Particles or particulate matter (PM) are small solid particles suspended in the air that are so small that they can be carried by the air. Their increased concentration can cause serious health problems. It also participates in important atmospheric events such as water precipitation and influences the Earth's temperature balance. 

In most cases, airborne dust or fine dust is synonymous with solid particles. In contrast, the concept of aerosol particles or atmospheric aerosols may already be different under the conditions: it includes all particles that are propagated by the flow of air - both solid and liquid. The terminology used to describe these particles is not uniform and depends on whether the particles are judged more by their origin or by their impact on health. Meteorologists use different terms, which follow rather physical and optical properties of air.

 Inhalation of solid dust particles damages the cardiovascular and lung systems in particular. The effect of these particles on the human organism (generally also on other animals or plants) depends on the duration of exposure of the organism to their action - the so-called exposure time.

Short-term exposure may result in a higher number of inflammatory lung diseases, adverse effects on the cardiovascular system and, as a result, an increase in hospital admissions and increased drug consumption.
Long-term exposure reduces the lung function of children and adults, increases the number of respiratory tract diseases, increases the incidence of chronic obstructive pulmonary disease and, as a result, reduces life expectancy.
In some, especially less developed countries, life expectancy is shorter by more than one year due to pollution or even much more, like in China, India or Ostrava.

 Observations have shown that the effect of solid dust particles on health depends primarily on their size. Larger particles get trapped on the hairs of the nose and do not cause more trouble. Particles smaller than 10 µm penetrating the larynx into the lower airways. Therefore, they are sometimes referred to as inhaled particles or thoracic particles - from the Latin thorax - chest. Here they can settle in the bronchi (PM2.5 - "fine particles"), penetrate into the alveoli (PM1) or into the blood (PM0,1 - ultrafine particles) and cause health problems. The effect of particle size was also confirmed by volunteer research, which found that soot from open hearths remained only one-fifth after inhalation in the lungs, while it was half for soot from diesel cars.  This is related to the finding that the soot from the engines has significantly smaller particle sizes than the soot from the fireplaces.

Because the dust particles are generally irregular in shape, the so-called aerodynamic diameter, which is the diameter of a spherical particle with a density of 1000 kg/ m³, has the same steady velocity due to gravitational force in a calm atmosphere. diameter of a water drop having the same weight as the measured dust particle).

In terms of health, different fractions of dust particles, have been defined depending on their size in micrometers. Usually particle sizes are determined

PM10 - particles smaller than 10 μm,
PM2,5 - particles smaller than 2,5 μm (not all-over),
PM1 - particles less than 1 μm (less frequently),
PM0.1 - particles smaller than 100 nm (exceptionally).
The concentration values ​​of these particles are usually given in units of bulk density (eg µg / m³) - ie they correspond to the total weight of the particles. However, the reactivity and hence the health effects of particles on organisms are more related to the surface or number of particles, which at a given volume concentration is larger for smaller particles. Therefore, the risk associated with PM2.5 and PM1 is also more significant. 

Carcinogen is demonstrably different types of dust, smoke or asbestos fibers. For substances (including viruses) with characteristic dimensions smaller than the cell (suspicious include carbon nanotubes), they cause pathological cell changes.

Smog is very visible on the horizon line in sun day (Brno, Lišen, Czech Republic)

More about PM please read here: https://cs.wikipedia.org/wiki/Pevn%C3%A9_%C4%8D%C3%A1stice