Particle SizesSizes of airborne particle as dust,
pollen bacteria, virus and many more
The size of contaminants and particles are usually described in microns,
a metric unit of measure where one micron is one-millionth of a meter.
The eye can see particles to about 40 microns.
(some reports say as low as 10 microns)
Online Metric Conversion Calculators
The size of some contaminants and particles are indicated in the table below.
Airborne particles are solids suspended in the air.
Larger particles - larger then 100 μmterminal velocities > 0.5 m/s
fall out quickly
includes hail, snow, insect debris, room dust,
soot aggregates, coarse sand, gravel, and sea spray
Medium-size particles - in the range 1 to 100 μmsedimentation velocities greater than 0.2 m/s
settles out slowly
includes fine ice crystals, pollen, hair, large bacteria,
windblown dust, fly ash, coal dust, silt, fine sand, and small dust
Small particles - less than 1 μm (submicron)falls slowly, take days to years to settle out of a quiet atmosphere.
In a turbulent atmosphere they may never settle out
can be washed out by water or rain
includes viruses, small bacteria, metallurgical fumes,
soot, oil smoke, tobacco smoke, clay, and fumes
Hazardous Dust ParticlesSmaller dust particles can be hazardous for humans. In many jurisdictions dust fractions at specified particle sizes in working environments are required to be measured.
Inhalable DustAirborne particles which can enter the nose and mouth during normal breathing. Particles of 100 microns diameter or less.
Thoracic DustParticles that will pass through the nose and throat, reaching the lungs. Particles of 10 microns diameter and less. Referred to as PM10 in the USA.
Respirable DustParticles that will penetrate into the gas exchange region of the lungs. A hazardous particulate size less than 5 microns. Particle sizes of 2.5 micron (PM2.5) are often used in USA