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Meteorological phenomenon that happens mainly in metropolises and main urban centers. The solar radiations heat up the soil and the heat that it is kept in the same is irradiated, heating up the layers more drops of the atmosphere. Those layers, since they are hot, they are less dense and they tend to arise, forming currents of convection of the air. The pollutant ones hotter than the air (therefore, less dense), they arise and they will be dispersed in the layers more discharges of the atmosphere. That is the normal phenomenon. But when there are two masses of different air, the air hot raisin on the cold air, being like this above him. For to be denser, the cold air that was below it doesn't arise and the hot air that it was on top of the cold doesn't go down, for being less dense. In the intersection of the hot and cold air, he/she is formed a layer that doesn't leave that the pollutant and poisonous gases pass for the layers more discharges of the atmosphere.

To that it feels the name of Thermal Inversion. Like this, those gases are dispersed in the troposphere, creating a fog on the city or municipal district. That fog is composed of poisonous and pollutant gases, that you/they are harmful to the health
. It usually happens in the cold days of the winter, where the formation of cold fronts is larger. When there is horizontal displacement of the winds, the layer of cold air is carried and the hot air goes down, like this putting an end to the thermal inversion.

Principals air pollutants

Pollutant	Principal source	Comments
Carbon monoxide (CO)	Escapes from the motorized vehicles; some industrial processes	Limit supported maximum: 10 mg/m3 in 8 h (9 ppm); 40 mg/m3 in a 1 h (35 ppm)
Dioxide of Sulfur (SO₂)	Central thermoeléctrics to petroleum or coal; sulfuric acid factories	Limit supported maximum: 80 mg/m3 in one year (0,03 ppm); 365 mg/m3 in 24 h (0,14 ppm)
Particles in suspension	Escape from the motorized vehicles; industrial processes; central termoeléctricas; reacção of the pollutant gases in the atmosphere	Limits supported maximum: 75 mg/m3 in one year; 260 mg/m3 in 24 h; composed of carbon, nitratos, sulfates, and several metals as the lead, copper, iron
Lead (Pb)	Escapes from the motorized vehicles; central thermoeléctrics; factories of batteries supported maximum	Limit: 1,5 mg/m3 in 3 months; being most of the lead contained in particles suspension.
Oxides of Nitrogen (NO, NO₂)	Escapes from the motorized vehicles; central hermoeléctrics; factories of fertilizers, of explosives or of acid nítrico	Limits supported maximum: 100 mg/m3 in one year (0,05 ppm) - for NO₂; it reacts with Hidrocarbonos and solar light to form fotoquímic .oxidizers
Oxidizers fotoquímics - (O₃) - Ozone	Formed in the atmosphere due to reacção of Oxides of Nitrogen, Hidrocarbonos and light solar	Supported maximum limit: 235 mg/m3 in one hour (0,12 ppm)
Ethane, Ethilen, Propane, Butane, Acetylene, Pentane	Escapes from the motorized vehicles; evaporation of solvents; industrial processes; solid garbages; use of fuels	Reacts with Oxides of Nitrogen and with the solar light to form oxidizers fotoquímics
Carbon dioxide (CO₂)	All of the combustions	Dangerous saint for the health when in superior concentrations to 5000 ppm in 2-8 h; the atmospheric levels increased about 280 ppm, there is one century behind, for 350 ppm actualmente, something that can be to contribute for the Greenhouse Effect

The problems of health caused by the thermal inversion are, among others: pneumonia, bronchitis, emphysemas, aggravation of the heart diseases, indispositions, irritation of the eyes...