In December 1952, a massive anticyclone settled over London, creating a weather inversion that trapped cold air beneath a blanket of warm air. Millions of Londoners stoked their home fireplaces against the freezing cold, but they weren't burning normal coal. In the stark post-World War II economy, Britain was desperate for foreign currency to repay wartime debts.
To achieve this, the British government exported its premium-grade, low-sulfur coal to overseas buyers. Domestic consumers were left with cheap, low-quality coal known colloquially as 'nutty slack.' This dirty fuel was packed with high concentrations of sulfur, ash, and moisture.
As millions of hearths blazed across the city, domestic chimneys churned out thousands of tonnes of sulfur-heavy smoke every hour, compounded by coal-burning power stations and diesel-fueled buses that had recently replaced electric trams. Trapped by the stagnant weather inversion, the sulfur dioxide emitted from this reject coal reacted with mist and water droplets in the stagnant air, converting into airborne sulfuric acid. The resulting toxic yellow haze, famously nicknamed a 'pea-souper,' was so thick that trains stopped running, movie theater screens were obscured indoors, and cattle collapsed in the streets from asphyxiation.
By the time the wind finally dispersed the fog five days later, over 4,000 people had died directly from respiratory complications, with modern epidemiological estimates putting the total casualty count closer to 12,000. The catastrophe eventually forced the passage of the Clean Air Act 1956, marking a pivotal turning point in modern environmental policy.