Understanding Dust
When people talk about dust, they are generally referring to small, dry, solid particles that slowly settle under their own weight. And small means up to 100𝜇m (0.1mm) diameter, though some give a smaller upper limit.
A measure used in environmental measurements is to represent the maximum particle size as PMn, where n is the size in 𝜇m. PM2.5 is the typical size of car exhaust particles, PM10 dust and pollen, and PM15 smoke particles from an open fire.
Terminal Velocity and Aerodynamic Diameter
Heavy lead particles will settle more quickly than lightweight lithium carbonate, for example. The lead particles will have a higher maximum speed at which they can settle, or terminal velocity. To correct for this, the concept of aerodynamic diameter was introduced. The aerodynamic diameter of a particle is the diameter of a particle with a density of 1 g/cm3 that has the same terminal velocity as the particle we're looking at. If our particles are close to spherical, the aerodynamic diameter varies with the density. To put this into perspective, the table below compares water (assumed solid), lithium carbonate and lead, giving the density, terminal velocity and aerodynamic diameter of a 10𝜇m diameter spherical particle.
| Material | g/cm3 | Vt cm/s | Vt in/s | Aerodynamic dia. |
| Water | 1.00 | 4.5 | 1.8 | 10 |
| Lithium carbonate | 2.11 | 6.5 | 2.6 | 21 |
| Lead | 11.34 | 15.2 | 6.0 | 113 |
It is the aerodynamic diameter that has been found to best determine how far particles penetrate the respiratory system, as well as how long they take to settle in air.
The terminal velocity, Vt, determines whether something is dust or not. Whilst 100𝜇m particles may be airborne very briefly, 50𝜇m particles have a Vt of about 7cm/sec, so settle quickly, and a 1𝜇m particles has a Vt of about 0.03mm/sec, so its movement is determined more by air currents than by gravity.
Fibres
The long, thin shape of fibres can cause specific health problems. For health purposes, fibres have a diameter less than 3𝜇m, a length greater than 5𝜇m, and a length:width ratio greater than 3.