 Experts cite coal mining, population growth, and the oil and gas industries as potential reasons for the Denver metro area becoming more polluted.In 2008, the EPA created “nonattainment areas” for ozone in which ozone levels shall not exceed the federal standard of 75 parts per billion averaged over the course of three years.The EPA put the revised ozone standard into effect on October 1, 2015 (Salley, 2015).