82 years ago today, on July 26, 1943 (June 25, 1943 lunar calendar), the Los Angeles photochemical smog incident in the United States was the largest source of air pollution. The Los Angeles photochemical smog incident in the United States is one of the world's famous public nuisance events. It occurred in Los Angeles in the early 1940s. Photochemical smog is produced by the chemical interaction of a large amount of hydrocarbons with other components in the air under the action of sunlight. This smog contains ozone, nitrogen oxides, acetaldehyde and other oxidants, which remain in the urban area for a long time. In a photochemical smog event in December 1952, more than 400 people over the age of 65 in Los Angeles died. In September 1955, due to air pollution and high temperatures, in just two days, more than 400 people over the age of 65 died, and many suffered from eye pain, headaches, breathing difficulties and other symptoms. Until the 1970s, Los Angeles was known as "America's Smog City." Poison Gas Attacked On July 26, 1943, during World War II, Los Angeles was attacked, not from enemy countries, but from domestic smog. According to the Los Angeles Times, a large number of smoke screens poured into the city center, and the visibility of the urban area was reduced to three blocks. Many people suffered from eye pain, headaches, and breathing difficulties. The government pointed the finger at the Aliso Street plant of Southern California Gas Company, which produced a large amount of butadiene, a synthetic rubber. Under public pressure, the factory was temporarily closed, but the smog continued, which also proved that it was not the culprit. At that time, thousands of people developed symptoms of coughing, tears, sneezing, and severe cases of stinging eyes, discomfort in breathing, dizziness and nausea. Then, Los Angeles often experienced severe pollution in the summer, becoming one of the first "fog capitals" in the United States to fall into air pollution. This "gas attack" fired the first shot in the battle against air pollution that has lasted for more than half a century. From banning backyard trash burning to newly formulated gasoline, from the invention of automobile catalytic converters to the development of zero-emission electric vehicles, the fight against air pollution has not only inspired technological innovation, but also triggered fierce political battles. How to control air pollution has become a hot topic of public debate. (File photo: Los Angeles under photochemical smog in 1943) At the same time, the rapid development of industry and population growth after the war, Los Angeles, like many other American cities, began to vigorously develop modern industries such as aircraft manufacturing and military industry. By the 1970s, the number of cars in Los Angeles had soared from 2.50 million 30 years ago to more than 4 million. Michael Hoffman, an expert on air pollution in the United States, said that during the worst period of air pollution in the United States, many people who did not have bad habits such as smoking would develop serious lung diseases. Several professors at the California Institute of Technology recalled that people who lived in colleges for months in the 1960s and 1970s did not even know that there was a mountain a few miles away, and the air pollution known as "killing dust" caused many people to fall ill and even die. In September 1979, the ozone level in the air of Los Angeles was a critical "danger point". In the fall of 1989, the Los Angeles Times published a front page photo of the outline of a high-rise building in the suburbs of Los Angeles. Smoke blanketed the whole of downtown Los Angeles, and visibility had dropped to three blocks. Finding the culprit The severe smog incident of 1943 greatly stimulated the nerves of the American people, and people began to reflect in shock and fear. After the incident, the city of Los Angeles quickly established a smog commission. At first, everyone blamed the production of "poisonous gas" on factory emissions. The health department began to investigate industrial emissions, and the factory that produced butadiene was subsequently ordered to temporarily close. However, the situation did not improve. From 1943 to 1946, air pollution experts were hired by the local government, the media and civil society to analyze the composition of smog. Raymond Tucker, an air pollution expert, bluntly said that the blame should not be blindly placed on a single industry, factory or group, and that all parties "have a share" in the problem. He pointed out that local officials lack the authority given by law to manage smog. What exactly is smog? The answer was not revealed until Ali Hagen-Smit, a professor of biochemistry at the California Institute of Technology, did a fruitful study of smog. Hagen-Smit and others published a study in 1952 that pointed out that "Los Angeles smog" is a photochemical smog, mainly caused by the photochemical reaction of nitrogen oxides emitted by cars under sunlight. In addition to nitrogen oxides and other pollutants, this harmful mixture of smog mainly contains ozone and aldehyde substances, as well as fine particles, which have a direct impact on the human respiratory system. Studies have shown that air pollution in the United States is mainly caused by six factors: gaseous pollutants, greenhouse gas effects, acid rain, ozone layer destruction, inhalable particulate matter (PM) and climate impact. According to Nesta Jones, a spokesperson for the US Environmental Protection Agency, particulate pollutants have a huge impact on public health. Since 1997, the United States has subdivided particulate matter into fine particles and coarse particles, and established a set of new standards for particulate matter in the air with a diameter of less than or equal to 2.5 microns (PM2.5). The research findings provide the basis for the development of air Quality Standards in California and the United States. Thus began a battle against air pollution in Los Angeles, where citizens afflicted by photochemical smog created an air pollution control zone in 1947 to study the nature of pollutants and their sources and to see what could be done to change the situation. The growing number of cars was a headache for the United States government, and even President Nixon said in frustration that "cars are the largest source of air pollution."