77 years ago today, October 26, 1948 (September 24, 1948 in the lunar calendar), the Donora Smoke Incident in the United States. Donora is a small town in Pennsylvania, USA. It is located inside a horseshoe bend. Hills about 120 meters high on both sides sandwich the town in the valley. Donora Town is a concentration of sulfuric acid mills, steel mills and zinc smelters. From October 26 to 31, 1948, the climate was humid and cold, the sky was cloudy, and there was not a single wind. The air lost its vertical movement up and down, and a temperature inversion occurred. A large amount of smoke emitted by the factory was enclosed in the valley. There is a pungent smell of sulfur dioxide (SO2) in the air, which is disgusting. What followed was a sudden onset of 6000 people in the town, with symptoms such as eye diseases, sore throat, runny nose, cough, headache, fatigue of limbs, chest tightness, vomiting, diarrhea, etc., and 20 of them died quickly. The main reason for this smog incident was that gases and metal particles containing toxic and harmful substances such as sulfur dioxide emitted by factories in the town gathered in the valley under abnormal weather conditions. These toxic substances adhered to suspended particulate matter and seriously polluted the atmosphere. People inhale large amounts of these toxic gases in a short period of time, causing various symptoms and causing epidemics. The Donora Smog Incident and the London Smog Incident, like the December 1930 Smog Incident in the Maas Valley in Belgium, were both air pollution incidents caused by industrial emissions of smoke. They have the same pollution-producing factories and power plants. Pollutants mainly come from the combustion of coal, petroleum and other fuels, as well as carbon monoxide, carbon dioxide, hydrocarbons, sulfur dioxide and nitrogen oxides produced by chemical plants. At the same time, they have the same polluted meteorological conditions, temperature inversion and dead wind environment. This means that the already heavily polluted air loses its ability to diffuse and cannot effectively dilute or take away pollutants. More pollutants add in and the concentration continues to increase, causing people living in it to suffer from bronchitis, asthma and other diseases that affect the lungs.