The unit of measure generally associated with filterable particles is microns. One micron equals 1/25,400 of an inch (or one millionth of a meter). Human hair for example has an average diameter of 90 microns. Particles smaller than this often times require the use of a microscope. However, even with a microscope, a particle less than 1 micron in diameter is difficult to see. Tobacco smoke, for example, typically ranges in size from 0.01 to 1.0 microns. When the naked eye "sees" tobacco smoke, it is actually seeing the light that is being reflected or scattered from millions of small particles. The wavelength of such light is in the ultraviolet range, hence the blue haze normally associated with a smoke-filled room.
As particles such as tobacco smoke become very small, they cease to behave as particles. Instead, they begin to behave more like gas phase molecules. Traditional methods of separation do not apply to particles below .01 microns and removing them from air requires techniques reserved for gaseous materials. Particles above 0.01 micron, however, are usually considered to be filterable and conform to the filtration methods explained in this discussion.
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