Residents of the Mon Valley, a cluster of townships along the Monongahela River 20 miles south of Pittsburgh, Pennsylvania, endured some of the stinkiest and most polluted air in the nation during Christmas.

Starting Dec. 21, the region’s air exceeded federal safety standards for daily levels of particulate matter — microscopic particles that penetrate the lungs and can trigger heart attacks and respiratory disease — for six consecutive days. One of the region’s air monitors recorded the worst air quality in the entire United States and registered in the “Code Red” range for several hours on multiple days.