The word smog itself came from a combination of two words - smoke and fog. Smog is the fog that occurs when air is polluted, by e.g. airborne dust PM10 and PM2.5 (which is the most harmful of all pollutants). Due to the conditions and place of occurrence, we can distinguish two types of smog - the London smog, consisting mainly of oxides of nitrogen, sulfur, heavy metals or aromatic hydrocarbons (including the carcinogenic benzopyrene) and photochemical smog observed in large agglomerations where smoke reacts with sunlight.

The smog with which we are dealing in the Czech Republic arises predominantly during the heating season, when the emissions of harmful substances produced by chimneys increase. The sources of pollution are mainly obsolete furnaces and boilers which are used in family and apartment houses. The quality of the used fuel is of key importance, and in many households the fuel becomes anything that comes along - from low-grade coal, to rubbish to old varnished furniture. The favorite fuel consists of PET bottles filled with sawdust and used engine oil!

Industrial installations and cars are also a major source of pollution. Cars do not only produce exhaust gases, they also create the so-called secondary dustiness, when they swirl the particles lying on the road.