A common measurement of particles is PM10 and PM2.5. Simplified, it’s the mass of the particles in the air between 0,001 to 10 micro metre (µm) in diameter.
Particles that are less than 10 micro metre in diameter can enter the lungs and cause lung diseases.
Particles come from different sources. The larger particles come from wear and tear from spiked tyres for example. The smaller particles come from the combustion industry.