Black Wednesday Demonstrates Dangers of Smog Summary

  • Last updated on November 10, 2022

“Black Wednesday,” a photochemical pollution episode in Los Angeles, demonstrated for the first time the extent of the health dangers posed by smog. It resulted in the first governmental actions designed to address the smog problem in Los Angeles.

Summary of Event

On Wednesday, September 8, 1943, Los Angeles, California, experienced a new—or more severe—type of air pollution that caused plant damage, eye irritation, cracking of stretched rubber, and a decrease in visibility. It was not the first occurrence of pollution over the Los Angeles basin; Spanish mariners were the first to look out over the basin and see what would later be known as smog. The name the sailors gave to the area was the Bay of Smokes. Smog Black Wednesday Pollution;United States [kw]Black Wednesday Demonstrates Dangers of Smog (Sept. 8, 1943) [kw]Dangers of Smog, Black Wednesday Demonstrates (Sept. 8, 1943) [kw]Smog, Black Wednesday Demonstrates Dangers of (Sept. 8, 1943) Smog Black Wednesday Pollution;United States [g]North America;Sept. 8, 1943: Black Wednesday Demonstrates Dangers of Smog[00940] [g]United States;Sept. 8, 1943: Black Wednesday Demonstrates Dangers of Smog[00940] [c]Environmental issues;Sept. 8, 1943: Black Wednesday Demonstrates Dangers of Smog[00940] [c]Health and medicine;Sept. 8, 1943: Black Wednesday Demonstrates Dangers of Smog[00940] Haagen-Smit, A. J.

By the early 1940’s, residents’ complaints of burning eyes and irritated throats were increasing. A streetcar strike in July, 1943, caused more people to drive, increasing the number of automobiles on the streets and highways. As a result, a heavy layer of smog developed. The streetcar strike ended on July 23, but the smog remained and even increased in concentration. On Monday, July 26, the air pollution reached its maximum concentration, and the episode was referred to as a “gas attack,” a term that had ominous overtones in connection with World War II.

The war had brought a large infusion of industry to California, and Los Angeles was experiencing increased effluents in the air from dirty smokestacks; at that time, few pollution controls existed in heavy industry. In 1943 alone, Southern California received $8 billion in war contracts. Manufacturing jobs in the region increased from 152,000 in 1940 to 446,000 in 1943. The increase in economic development also meant a substantial increase in Los Angeles’s population. The increase in population entailed an increase in automobile traffic as well. However, most observers assumed that the evident rise in air pollution in the city was a function of industrial pollution rather than automobile exhaust.

On September 8, 1943, a dense cloud of pollution hovered over Los Angeles, and the Los Angeles Times dubbed the day Black Wednesday. People realized that something was wrong and that action must be taken to alleviate the problem. The source of the pollution was quickly but erroneously identified as a synthetic rubber factory in Boyle Heights.

During World War II, one of the major supply shortages was of natural rubber. By 1943, 97 percent of the nation’s previous sources of crude rubber were controlled by the Japanese. Brazil’s rubber plantations were the only sources left outside Axis control, and Brazilian production was insufficient. The solution to this problem was a fairly new invention, synthetic rubber Rubber, synthetic . A gas plant in Boyle Heights was converted to produce a synthetic called butadiene Butadiene , an essential constituent in the production of synthetic rubber. Butadiene is a flammable, gaseous hydrocarbon derived from petroleum, and one result of its production was a highly malodorous effluent. The plant’s production of butadiene seemed the most likely cause of Black Wednesday.

With the apparent source of the smog identified, officials attempted to correct the situation. The rubber plant’s hours of operation were reduced, and special filters were installed. The smog, however, persisted. The plant was closed so that built-in pollution controls could be added. Installation was completed in mid-December and appeared to be a success: The smog was gone. Officials were as yet unaware, however, of the influence of seasonal meteorological conditions—particularly of temperature inversions—upon smog levels.

A temperature inversion Temperature inversions;Los Angeles is a situation in which the air temperature is higher at higher altitudes than it is at lower altitudes. Southern California is less likely to experience a temperature inversion in winter than in summer or early fall, because the Pacific high-pressure system is farther south during the winter than it is at those other times of year. Moreover, in other parts of the world, a pollution episode would be caused by the combination of a temperature inversion and light winds occurring over a given area for four days or more. An unusual set of topographic and climatic factors exists in Los Angeles, however, making the area prone to persistent pollution concentration.

Topographically, Los Angeles lies in a basin with high mountains to the north and east. Offshore, a persistent high-pressure system is extant most of the year, especially during summer and early fall. Subsidence in this air mass produces hot, dry air in the upper levels. Below, a cold ocean current of upwelling bottom water parallels the coast slightly offshore. Westerly winds, moving across this upwelling, are chilled from below, creating a cool, moist layer of air near the surface.

Weak southerly and southwesterly winds move the cool, moist air up the low, sloping plain of the Los Angeles Basin. Mountain barriers around the city prevent further movement of air inland. With cool, moist air overlaid by hot, dry air, a temperature inversion is produced. Because the temperature inversion is trapped by mountains, the potential exists for it to last for several days. The probability of a pollution episode is higher under such topographical conditions than it would be in areas where a stagnating high-pressure system alone produces a temperature inversion.

