Particulate matter is often a consequence of incomplete burn of wood, fuel, etc. It is also frequently due to abrasion or mashing of material, typically brake blocks, or ground erosion. It can sometimes have a biological origin: pollens, bacteria, mold, …
The vast majority of such particulate matter is produced outside, and it enters into homes with the fresh air, through windows, doors, HVAC systems, etc.
Moreover, our activities generate numerous particulate matter (cigarette smoke, candles, cooking fat, abrasion of clothes on the skin, …).
Also, we transport particulate matter through our clothes. For instance, if you spend even a short time in a room full of smoke, your clothes will absorb the particulate matter and will release them over many days: you will “smell like smoke”.
These particulate matter are ranked by size, expressed in microns: less than 10 µm, less than 2.5 µm, less than 1 µm.