Originally, the term smog was coined to describe the mixture of smoke and fog that lowered visibility and led to respiratory problems in industrial cities. More recently, the term has come to mean any decrease in air quality whether associated with reduced visibility or a noticeable impact on human health.

Smog occurs when emissions of gases and particles from industrial or transportation sources are trapped by the local meteorology so the concentrations rise and chemical reactions occur. It is common to distinguish between two types of smog: London smog and Los Angeles smog.

London, or sulphurous, smog was noted following the introduction of coal into cities. It is most prevalent in the fall or winter when cool conditions naturally produce a thick surface fog. This fog mixes with the smoke and gases from burning coal to produce a dark, thick, acrid sulphurous atmosphere.

Normally, the unpolluted fog would disperse during the day and be reformed at night. However, the presence of smoke particles makes the fog so thick that sunlight cannot penetrate it and so only a major change in meteorology can disperse it. The smog has been shown to contribute to an increased death rate, primarily due to respiratory problems.

The most notable example of this kind of smog occurred in London, from December 4 to 10, 1954, when some four thousand deaths in excess of normal averages resulted. A similar episode in Donora, Pennsylvania, in 1948 involved approximately twenty excess deaths. Most jurisdictions have instituted control measures to prevent this level of disaster from happening again.

They have moved industries out of cities, demanded lower industrial emissions, and increased the heights of smokestacks so emissions are not trapped by local meteorology. These approaches have been largely successful, at least in controlling the most extreme events.

Los Angeles, or photochemical, smog first became apparent in the late 1940s in warm sunny cities that did not have significant coal-burning industries. It is a daytime phenomenon characterized by a white haze and contains oxidants, such as ozone, that cause eyes to water, breathing to become labored, and plants to be damaged.

It results from the action of sunlight on the combination of hydrocarbons and nitrogen oxides (NOx), known as precursor gases. These are emitted from combustion sources to produce a range of oxidized products and oxidants.

These compounds have been shown to produce respiratory and cardiac problems in individuals sensitive to pollution, and the damage inflicted on crops can cause significant decreases in yield. In most cities, the automobile is the primary contributor of smog’s precursor gases.

As the name would suggest, the most notable example of this type of smog occurs in Los Angeles, California, but it has also been experienced in a large number of cities where the weather is dry, sunlight is plentiful, and there are many automobiles or petroleum industries (e.g., Houston, Athens, and Mexico City.)

The control of photochemical smog is more difficult than for sulphurous smog because the compounds responsible for human and crop impacts are not directly emitted, but produced by chemistry in the atmosphere. Thus, greater knowledge on the emissions of gases, their reactions in the atmosphere, and their lifetime is needed. Most jurisdictions continue to focus their control strategies on reducing ozone concentrations, although particle concentrations are receiving increasing attention.

Because smog results from the sunlight-initiated chemistry of hydrocarbons and nitrous oxides, the most common approach to smog control is to decrease the emission of these compounds at their source. Lower volatility gasolines and systems to capture gasoline vapors are used to reduce hydrocarbon emissions while tailpipe controls (catalytic converters) reduce emissions of both hydrocarbons and nitrogen oxides.

The emission control systems of the twenty-first century mean that a car typically emits 70 percent less nitrogen oxides and 80 to 90 percent less hydrocarbons than the uncontrolled cars of the 1960s. The expected improvement in air quality, as a result of increasing controls, is estimated by using computer models of the atmosphere and its chemistry.