The Environmental Protection Agency has listed metro Denver and 10 other cities as failing to meet federal smog standards after seven years.
On Thursday, the federal government tightened the limit.
EPA officials said metro Denver and the northern Front Range won't be penalized for missing a July 20 deadline for meeting the old standard set in 2008.
The Denver Post reports
state-run air monitoring shows persistent ozone pollution in metro Denver and along the northern Front Range at unhealthy levels, despite cleaner vehicles and industry.