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Table 1 Classification of particles based on size

From: Particulate matter and atherosclerosis: role of particle size, composition and oxidative stress

Particle Aerodynamic diameter (μm) Sources Mode of generation Atmospheric half-life
Thoracic particles(PM10) < 10 - - -
Coarse particles(PM2.5-10) 2.5 - 10 Suspension from disturbed soil (farming, mining, unpaved roads), construction, plant and animal fragments Mechanical disruption (crushing, grinding, abrasion of surfaces), evaporation of sprays, suspension of dusts. Minutes to hours
Fine particles (PM2.5) < 2.5 Power plants, oil refineries, wildfires, residential fuel combustion, tailpipe and brake emissions Gas-to-particle conversion by condensation, coagulation (accumulation mode) Days to weeks
Ultrafine particles (UFP) < 0.1* Fuel combustion (diesel, gasoline) and tailpipe emissions from mobile sources (motor vehicles, aircrafts, ships) Fresh emissions, secondary photochemical reactions (nucleation mode) Minutes to hours
  1. PM10 includes all other PM fractions. * While the UFP size cutoff is by definition considered < 0.1 μm, UFP can include particles up to 0.2 μm in experimental studies using particle concentrators, because the distinction between the ultrafine and the accumulation modes can vary from 0.1 to 0.2 μm depending upon location and season for reasons that include among others, variability in the size of particulate derived from "fresh" emission sources and deviation from a spherical shape [101]. Table based on U.S. EPA [8].