The fog that blanketed London, England, on December 5, 1952, seemed quite ordinary at first. But over the next two days, the fog turned yellow and people began to die. By the end of the week 4,000 Londoners lay dead or dying. Evidence suppressed for fifty years put the final death toll at 12,000 victims. Killer Fog reveals how air pollution was the real cause of one of the worst natural disasters of the 20th century.