The ground-level ozone monitor in Perry County has gone four consecutive weeks without exceeding the standard, after topping it 14 times this summer.
With the summer heat wave over, cooler temperatures have helped keep ozone levels in check.
"Ozone readings are directly tied to temperatures. It's really that simple," said David Grimes, deputy director of the Southeast Missouri Regional Planning and Economic Development Commission in Perryville, Mo. "Concentrations of volatile organic compounds and oxides of nitrogen are necessary, but it is the temperature and the sunlight that get things going."
This year was an ideal summer for ozone formation and it shows up in area readings. The temperature surpassed 100 degrees on three of the 14 days when exceedances of EPA Ambient Air Quality Standards for ground level ozone were recorded on the monitor in Farrar, Mo.
While the cooler weather is keeping the monitor under the standard for now, the damage to the area's three-year rolling average, referred to as a "design value," on which designations are based, has already been felt. The average is over the current standard of 75 parts per billion.
Last year's design value was in compliance at 73 parts per billion, this year, at 77 parts per billion, it is not.
Counties that fail to meet ozone standards are designated "nonattainment zones" by the EPA and face restrictions on their emissions of pollutants that contribute to ozone, which worries economic developers and businesses that generate these substances.
This year's number of exceedances is unprecedented. Five is the most ever recorded in one year and those occurred in 2010, according to the Missouri Department of Natural Resources monitoring data.
The highest reading in 2012 was 89 parts per billion June 29, when the temperature climbed to 107.
There are now 12 of the state's 23 ozone monitors with design values exceeding the current standard.
"The ozone design values statewide have been generally trending upward compared to the most recent three ozone monitoring seasons," said Renee Bungart, director of communications for the Missouri Department of Natural Resources.
Ozone standards are re-evaluated every five years, with the EPA scheduled to review ozone standard and boundary designations again in 2013.
The Obama administration was considering reducing allowable ozone levels from 75 parts per billion to somewhere in the range of 60 to 70 ppb, but last fall, President Barack Obama announced his administration would keep the existing standard set during President George W. Bush's term.
Grimes expects any revision to the standards will depend on the November election.
"I would anticipate that if Mr. Obama is re-elected that will be the target for the review. If Mr. Romney is elected, given the candidate and the party's focus on regulation -- or, rather, over regulation in their view -- that the standard will be retained at 75 ppb for at least the next five-year cycle," Grimes said.
The evaluation of the ozone standard will be conducted by the U.S. EPA and Clean Air Scientific Advisory Committee, an independent body that advises the EPA based on a review of the latest health studies, Bungart said.
The possibility of portions of Southeast Missouri being designated nonattainment is on the radar of both local companies and economic development officials. John Mehner, president and CEO of the Cape Girardeau Area Chamber of Commerce, is hopeful the implementation of new standards will be delayed.
"Depending on when new standards are implemented, if we have another summer under our belt and we have two good years surrounding a bad, we could still be under," Mehner said. Mehner is serving as interim director of Cape Girardeau Area Magnet.
"If we end up being nonattainment, then that carries consequences, but they haven't clearly defined what those consequences are in small areas," Mehner said.
In larger cities, companies can trade or purchase emissions "credits," but it's not clear how that would work in more rural areas like Southeast Missouri where there aren't many companies to trade with, he said.
Being designated a nonattainment area is also seen as a negative by industrial site selectors, he said.