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The size of particulates is generally described by the "aerodynamic" diameter of the particles. Common size designations are PM 10 = particles that are 10 micrometers (um) and smaller, PM 4 = particles 4 um and smaller (these are often referred to as "respirable"), and PM 2.5 = particles 2.5 um and smaller (these are often called "fines").

The size of the particle is the principal factor determining where in the lung the particle is deposited, as seen in the figure below: 

Sand used for water purifying is in the 500-1000 um (1 mm) diameter range. The smaller "cementing" particles in intact sandstone of greatest concern to human health are in the 2.5 and lower range (PM 2.5, or "fines"), about 300 times smaller.


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