In December 1952, a severe anticyclone combined with freezing weather trapped toxic emissions from coal burning over London. While the visible outdoor yellow fog was terrifying, recent atmospheric research reveals an even deadlier indoor chemical reaction. High concentrations of sulfur dioxide reacted with water vapor, but this conversion was drastically accelerated by trace metals like manganese and iron present in coal ash.
Acting as catalysts, these metals facilitated the rapid oxidation of sulfur dioxide into microscopic sulfuric acid droplets. Because fog droplets evaporated indoors due to household heating, the remaining acid mist became highly concentrated, penetrating deeply into lungs. Residents seeking refuge inside unsealed homes unknowingly inhaled elevated levels of corrosive aerosol, causing acute respiratory distress, severe lung damage, and thousands of excess deaths over a matter of days.