U.S. cities are facing a growing threat that goes beyond hot weather or hazy air. New research from the University of Oklahoma reveals that "compound events"—periods when heat wave conditions coincide with high air pollution levels—are becoming more frequent and intense in urban areas across the United States.

U.S. cities are facing a growing threat that goes beyond hot weather or hazy air. New research from the University of Oklahoma reveals that “compound events”—periods when heat wave conditions coincide with high air pollution levels—are becoming more frequent and intense in urban areas across the United States.