More U.S. communities might be required to crack down on ozone, a smog-causing pollutant linked to asthma and heart disease, as part of a controversial federal proposal announced Wednesday.

After years of delay, the Obama Administration faced a court-ordered December 1 deadline to update the current 2008 standard on ground-level ozone, a by-product of burning fossil fuels. Its proposal to toughen the ozone limit is one of several sweeping environmental efforts that have drawn opposition from Republicans in Congress and business groups because of potential economic costs.

"Bringing ozone pollution standards in line with the latest science will clean up our air, improve access to crucial air quality information, and protect those most at risk," Environmental Protection Agency Administrator Gina McCarthy said in announcing the standard. The EPA said that the people most affected by smog are children, older adults, and those who suffer from asthma or spend a lot of time outdoors.

Industry groups say a stricter standard would force many cities and counties to require pricey pollution-control equipment for vehicles, factories, and power plants, all of which would hike prices for consumers and manufacturers.

"It will be the most expensive regulation of all time," says Ross Eisenberg, vice president of energy and resources at the National Association of Manufacturers. "This is not just a power plant rule. It hits everyone." He points to an NAM-commissioned study, released in July, which estimated that a stricter standard could cost the economy $270 billion a year from 2017 to 2040.

Public health and environmental advocates say industry has cried wolf too many times.

"There is a long history of opponents overestimating the costs of compliance and underestimating the health benefits of cleaning up the air," says Elena Craft, a scientist at the Environmental Defense Fund. She says the past 40 years have shown that air pollution can be cut dramatically without slowing economic growth.

High in the atmosphere, ozone is not a problem. In fact, it can protect people from the sun's harmful ultraviolet rays, which is why there have been global efforts to protect that ozone layer. (Related: "Whatever Happened to the Ozone Hole" and "Healing the Ozone Layer: Chemist Says Treaty Is Working.")

But at ground level, ozone is a corrosive gas that forms when tailpipe or smokestack emissions of nitrogen oxides and volatile organic compounds are baked by the sun. It's usually worst in the summer.

As a new fight looms in Washington, D.C., here's what you need to know about the EPA's ozone proposal, slated to be finalized in October.

1. How would the standard change?

The EPA is proposing to limit ozone in the air to between 65 and 70 parts per billion—down from the current 75 ppb—and will take public comment on a standard as low as 60 ppb.

"Ozone is the most pervasive pollutant," says Paul Billings, a senior vice president of the American Lung Association. He says scientists have learned more about its hazardous health effects since the current standard was last updated, and he notes that research also links it to premature deaths, low birth weight, and neurological problems.

This year the EPA's own staff and its advisory commission of independent scientists recommended a range of 60 to 70 ppb. Health and environmental groups, including the American Lung Association, have pushed for 60 ppb. NAM and other industry groups have urged no change.

2. How many communities would be affected?

More communities might not meet a stricter standard, but it's difficult to say how many. Forty-six areas—home to 123 million people, or 40 percent of the U.S. population—do not meet the current standard. Most of them are urban and include the nation's capital, Atlanta, Chicago, Dallas, Houston, Phoenix, and 15 areas in California, Los Angeles and San Francisco among them.

Areas that don't take steps to meet the ozone standard risk several penalties, including the loss of federal highway funds. They would be given years, depending on the severity of their problem, to comply.

Most of the United States would not meet a 60 ppb standard, according to a map compiled by the American Petroleum Institute. Yet its map looks at a tougher standard than EPA is likely to adopt and is based on 2011-2013 air quality data whereas the EPA is expected to use later data.

EPA officials told reporters Wednesday that, beginning in 2025, the agency projects 9 counties outside of California will violate the 70 ppb standard and 68 counties outside the state will violate the 65 ppb limit. Its projections are far lower than the number of counties whose monitors measured ozone above these levels based on 2011-2013 figures.

The officials said other upcoming EPA rules could help reduce ozone-related emissions, including more stringent standards for power plants and vehicles.

"Whether these [rules] will be sufficient to help areas reach attainment is a key question," writes James E. McCarthy, an environmental policy specialist at the Congressional Research Service, in an October 2014 report. He says controlling ozone is more complicated than many other pollutants, because it is not directly emitted by power plants or other sources. So its amount depends not only on temperature and amount of sunshine but also the presence of precursor gases such as nitrogen oxide.

The EPA says ozone standards that have required scrubbers and other pollution-control equipment have yielded dramatic results, helping to slash average ozone levels 33 percent from 1980 to 2013. "We fully expect this progress will continue," McCarthy said, noting California has become a "birthplace of innovation" that could help other parts of the country.

3. Why Has the Update Taken So Long?

Politics. The ozone standard has long been contentious. First established in 1971 and updated in 1979 and 1997, the current one was established during the George W. Bush administration. But public health and environmental groups have sued the federal government, saying it does not adequately protect human health as required by the Clean Air Act.

In January 2010, the EPA said it would revisit the standard and subsequently submitted a 65 ppb update for White House review. In September 2011, after hearing industry complaints about the economic costs, President Obama ordered the EPA to withdraw its proposal, delaying an update until after his 2012 re-election and the 2014 mid-term elections. (See related story: "Obama: A No-Go on Ozone.")

4. How Costly Would the Standard Be?

The amount is fiercely debated. The EPA estimates that a 70 ppb standard will cost about $3.9 billion annually, beginning in 2025, and a 65 ppb standard will cost $15 billion.

Eisenberg expects the costs will be much higher. He says most power plants already have equipment to reduce emissions of nitrogen oxides, so a tougher standard could force the closure or scrappage of not only power plants but also factories, heavy-duty vehicles, and even passenger cars.

"Significant investments have already been made to reduce emissions, leaving few low-cost options as the ozone standard tightens," says the NAM report, conducted by NERA Economic Consulting.

Billings, of the American Lung Association, says many power plants don't use what he calls "mature, cost-effective" technology to reduce emissions. He says the health benefits of a stricter standard are enormous and could outweigh the costs.

On Wednesday, the EPA said that every dollar spent to meet a tougher standard will return up to three dollars in health benefits. "These large health benefits will be gained from avoiding asthma attacks, heart attacks, missed school days and premature deaths," the agency said, estimating the annual benefits would range from $6.4 billion to $13 billion for a 70 ppb standard and $19 billion to $38 billion for 65 ppb.

The EPA said most U.S. counties with ozone monitors would meet the more protective standards "just with the rules and programs now in place or underway."

The problem: these monitors are concentrated in cities, even though rural areas also have smog. At present, the Congressional Research Service reports, only 675 of the nation's 3,000 communities have ozone monitors in place.

Also, the CRS reports, ozone concentrations as high as those in Los Angeles, the nation's smoggiest city, have been found near oil and gas fields—during winter—in rural areas of Wyoming, Utah and Colorado. The sunlight needed to create ozone is magnified when reflected off heavy snow cover.

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