3.No. 8 (Tie): Salt Lake City
Utah’s capital, along with most of the state’s population, is positioned in front of the Wasatch Mountains. In winter, temperature inversions — a layer of high warm air aloft — traps cooler air below. That means pollution created by the Salt Lake-Provo-Orem area’s 2.5 million residents has nowhere to go.
4.No. 8 (Tie): Denver
Colorado is working to meet a June 2021 EPA compliance deadline for lowering ground ozone levels. Strict oil and gas regulations helped, as did having fewer of the Denver-Aurora metro area’s 3.6 million residents on the road during the COVID-19 pandemic. But rising temperatures and drier weather have also led to a jump in wildfires throughout the state.
