Sub-micrometre particles are optically active. Such large particles tend to have very short residency times. Table 1 summarises properties of atmospheric aerosols. From a meteorological perspective, PM 10 will generally remain entrained in the atmosphere until some process acts to change the regional air mass as a whole, at which point they are either moved from an area or removed from the atmosphere. Such processes include the passage of a frontal or pressure system.
Fronts define the boundaries between air masses.
Therefore, when they pass through an area they bring with them a new airmass. Generally, these particles are not associated with human health impacts. One obvious exception is pollen, which triggers allergic reactions among many people. Therefore, environmental scientists and regulators are concerned with the PM 10 subset of total suspended particulate. These particles contain materials common to the earth's crust and the ocean, reflecting that natural sources are contributors to the coarse fraction.