The smog started building up in Donora on October 27, 1948. By the following day it was causing coughing and other signs of respiratory distress for many residents of the community in the Monongahela River valley. Many of the illnesses and deaths were initially attributed to asthma. The smog continued until it rained on October 31, by which time 20 residents of Donora had died and approximately a third to one half of the town's population of 14,000 residents had been sickened. Sixty years later, the incident was described by The New York Times as "one of the worst air pollution disasters in the nation's history". Even ten years after the incident, mortality rates in Donora were significantly higher than those in other communities nearby.^[2]

• http://en.wikipedia.org/wiki/Donora Smog of 1948