Particle Sizes

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
• 1 micron = 10^-6 m = 1 μm
  

In imperial units

• 1 inch = 25400 microns
• 1 micron = 1 / 25400 inch

The eye can in general see particles larger than 40 microns.

Typical size of contaminants and particles are indicated below. Note that the values varies widely depending on how the products are processed. By example milling corn starch in 30 minutes can reduce the average diameter of starch particles from 10 to 0.3 microns (μm, 10^-6 m). Further milling may produce particles even smaller than 0.1 microns.

• one micron is one-millionth of a metre
• 1 micron = 10^-6 m
• 1 micron = 1000 nano metre

Airborne particles

Airborne particles are solids suspended in the air.

Larger particles - larger then 100 μm

• terminal 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 μm

• sedimentation 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

• 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 Particles

Smaller 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 Dust

Airborne particles which can enter the nose and mouth during normal breathing. Particles of 100 microns diameter or less.

Thoracic Dust

Particles 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 Dust

Particles 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 (PM_2.5) are often used in USA.

The total allowable particle concentration - building materials, combustion products, mineral fibers and synthetic fibers (particles less than 10 μm) - specified by EPA (U.S. Environmental Protection Agency)

• 50 μg/m³ (0.000022 grain/ft³) - allowable exposure per day over the course of 1 year
• 150 μg/m³ (0.000022 grain/ft³) - allowable exposure over 24 hours