Deadly mixture of smoke and fog peaking in the midtwentieth century in large cities. A smog episode in London in 1952 led to 4000 deaths. The sulfuric acid produced from the fossil fuel sources in use at that time led to a choking mixture when incorporated into fog droplets. It is associated with low temperatures, low actinic flux, and high humidity. This form of air pollution was largely eliminated by legislation in the 1950s that led to reduced emissions of SO2 and smoke. See Los Angeles (photochemical) smog.