These units express the concentration of air pollution in terms of the mass or volume of the pollutant, and they are commonly used for measurements of both gaseous pollutants, such as nitrogen dioxide, and coarse (PM10) and fine (PM2.5) particulates. An alternative measurement for particulates, particle number, expresses the concentration in terms of the number of particles per volume of air instead, which can be a more meaningful way of assessing the health harms of highly toxic ultrafine particles (PM0.1, less than 0.1 μm in diameter).
