Particulate matter (PM)
PM stands for particulate matter, which is the term for a mixture of solid particles and liquid droplets found in the air. Particles have different sizes and or are more or less visible.

PM10 are particles with a diameter of 10 micrometres and smaller. Examples of PM10: pollen, dust (settling dust, cement dust), mould spores.

PM2.5 has a diameter of 2.5 micrometres or smaller. (Human hair is approximately 70 micrometres in diameter). Example of PM2.5: house dust, animal dander, bacteria, cooking oil smoke/grease, smog, fly ash, settling dust. 

PM1 have a diameter of 1 micrometre or smaller. Example of PM1 (from 0.3 to 1 micrometre): Bacteria, suspended dust, house dust, animal dander, smog, tobacco smoke, soot.