The word "smog" results from the combination of the English words "smoke" and "fog", describing a condition of widespread airborne contamination with partially visible air pollution above urban and industrial areas. The term smog is today used in general to refer to high levels of pollutants in the air, with or without the presence of fog.

The problem of polluted air in cities is not new. For example, in the year 66 AD Seneca exclaimed: "As soon as I escaped the heavy air of Rome, the stink of the smoking chimneys, the soot they spewed forth, and the pestilential steam, I felt a complete change in my well-being."

The connection between air pollution and health has long been well known. This became clearest, however, during smog catastrophes with exceptional numbers of deaths, such as when over 4,000 people died in London in 1952.

Today, it is assumed that the synergistic effect of particulate matter and sulfur dioxide has played the most important role in the majority of smog cases. This realization led eventually to the introduction of total threshold values for particulate matter and sulfur dioxide in the smog ordinances of the Federal Republic of Germany.

Different types of smog can be principally differentiated based on their origin: The winter smog of type "LONDON" or "LONG-DISTANCE TRANSPORT" and the summer smog of type "LOS ANGELES."

The smog type "LONDON" forms in winter during inversion weather conditions with cold air near the ground, when the low temperatures require intense artificial heating. Since coal and fuel oil are also used as heating material (though less frequently today), the primary pollutant components that arise are SO₂, CO, particulate matter, sulfates, and H₂SO₄.

The German state governments are empowered by § 40 Paragraph 1 of the BImSchG to establish smog ordinances for areas where a strong increase in damaging environmental effects from air pollution can be expected during low-exchange weather conditions. The states are also empowered to issue smog alarms under specific (nationwide) criteria. In such a case, large polluters are subject to emissions-reducing ad-hoc devices. Smog ordinances are enacted according to state law. A universal condition is that the respective responsible state ministry gives notice of a low-exchange weather condition under the indication of the forewarning level or alarm level for a smog area. In addition, under § 40 BImSchG it is possible to establish areas where vehicle traffic can be limited or forbidden during weather conditions with smog alarms.

The substitution of coal and oil by gas and steam heating, a clear reduction (2008) of 0.1% by volume in the sulfur content of light heating oil, and the de-sulfuring of powerplant exhaust gases since 1990 have all contributed to a significant decrease of the component sulfur dioxide. The need in the 1980s to nevertheless issue smog alarms lay on the one hand with intensified alarm values and on the other with long-distance pollution from the East. The smog type "LONDON" has since lost much of its importance, so much so that many states have lifted the corresponding smog ordinance, including Baden-Württemberg in 1997.

The smog type "LONG-DISTANCE TRANSPORT" is likewise a winter smog that in Germany can form under easterly wind conditions with inversions, since the areas east of Germany still use substantial amounts of coal and brown coal for heating purposes and energy generation. This type of smog can also appear with high wind velocities.

The summer smog "LOS ANGELES" is an entirely different type of smog. The source here is first and foremost automotive traffic with its exhaust gases (e.g. nitrogen oxides, hydrocarbons, and carbon monoxide). As a result of poor air exchanges connected with strong sunshine, a chemical transformation of the gases in the atmosphere results in the formation of photo-oxidants. The most important compounds here include ozone (O₃) and peroxiacetylnitrate (PAN) next to peroxides, aldehydes, etc..