• Primary sources: direct emission of particulate matter into the atmosphere. In this case, they can originate from both natural sources (volcanic eruptions, spontaneous combustion of forests, pollen…) and anthropic sources (traffic, industrial processes, heating, etc.);
• Secondary sources: derived from the condensation and subsequent transformation into aerosols of molecules present in the gaseous phase. In this case, we have the formation of small-sized particles resulting also from the chemical transformation of particulate. Examples include sulfur dioxide, ammonia, nitrogen monoxide, etc..

Regarding human and animal health, we can say that dust has different effects depending on its origin (natural or anthropogenic) and particle size. Generally, health risks are assessed by considering the particles’ ability to penetrate the respiratory system. As you can imagine, this factor is determined by the size of the powders. In particular:

• coarse particulate matter – sedimentable particles larger than 10 µm generally resulting from erosion or industrial processes. They settle on the ground fairly quickly and are unable to penetrate beyond the larynx into the respiratory tract, except in small amounts;
• PM10 – particulate matter consisting of particles smaller than 10 µm (i.e., smaller than one-hundredth of a millimeter). These are thoracic particles capable of reaching trachea and bronchi. Particles between about 5 and 2.5 µm deposit before reaching the bronchioles;
• PM2.5 – ultrafine particulate matter with a diameter smaller than 2.5 µm (a quarter of a hundredth of a millimeter). This is inhalable dust, able to penetrate deeply to reach the alveoli and transmit into the bloodstream through them.