In terms of composition, two types of urban atmospheres are recognized. The traditional atmosphere, exemplified by that of New York City, is characterized by particulate smog, a combination of smoke and fog composed largely of suspended particulate matter. The modern atmosphere is epitomized by that of Los Angeles, which experiences photochemical smog rather than particulate smog. Photochemical smog is a complex mixture of products formed by the interaction of sunlight with nitric oxide, hydrocarbons, and other air pollutants found in automobile Automobile emissions exhaust, as well as power plant emissions and the effluvia of petroleum refining and storage facilities.

Ensuing years brought the realization that negative effects of smog were as bad upwind of the Boyle Heights plant as downwind. Moreover, the presence of irritant gases miles away from the rubber plant indicated that the problem existed region-wide. The first reaction to this realization was to ban stationary pollution sources, such as outdoor burning, backyard incinerators, and smudge pots. Industries were ordered to install scrubbers on their stacks. Still, smog remained.

By 1947, the problem was severe enough that civic organizations protested through the news media. In 1948, the state legislature passed a law permitting the formation of air-pollution control districts. That same year, Los Angeles formed a pollution control district. The agency first attempted to control industries and open burning. As a result, visibility in Los Angeles improved, but eye irritation and other problems persisted. Studies of Los Angeles smog were undertaken to find the sources of the remaining problems.

By 1950, Los Angeles smog was identified as photochemical. In 1951, Professor A. J. Haagen-Smit of the California Institute of Technology published a paper in which he demonstrated that in the presence of sunlight, hydrocarbons and nitric dioxide reacted to form a variety of oxidation products such as ozone, which could account for the effects in Los Angeles.

Following Haagen-Smit’s work, the officials set out to control the leakage of gasoline vapors from storage tanks in order to rectify the situation. When this action failed to achieve the desired effect, the officials finally concluded that the approximately 2.5 million automobiles in the Los Angeles area, using some five million gallons of gasoline daily, had to be the primary source of Los Angeles’s smog. At that time, automobiles were releasing more than one ton of hydrocarbons per day into the Los Angeles atmosphere.


With the automobile identified as the probable major cause of photochemical smog, the Automobile Manufacturers Association formed the Vehicle Combustion Products Committee Vehicle Combustion Products Committee in 1953. The committee further defined the problem and sought solutions. Concurrently, an effort was made to reduce hydrocarbon emissions from stationary sources such as refineries.

By 1958, a substantial reduction in hydrocarbons from stationary sources was achieved, but smog was still a serious problem. It was apparent that some type of automotive controls were needed. In 1959, the California legislature added air quality standards to the health and safety code. Then, in 1960, the California Motor Vehicle Pollution Control Board California Motor Vehicle Pollution Control Board Motor Vehicle Pollution Control Board, California was created to implement the new standards.

The California regulations had some effect, but it remained for the federal government to implement environmental policies that would have a real impact on air pollution. In 1955, the Air Pollution Control Act Air Pollution Control Act (1955) was introduced. This act stipulated that the federal government would provide research and technical assistance, but state and local governments would be responsible for controls. This law was followed by the Clean Air Acts of 1963 and 1967, as well as by the creation of the Environmental Protection Agency by President Richard M. Nixon in 1970. Over time, progress began to be made both in Los Angeles and throughout the nation in controlling automotive air pollution. Such progress was hampered, however, by the competing interests of health, business, and consumer convenience. Smog Black Wednesday Pollution;United States

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">American Association for the Advancement of Science. “Photochemical Air Pollution.” In Air Conservation: The Report of the Air Conservation Commission of the AAAS. Washington, D.C.: Author, 1965. A very good account of photochemical air pollution, which uses Los Angeles as an example. Covers sources, reactions, impacts, and controls.
  • citation-type="booksimple"

    xlink:type="simple">Elsom, Derek. Smog Alert: Managing Urban Air Quality. London: Earthscan, 1996. Study addressing possible solutions to air polution. Includes case studies of different cities, which explain how Los Angeles differs from other urban areas and the implications of that difference.
  • citation-type="booksimple"

    xlink:type="simple">Leighton, P. A. Photochemistry of Air Pollution. New York: Academic Press, 1961. A technical treatment of the chemistry involved in the formation of photochemical smog.
  • citation-type="booksimple"

    xlink:type="simple">Middleton, J. T., and A. J. Haagen-Smit. “The Occurrence, Distribution, and Significance of Photochemical Air Pollution in the United States, Canada, and Mexico.” Journal of the Air Pollution Control Association 11 (1961): 129-134. A good analysis of the geography of photochemical air pollution in 1961.
  • citation-type="booksimple"

    xlink:type="simple">Stern, A. C., ed. Air Pollution. New York: Academic Press, 1976. A good account of air pollution and its effects on the environment, humans, plants, and animals.
  • citation-type="booksimple"

    xlink:type="simple">Strauss, W., and S. J. Mainwaring. Air Pollution. Baltimore: Edward Arnold, 1984. An excellent treatise on air pollution covering pollution sources, effects, measurement, and control.
  • citation-type="booksimple"

    xlink:type="simple">VanNijnatten, Debora L., and W. Henry Lambright. North American Smog: Science-Policy Linkages Across Multiple Boundaries. Orono: University of Maine Press, 2001. This pamphlet argues for attacking North American smog on a continental level and discusses the scientific and environmental-policy implications of that position. Bibliographic references.

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Mumford Warns of the Dangers of Growing Cities

Clean Air Act Grants Federal Authority to Regulate Air Pollution

Bookchin Publishes Crisis in Our Cities

Congress Passes the Motor Vehicle Air Pollution Control Act

Greenhouse Effect Is First Predicted

Pittsburgh Residents Form the Group Against Smog and Pollution

Congress Amends the Clean Air Act

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