Fine particles are particles with a diameter of less than 2.5 micrometers. They are referred to by the term PM 2.5 (PM is an abbreviation for the term particulate matter).
Given the small size of these particles, they cannot sediment by gravity alone. They can only fall to the ground by aggregating or by precipitation.
Fine particles are found suspended in the atmosphere and remain there for several days. These small aerosols are therefore able to penetrate deeply into lungs, up to the alveoli.
Another particularity of these aerosols: they can become condensation nuclei which adsorb humidity and pollutants (lead, sulphur dioxide, etc.), making them even more toxic.
PM 2.5 are therefore a major carrier of poisons through the respiratory tract.
Air quality analysis device. Fine particles are among the parameters studied. © brewbooks CC by-sa 2.0