"Particulate matter," also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles. Minute solid and liquid particles are suspended in a gas. The term aerosol is also used as a synonym. The particulates dispersed in the atmosphere are of different origin, both natural and anthropogenic. The most important natural sources are dust, volcanoes, forest fires and marine spray. Particulates originating from human activities currently amount to approx. 10% of the total amount and derive mainly from the use of fossil fuels in vehicles and power plants. The classification of atmospheric particulates is generally done in terms of aerodynamic diameter. A particle with an aerodynamic diameter of 1 micron (1μm) moves in the air like a sphere of density 1g/cm3 of diameter equal to 1 μm. The commonly used notation PM10 indicates particulates of aerodynamic diameter less than or equal to 10 μm, and similarly for PM2,5 ( 2,5 μm) and PM1 ( 1 μm). Particles with a diameter of less than 100 nm ( 0,1 μm) are called ultrafine particulates (UFP, Ultra fine particles). All particulates absorb and diffuse sunlight, in percentages that depend on the type of substances that compose them, and can therefore have an effect on the climate by changing the transmissibility of the atmosphere to solar radiation. However, the overall effect in terms of heating or cooling is not yet clear due to the complexity of the interactions involved and the different behaviour of the different types of particulates. 

The effect on human health is less controversial. From this point of view the main discriminating one is the size of the particulates: the larger particles are filtered into the nose and into the first part of the respiratory system, and do not cause major damage. The smallest particles (PM10 and lower) can reach the bronchi, while the UFP can pass from the lungs to the organism. Particulate inhalation was statistically linked to an increased incidence of asthma, lung cancer and cardiovascular disease. 

SOURCES : United States Environmental Protection Agency, Treccani.

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