In a decision criticized by both environmental activists and business concerns, the Bush administration on Wednesday imposed stricter ozone limits affecting air quality in Atlanta and other cities.

Even under the old standards — allowing for a maximum concentration of .08 to .084 parts per million units of ozone, a component of smog — metro Atlanta's air was considered hazardous to public health.

The new Environmental Protection Agency mandate is .075 parts per million. Nationwide, 345 counties — including Cobb, DeKalb, Fulton, Gwinnett and 10 others in Georgia — will fail to meet the new standard.

One immediate consequence is that Atlantans can expect more smog alerts.

Metro Atlanta violated the old standard 29 days in 2007. The new requirements could place the area in violation every other day during the summer — meaning, on the worst days, more Air Quality Index warnings for people to avoid outdoor physical activity.

"Clearly, the EPA and the executive branch were lobbied pretty heavily by industrial groups," said Brian Gist, an attorney with the Southern Environmental Law Center. "This continues a pattern of unwillingness by this administration to make the hard decisions required to ensure clean air."

Gist said he was disappointed that EPA Administrator Stephen Johnson rejected suggestions by an independent panel of scientists and public health experts to allow ozone concentrations of no more than .070 parts per million.

Business interests had argued against lowering the standard at all.

"The costs are too high and the benefits too unclear to impose this new burden on America's manufacturers and employees," said John Engler, president of the National Association of Manufacturers. "Anyone interested in preserving high-paying U.S. jobs in manufacturing and keeping a lid on energy prices should be disappointed in today's ruling."

Johnson told Congress last year that the impact of ozone is more harmful than previously believed. He has said that existing air quality standards, dating to 1997, were not stringent enough. More than half of the U.S. population, including more than 4 million metro Atlantans, lives in areas whose summertime air is considered hazardous by the EPA.

Already struggling to meet the old ozone standards, the state will now have to find ways to meet the new directive. These Georgia counties are in violation of the new EPA standards: Bibb, Athens-Clarke, Cobb, Coweta, DeKalb, Douglas, Fayette, Fulton, Gwinnett, Henry, Murray, Paulding, Richmond, Rockdale.

Expect tougher regulations for coal-fired power plants, engines and fuels, as well as bans on outdoor burning and other controls. Heather Abrams, chief of the Air Protection Branch of the state Environmental Protection Division, said one area that can improve is diesel-powered vehicles and rail yards

Individual communities would have to submit their plans to meet the new standard. Failure to meet them could lead to a loss of federal funds.

Scientists lists ozone among the most harmful air pollutants. A gas created by a chemical reaction, it can be found in the stratosphere high above the earth, or at ground level.

At high altitudes, the substance, often called "good ozone," occurs naturally and forms a protective layer that helps block potentially harmful ultraviolet rays.

At ground level, the so-called "bad ozone" arising from motor vehicle emissions, factories and chemicals solvents is a component of smog and can be unhealthy, especially in warmer and sunny weather.

Ground-level ozone can increase the likelihood of respiratory illnesses including bronchitis and pneumonia, worsen an asthmatic condition and harm lungs, studies have shown. It especially threatens children and the elderly.

The EPA, as required under the Clean Air Act, established national air quality standards for six air pollutants including ozone to protect the public and environment. The law also requires the agency to review and, if necessary, change those standards.

The agency was under court order to review its standard for ground-level ozone by Wednesday.

Communities violating the new standard will be required to install tighter pollution controls, including cleaner-burning gasoline and yearly vehicle emissions tests.

An EPA analysis of the proposed standards concluded that reducing smog nationally could cost as much as $3.9 billion annually to pay for cleaner gasoline, cleaner vehicles and industrial pollution controls. Most of that would be spent in eastern states, including Georgia.

The study said reductions in pollution could save as many as 5,400 lives a year in the U.S. by 2020.

— The Associated Press contributed to this article.