Concentration is the amount of material (for example a pollutant) in a given unit volume of liquid or gas, usually measured and expressed in milligrams per litre (mg/L) or micrograms per cubic metre of air (µg/m3).

Particle concentrations, for example, tell you how much particulate matter is in a given amount of air.  The concentrations of the measured indicator can be compared to guidelines to determine whether the level is within an acceptable range or not.