Particle pollution refers to a mixture of solid particles and liquid droplets found in the air. Some particles such as dust, dirt, soot or smoke are large or dark enough to be seen with the naked eye, while other fine particles, such as acids, organic chemicals, metals, soil and dust particles and allergens, are so small they can only be detected by using an electron microscope.
With diameters of 2.5 micrometers and smaller, the particles are a size nearly 30 times smaller than a single strand of human hair.