This article has been revised since it was published on March 26.

Mention the word "ozone" and many people think of the ozone layer, the part of the atmosphere that deflects ultraviolet rays.

Closer to the ground, ozone is the main ingredient in smog. The air pollutant is caused by manmade chemical reactions and can cause respiratory issues.

Experts say the Austin area meets all federal air quality standards but has struggled to stay under the accepted ozone levels.

A group of local governments known as the Central Texas Clean Air Coalition has spent years working on voluntary programs to reduce air pollution. In April, the coalition will receive the 2014 Clean Air Excellence Award from the U.S. Environmental Protection Agency for its efforts.

Officials say the work to improve Austin's air is ongoing. The area is still close to the federal ozone limit, and the EPA could tighten restrictions later this year.

"If we fail to meet [EPA] requirements, we face possible loss of federal funding for transportation projects. And Austin and Central Texas have been flirting with this federal nonattainment status for years," Mayor Lee Leffingwell said in his State of the City speech Feb. 25. "If traffic congestion continues to grow like this, we'll reach it soon—and pay a price when we do."

Acceptable levels of ozone

Every five years the EPA is required to review the National Ambient Air Quality Standards as part of the Clean Air Act.

In 2008 the EPA lowered the national eight-hour ozone average standard from 85 to 75 parts per billion of total gas in the air.

How the EPA measures ozone is that cities such as Austin can exceed the standard three days a year, said Andrew Hoekzema, air quality program specialist for the Capital Area Council of Governments.

"They use the fourth-highest readings each year and average that across three years," he said. "If the three-year average of the fourth highest [readings] is above 75 parts per billion, the area is considered in nonattainment."

If the EPA declares an area is in nonattainment, the state must enact a State Implementation Plan to meet the standards.

Securing federal funding would become more complicated, Hoekzema said.

"The region's transportation plan is required to be consistent with its air quality plan," he said. "There's a cap on road emissions, and the transportation plan must stay under the cap."

Meeting the standards

CAPCOG's Central Texas Clean Air Coalition comprises Bastrop, Caldwell, Hays, Travis and Williamson counties and 13 city governments.

From 2008 to 2013, the coalition enacted the Eight-Hour Ozone Flex Plan.

The coalition secured voluntary emission reduction commitments from local government entities on items such as reducing commutes and vehicle fleet efficiency measures, according to CAPCOG.

The award-winning plan helped to bring the average readings down from 74 to 73 parts per billion as of 2013, Hoekzema said.

The flex plan has since evolved into a fourth version, the Ozone Advance Program, that expands on what types of emission reduction commitments are possible, Hoekzema said.

"The region is poised to be in a position to meet a new ozone standard if it is set as high as 70 parts per billion by 2018 but would be challenged to meet one as low as 65 parts per billion," according to the program's action plan.

Meanwhile the EPA had not completed its review of the 2008 standards by the end of 2013. It is roughly halfway through the process, EPA Region 6 spokesman Joseph Hubbard said.

The city expects the EPA to issue stricter standards by December, Environmental Program Coordinator Pharr Andrews said.

She said the area's population growth is a big issue and that the city and its partners are taking steps to address it (see sidebar).

"There is no real silver bullet, but all of these efforts add up to cleaner air for Austin," she said.

Environmentalists

Neil Carman, clean air program director for the Lone Star Chapter of the Sierra Club, said the organization advocates stricter standards for vehicle tailpipe emissions and industrial smokestacks.

The Sierra Club supports telecommuting, carpooling, public transportation and bicycle riding as ways to address the issue.

Carman called the federal 75 parts per billion standard inadequate to protect human health and said the standard should not exceed the 70 parts per billion that was recommended by the EPA's advisory group.

Activist Scott Johnson said vapor recovery for gas stations should be a priority.

He advocated restricting vehicle idling, reducing industrial emissions and promoting cleaner fuels for construction equipment. He said the Texas Commission on Environmental Quality has limited resources to implement these programs.

"Look at construction equipment," he said. "Cumulatively there are hundreds to even the low thousands of pieces of equipment [in Austin], and that's very consequential for ground-level ozone."