Colorado is working on reducing the overall amount of ozone throughout the state. The Denver Metropolitan and North Front Range areas specifically have violated the national ozone standards in the last several years. All other counties in Colorado are in compliance with the 75 ppb standard set by the EPA in 2015. Denver is currently the 8th most polluted area due to ozone in the United States (Finley, 2016). In 2015, Denver was ranked the 13th most-polluted city in the United States. Experts cite coal mining, population growth, and the oil and gas industries as potential reasons for the Denver metro area becoming more polluted.