PM2.5 Particulate Matter

PM2.5 denotes fine particulate matter with an aerodynamic diameter of 2.5 micrometres or less. Particulate matter is a mixture of solid particles and liquid droplets suspended in air, rather than a single chemical substance.

What the size classification means

The number in PM2.5 indicates particle size, not the amount of pollution in the air. One micrometre is one-thousandth of a millimetre.

• Aerodynamic diameter describes a particle's behaviour in air by comparison with a spherical particle of standard density; it accounts for differences in shape and density.
• PM10 includes particles with an aerodynamic diameter of 10 micrometres or less, so PM2.5 forms its finer-size fraction.
• The coarse fraction, commonly written as PM10-2.5, consists of particles larger than 2.5 micrometres and up to 10 micrometres.

How fine particles enter the air

PM2.5 can be emitted directly or formed within the atmosphere. Its chemical composition varies with its sources and atmospheric conditions.

• Primary particles are released directly through processes such as fuel combustion in vehicles, industries and biomass burning.
• Secondary particles form through atmospheric reactions involving gases such as sulphur dioxide and nitrogen oxides, producing particulate sulphates and nitrates.

Why PM2.5 is monitored separately

Fine particles can penetrate deep into the lungs, and some can enter the bloodstream. Exposure is associated with respiratory and cardiovascular disease.

• Ambient PM2.5 concentration is generally expressed in micrograms per cubic metre, which measures particle mass per volume of air.
• The size label and concentration are different: PM2.5 identifies the particle-size category, while its measured concentration indicates how much is present.