The U.S. Environmental Protection Agency approved a state plan Friday that puts much of the Front Range on track to meet a tough new federal standard for ozone and to reclaim its clean-air status.

Now all the region has to do is get there.

Under the ozone Early Action Compact, state regulators are counting on lower-emission vehicles and new pollution-control programs to help Denver come into compliance with the new standard by 2007.

However, success - in part - will hinge on the weather, state officials say.

"We've always been right above or right below the threshold," said Mike Silverstein, manager of planning in Colorado's Air Pollution Control Division. "There's a very thin margin between attaining the standard and not attaining it. We're not out of the woods."

The metro area's shallow- bathtub topography is perfect for trapping and cooking the chemical soup of exhaust and fumes to create ozone. Easterly winds that accompany summer heat waves concentrate the lung-irritating pollutant against high ground to the west and south of the city.

The ozone compact covers the seven-county Denver area, plus Weld and Larimer counties. The EPA's formal approval of the plan, published Friday in the Federal Register, is a key milestone in the process.

The new standard is based on a three-year rolling average of the fourth-highest ozone reading at any monitor. Readings may not exceed 84 parts per billion.

If any of the 12 pollution monitors are out of compliance by the end of 2007, Denver would again be tagged with a nonattainment designation. That could bring federal sanctions, such as the loss of federal highway funds or limits on new industrial plants.

Despite the July heat wave, ozone levels remained relatively low this summer. The 2005 data are significant because they will be included in the three-year average.

"We came out pretty well," said Ken Lloyd, director of the Regional Air Quality Council. "That was a little surprising."

Ozone levels typically diminish in August, when clouds and rain reduce heating and help break up stagnant air masses.

In addition to lower emissions from new vehicles, pollution limits on oil and gas wells, off-road diesel engines and power plants and cleaner fuels are expected to combine to keep Denver below the new limits through 2010, Silverstein said.

Three years ago, the EPA finally agreed that Denver had complied with the old one-hour ozone standard. That standard required that no monitor exceed 125 parts per billion during any one-hour sampling period. But on July 10, 2003, ozone levels at three air monitors across the city spiked, putting the metro area in technical violation of the new standard.

State officials immediately began charting a course to bring Denver back into compliance. Regulators ramped up their public-education efforts. They also clamped down on hundreds of natural-gas wells northeast of the city, whose burps of petroleum vapor are a significant contribution to the overall load of ozone-forming chemicals. In addition, gasoline retailers now sell less-volatile fuels, which adds several cents to the price of a gallon.