95 years ago today, on December 1, 1930 (October 12, 1930), the Mas Valley in Belgium was a smog event. The Mas Valley is a 24-kilometer-long section of the river valley next to the Mas River in Belgium. This section is low-lying in the middle, with 100-meter-high mountains on both sides facing each other, making the valley area in a narrow basin. The Mas Valley area is an important industrial area, with 3 oil refineries, 3 metal smelters, 4 glass factories and 3 zinc smelters, as well as electricity, sulfuric acid, fertilizer plants and lime kilns. The industrial area is all in a narrow basin. From December 1 to 15, 1930, the whole of Belgium was shrouded in heavy fog, and the climate was abnormal. Due to the special geographical location, a strong inversion layer appears over the Maas Valley. Usually, the higher the airflow rises, the lower the temperature. But when the climate is abnormal, the temperature of the lower layer of air will be lower than that of the upper layer, and the phenomenon of "temperature reversal" occurs. This reversal layer will inhibit the rise of smoke, so that the smoke and dust in the atmosphere will not be dispersed, and will accumulate under the reversal layer, which cannot be exchanged by convection, resulting in air pollution. Under the action of this inversion layer and heavy fog, a large amount of smoke emitted by 13 factories in the Maas Valley Industrial Zone permeates the river valley and cannot be diffused. Harmful gases accumulate thicker and thicker in the atmosphere, and their accumulation is close to the limit of harming health. On the third day, under the combined action of sulfur dioxide (SO2) and several other harmful gases and dust pollution, thousands of people in the industrial area of the river valley developed respiratory diseases, with symptoms such as chest pain, cough, tears, sore throat, hoarseness, nausea, vomiting, and breathing difficulties. More than 60 people died within a week, more than ten times the normal death toll in the same period. Among them, patients with heart disease and lung disease had the highest mortality rate. Many livestock were not spared and died one after another. This incident was a sensation. Although similar smog pollution incidents have occurred in many parts of the world in the future, the Mas Valley smog incident was the first recorded air pollution tragedy in the 20th century.