Particulate matter is the sum of all solid and liquid particles suspended in air, many of which are hazardous. Some particulates occur naturally, originating from volcanoes, dust storms, and sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes, also generate significant amounts of particulates.
PMx are the particles having a diameter of x µm (commonoly assessed ranges for x are 1 micron or less, 2.5 microns or less and 10 microns or less), and are a complex mixture of organic and inorganic substances such as ammonia, black carbon, mineral dust, nitrates, sodium chloride, sulphate, and water either in liquid or in solid form.
Among the different size classes of particulate matter, PM2.5 (particulate matter with an aerodynamic diameter <2.5µm) shows the strongest and most consistent association with adverse health effects, as air particles decrease in aerodynamic diameter they can penetrate deeper into human lungs, and these small particles have longer residence time in the body.