It is an air pollutant that contains both solid microparticles and tiny droplets of liquids. Both those and the size of about 10 nm to 2.5 microns. Other designations and names of PM2.5 particles: FSP (fine suspended particles), fine particles, fine particulate matter, fine suspended particles, fine dust.

By source type, PM2.5 particles are divided into:
Artificial (man-made)
The main anthropogenic source of particles is transport. Internal combustion engines and industrial processes with the burning of solid fuels (coal, brown coal, oil), construction, mining, many types of production (especially the production of cement, ceramics, brick, smelting), in cities the source may be road surface erosion and erasing brake pads and tires. Even agriculture is a source of ammonia, from which secondary PM2.5 can form.
Natural (non-anthropogenic)
Sources: soil erosion in arid areas and organic fumes.

Mass concentration of PM2.5 is a key parameter for assessing air quality and its threat to human health. According to the standards of the World Health Organization (WHO), the average annual level of PM2.5 should be no more than 10 μg / m3

In urban air, in principle, there are many different particles: small and large, light and heavy. Only heavy particles "fall" to the ground over time (remember the black snow next to some plant), and light PM2.5 practically do not settle. It is more difficult for small particles to overcome the resistance of the medium and "fall" to the ground. And for the smallest particles, the Brownian motion also exerts resistance.

diameter, um míra vypořádání/ settling rate, m/sek
0,1 8,71-10-7
0,2 2,27-10-6
0,4 6,85-10-6
1,0 3,4910-5
2,0 1,29 10-4
2,5 1,98 10-4
4,0 5.00 10-4
10 3.03-10-3
20 2,20-10-2
40 4,71-10-2
100 2.47-10-1

As can be seen from the table, for PM2.5 particles, the sedimentation rate is 15 times lower than for PM10 and is approximately 0.2 mm / s. This value is compensated even by a slight upward flow of air. And for the so-called PM0.1 ultrafine particles (with a diameter of up to 0.1 μm), the Brownian motion completely prevails over the sedimentation rate. Therefore, this smallest fraction of particles can never settle at all.

PM2.5 particles are also called respirable, respirable fraction. They are so small that they pass through biological barriers in our body: nasal cavity, upper respiratory tract, bronchi. PM2.5 together with air enter directly into the alveoli - vesicles in which gas exchange occurs between the lungs and blood vessels.

see more:

https://habr.com/ru/company/tion/blog/396111/