On this day in 1948, an asphyxiating cloud of smog enveloped Donora, Pa., a Monongahela River town of 14,000 people and the location of the Donora Zinc Works and the American Steel and Wire Co. The sulfur dioxide-laden smoke blighted yards and hillsides, and sometimes even made driving difficult. Doctors urged people with breathing problems to leave town, but this was tough because driving was already prohibited. Twenty people were killed, and more than 7,000 were hospitalized or became ill. The event remains one of America's greatest environmental disasters. It spurred federal and state laws, such as the Clean Air Act of 1970, to curb air pollution and deal with health threats caused by pollution.

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