Skyline of Downtown Los Angeles Shrouded and Obscured by Smog

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Skyline of Downtown Los Angeles Shrouded and Obscured by Smog


By: Anonymous

Date: 1956

Source: "Skyline of Downtown Los Angeles Shrouded and Obscured by Smog." Hulton Archive/Getty Images, 1956.

About the Photographer: This picture was taken on a particularly bad day for smog in 1956. The tall building at the center of the picture is the Los Angeles City Hall. The photographer is unknown.


Los Angeles is the U.S. city most notorious for smog. The word "smog" is a combination of "smoke" and "fog" first coined in 1905 to describe chronic severe ground-level air pollution in London caused by the burning of coal. In many modern cities, smog is mostly caused by exhaust from motor vehicles, although coal can also contribute.

The first recognized episodes in Los Angeles occurred in the summer of 1943, when California had about 2.8 million registered motor vehicles that were being driven some 24 billion miles (38.6 billion kilometers) per year. During these smog attacks it was only possible to see clearly for about three blocks, and people suffered breathing problems, vomiting, and smarting eyes. Two years later, Los Angeles established its own state Bureau of Smoke Control. The state of California passed the Air Pollution Control Act in 1947. Despite these and other legislative efforts, the smog problem continued. With each passing year there were more cars in California (as in the rest of the United States). By 1950, there were 4.5 million vehicles in the state traveling a total of 44.5 billion miles (71.6 billion kilometers) per year. By 1960, there were 8 million vehicles.

The air in Los Angeles got somewhat better when California mandated standards and controls for motor vehicle emissions starting in 1959. In 1960, California mandated the first automotive emissions control technology in the world, positive crankcase ventilation. In the 1970s, California was the first U.S. state to mandate catalytic converters to reduce vehicle pollution, and in 1990 was the first state to establish standards for Cleaner Burning Fuels and Low and Zero Emission Vehicles. (Zero Emission Vehicles include electric cars, though this is something of a misnomer: an electric car is only as "low emission" as the generating plant that made the electricity to charge its batteries.)

California's efforts to improve air quality—especially in the Los Angeles metropolitan area, where about half of the population of California lives—have been partly successful. By 1990, when there were 23 million vehicles and 242 billion vehicle miles (389 billion kilometers) traveled per year, the number of Stage 1 Smog Alert days stood at forty-two, half as many as in 1985. Yet in 2001 Los Angeles once again became the U.S. city with the most high-ozone days per year, and could not meet Federal pollution standards for carbon monoxide, particulates, and ozone.



See primary source image.


Los Angeles is located in a broad valley or basin surrounded by mountains. This, plus the proximity of the ocean, encourages the formation of temperature inversions over the city. A temperature inversion is a weather condition in which warm air is layered above cool air. Usually, temperature goes down with altitude: that is, the air is warmest at the ground and cooler higher up. Since warm air is less dense than cool air, it tries to float, convect upward through the cool air. This produces constant mixing of low-altitude air with high-altitude air. When an inversion occurs, however, cold air sits near the ground beneath a layer of warm air and has no tendency to mix with higher-altitude air. Under these conditions, air pollution produced at ground level by cars, power plants, and other sources does not rise and mix with cleaner air, but accumulates at low altitude like steam under a pot lid.

The pollutants that form smog are transformed by sunlight into even more harmful chemicals. Nitric oxide (NO) from automobile exhaust, a toxic gas, reacts with ordinary molecular oxygen in the air (O2) to form nitrogen dioxide (NO2, another toxic gas), which then breaks back down into nitric oxide and free oxygen (O). This free oxygen combines with O2 to form ozone, or O3 (O + O2 = O3). Ozone is beneficial in the stratosphere, where it blocks ultraviolet radiation from the Sun, but it is a pollutant at lower altitudes, where it is poisonous to plants and animals. As the sun shines, then, air pollution in the form of nitric oxide breeds additional air pollution in the form of ozone, and the air gets worse as the day goes on. Smog of the type seen over Los Angeles is thus sometimes called "photochemical (light-chemical) smog."

Vehicles emit other pollutants, too, particularly hydrocarbons. These also react with nitric oxide to form peroxyacetyl nitrate and other poisonous chemicals that are also part of smog.

Smog has long-term health effects and can also kill people with preexisting health problems. In 1930, in the Meuse Valley, Belgium, an inversion trapped industrial air pollution, killing sixty people and making thousands sick. In 1952, a smog inversion killed 12,000 people in London. (Estimates at the time were only 4,000 dead, and this number is still often cited; however, the British government raised its official estimate to 12,000 in 2002.) Another "Killer Fog" killed well over 1,000 people in London in 1956, the year this picture of Los Angeles was taken.

Hong Kong has particularly acute smog, fed largely by coal-burning factories and electrical power plants in nearby Guangdong Province, China.



Knipp, Steven. "Hong Kong Fades Under China's Smog." Christian Science Monitor (December 23, 2004). 〈〉 (accessed March 10, 2006).

Web sites

"California's Air Quality History Key Events." California Air Resources Board. 〈〉 (accessed March 10, 2006).

"Deadly Smog." Public Broadcasting Service (PBS), January 17, 2003. 〈〉 (accessed March 10, 2006).