The U.S. Environmental Protection Agency on Thursday downgraded metro Denver’s air quality to “severe,” triggering a requirement that Coloradans living in the Front Range buy a more more expensive blend of gasoline.

The EPA declaration means Colorado will need to submit a plan that demonstrates how the Denver metro will achieve the air quality standards under the Clean Air Act. Crucially, the federal law requires the region to sell “reformulated gasoline” within a year of the “severe reclassification.“

The Denver metro/Northern Front Range ozone non-attainment region is among five areas that failed to attend 2008 air quality standards, according to the EPA.

The federal agency says the regions will have until July 2027 to achieve the standards.

The Polis administration on Thursday threatened the EPA with legal action over pending plans to impose a more expensive blend of gasoline to reduce ozone pollution in the Denver metro area. In a letter to the EPA, Gov. Jared Polis called it “frustrating” that a federal law governing clean air standards attempts to impose a “decades-old, one-size-fits-all approach” that also “does not accurately account for Colorado’s unique situation.”

Noting the spike in gas prices and high inflation, the governor told EPA Administrator Michael Regan that Coloradans, particularly those of limited means, “should not bear the burden of this requirement.”

“Colorado hereby reserves the right to pursue any legal means we identify to correct this injustice and inequity, particularly when it leads to impacts like reformulated gasoline that impact hardworking Coloradans without significant benefit with regard to ozone attainment,” Polis said in his letter to Regan.