UFP or UP) refers to respirable dust, that is able to penetrate deeply into the lungs up to the alveoli and there are discrepancies between the sources with regard to their definition, although it is most common and accepted the definition of UFP as PM0, 1 rather than as PM1 (of which, however, are a subset):
• PM1, with diameter less than 1 mM
• PM0, 1, with a diameter of less than 0.1 mM
• nanopowders, with diameter of the order of magnitude of nanometers (one nanometer would PM 0.001).