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December 5, 1952, was a quiet day. Brian Bone, 9, was at home with his parents and his 15-year-old brother, Hugh. When Brian opened the backdoor to let out his dog, a German shepherd named Tarzan, he noticed that the morning was damp and hazy. That wasn't unusual for London at that time of year though. Tarzan trotted into the fenced-in yard, and Brian shut the door and went about his day.

It was only later, when Brian went to call Tarzan in, that he realized something was horribly wrong. The hazy morning had turned midnight black. The air had a sharp smell-a mixture of chemicals and rotten eggs. Brian and his parents called for Tarzan, but the dog had escaped through a hole in the fence. Normally, Tarzan would have been able to sniff his way home. But even a dog's powerful sense of smell was no match for the smothering smog. For hours the family searched the neighborhood, braving the darkness as they called for Tarzan. Yet he remained hopelessly lost.

Across London, millions of people had been plunged into the same darkness. Buses screeched to a halt. Trains stopped on their tracks. Cars crashed. People stumbled along the streets, unable to find their way home. A few got so lost they fell into the Thames River and drowned. Staying indoors provided no escape. Black air crept under doors and through keyholes, filling up homes and offices and hospitals.

What was happening?

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How a deadly environmental disaster changed our ideas about the air we breathe