Particulate mater, aka PM, is a pollutant that affects more people than any other pollutant in the world. It is impossible to avoid them in our daily life and sometimes we actually may see them as it is the particles floating in the air, including dusts, pollens and acarids faeces.
Sources are transportation, industrial processes or pollens when coming from the outside. You are also generating PM with combustion while cooking at home. Cumulated dust or unfiltered vacuum cleaner are also major sources.
What is measured, how dangerous
Particules with size around 2.5 micrometers are measured. The smallest are the most dangerous as they can get deep into your lungs, and some may even pass into the bloodstream.
Threshold: WHO recommends that people not be exposed to an average concentration higher than 50 µg/m3 over 24h for PM10 and smaller. As PM concentration is volatile, it is relevant to check average values over at least 2h period of time.