The Donora Smog was a severe air pollution event that occurred in October 1948 in the town of Donora, Pennsylvania. It was caused by a combination of industrial emissions from local factories, particularly the Donora Zinc Works, and weather conditions that trapped pollutants close to the ground. Over several days, a thick fog mixed with smoke and sulfur dioxide created hazardous air quality. The smog led to respiratory issues for many residents, resulting in 20 deaths and thousands of illnesses. This event highlighted the dangers of air pollution and contributed to increased awareness and regulation of environmental standards in the United States, paving the way for future legislation like the Clean Air Act.