The Smog of Denora, Pennsylvania

In October of 1948, a temperature inversion trapped cold air close to the Earth's surface. The result was a thick yellow fog that covered Denora, Pennsylvania for several days. 19 people died from asthma or related conditions due to the smog and thousands more were left with breathing problems. A government investigation did not blame U.S. Steel but instead blamed the temperature inversion as one of the main causes. Some claim this event kicked off the focus on clean air which led to the enactment of the Clean Air Act in 1963.