Smog is often categorized as being either summer smog or winter smog. Mexico City was choking in smog and pollution. The smelting is directly responsible for severe pollution, including acid rain and smog. VOCs contribute to the formation of Tropospheric ozone and smog. Sulfur and nitrogen oxides contribute to smog and acid rain. Noida also has fog and smog problems. Isocyanic acid is also present in various forms of smoke, including smog and cigarette smoke. This releases carcinogens and neurotoxins into the air, contributing to an acrid, lingering smog. The composition and chemical reactions involved in photochemical smog were not understood until the 1950s. The deadly London smog was caused primarily by the heavy use of coal. The other was to perform inspections immediately following certifications at smog check stations. A hybrid is not exempt from smog inspection. There are certain reactions that can limit the formation of O in smog. Much of the area is frequently covered with a brown smog. The population of the area affected by the smog has been estimated at 16 million.