Detrimental health effects of smog

DEFINITION: Air pollution resulting from the combination of smoke with fog or from sunlight acting on unburned hydrocarbons emitted from automobiles

Severe smog episodes have caused many deaths and widespread illness in cities around the world. Growing recognition of the detrimental health effects of smog has led many governments to pass laws to reduce air pollution.

Originally a blend of the words “smoke” and “fog,” the term “smog” was coined to describe the severe air pollution that results when smoke from factories combines with fog during a temperature inversion. As one ascends upward from Earth’s surface, the air temperature drops by about 3 degrees Celsius (5.5 degrees Fahrenheit) every 300 meters (1,000 feet). Temperature inversions occur when this normal condition is reversed so that a blanket of warm air is sandwiched between two cooler layers. A temperature inversion restricts the normal rise of surface air to the cooler upper layers, in effect placing a lid over a region. When the air above a city cannot rise, the air currents that carry pollutants away from their sources stagnate, causing pollution levels to increase drastically. A combination of severe air pollution, prolonged temperature inversion, and moisture-laden air may result in what has been termed “killer fog.”

Killer Fogs

Several acute episodes of killer fog occurred during the twentieth century. One was in the Meuse Valley of Belgium. During the first week of December 1930, a thick fog and stagnant air from a temperature inversion concentrated pollutants spewing forth from a variety of factories in this heavily industrialized river valley. After three days of such abnormal conditions, thousands of residents became ill with nausea, shortness of breath, and coughing. Approximately sixty people died, primarily older adults and persons with chronic heart and lung diseases. The detrimental effects on health were later attributed to sulfur oxide gases emitted by combusting fossil fuels; the gases were concentrated to lethal levels by the abnormal weather. The presence of coal soot, combined with moisture from the fog, exacerbated the effect.

A second episode occurred in Donora, Pennsylvania, during the last week of October 1948. Donora is situated in a highly industrialized river valley south of Pittsburgh. A five-day temperature inversion with fog concentrated the gaseous effluents from steel mills, with the sulfur oxides released by burning fossil fuels. Severe respiratory tract infections began to occur, especially in older adults, and 50 percent of the population became ill. Twenty people died, a tenfold increase in the normal death rate.

A third major episode occurred in London, England, in early December 1952. At that time, many residents burned soft coal in open grates to heat their homes. When a strong temperature inversion and fog enveloped the city for five consecutive days, Londoners began complaining of respiratory ailments. By the time the inversion had lifted, four thousand excess deaths had been recorded. In this case, it was not only older adults who were affected—deaths occurred in all age categories. During the next decade, London experienced two additional episodes: one in 1956, which claimed the lives of one thousand people, and one in 1962, which caused seven hundred deaths. The decline in mortality rates resulted from the restriction of the use of soft coal, with its high sulfur content, as a source of fuel. Sulfur oxide compounds are responsible for causing lung problems during such episodes; therefore, the term “killer fog” has come to be replaced by the more accurate “sulfurous smog.”

Photochemical Smog

Photochemical smog, first noticed in the Los Angeles basin in the late 1940s, has been an increasingly serious problem in cities around the world. Moisture is not part of the equation in this type of air pollution, and smoke-belching factories dumping tons of sulfur oxide compounds into the atmosphere are not required. Rather, photochemical smog results when unburned hydrocarbon fuel, emitted in automobile exhaust, is acted upon by sunlight. The Los Angeles basin, hemmed in by mountains to the east and ocean to the west, has a high density of automotive traffic and plenty of sunshine. Varying driving conditions mean that gasoline is never completely consumed by automobile engines; instead, it is often changed into other highly reactive substances. Sunlight acts as an energy catalyst that changes these compounds into a variety of powerful oxidizing agents that constitute photochemical smog. This type of smog has a faint bluish-brown tint and typically contains several powerful eye irritants. The chemical reactions also produce aldehydes, a class of organic chemicals best typified by an unpleasant odor.

The complicated chemistry of photochemical smog also produces ozone, which is extremely reactive; it damages plants and irritates human lungs. Because ozone production is stimulated by sunlight and high temperatures, it becomes a particularly pernicious problem during the summer, especially during morning rush hours. Under temperature inversion conditions, the ozone created in photochemical smog can increase to dangerous levels. Ozone is highly toxic. It irritates the eyes, causes chest irritation and coughing, exacerbates asthma, and damages the lungs.

Photochemical smog and ozone are now common ingredients in urban air. Although acute episodes of ozone-induced mortality are rare, concerns have grown about the detrimental long-term consequences of the brief but repetitive exposures to ozone consistently inflicted on commuters. A 2025 report by the American Lung Association (ALA) found that over 125 million Americans are still exposed to unhealthy air caused by smog and soot. The organization also found that people of color are nearly four times as likely to live in polluted places as White people. People of color account for over 50 percent of those living in regions with poor air quality, even though they make up only 40 percent of the general population. Nine of the twenty-five most polluted cities are in California, where frequent wildfires are reversing the improvements made in air quality. Other states with heavily polluted cities included Arizona, Colorado, Nevada, New Mexico, Oklahoma, Texas, and Utah.

The State of Global Air 2025 report indicated that 36 percent of the global population was exposed to PM2.5 (fine particulate matter) pollution at levels that negatively impacted human health. For the first time, the report included the impact of air pollution on dementia, finding that 625,000 cases were related to pollution in 2023.

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