The air in the Philadelphia region, one of the smoggiest in the nation, is worse than people think, environmental groups said Wednesday.

Under the tougher standards for ground-level ozone that President Obama rejected three weeks ago, an estimated 25 percent more days would be considered unhealthy for sensitive groups.

In a news conference at City Hall, PennEnvironment, an advocacy group, sought to keep alive a contentious issue that doctors say may have a direct impact on public health. The group's affiliates are making the same argument across the country.

The Philadelphia region ranked fifth among large metro areas last year, with 29 days exceeding the current standard for ground-level ozone, the group said.

A tighter standard - rejected by the U.S. Environmental Protection Agency under President George W. Bush and then again by Obama, who overruled his own EPA administrator - would have meant an estimated 36 orange days on the government's color-coded air quality index, which translates to "unhealthy for sensitive groups."

Patients with asthma and other respiratory conditions are put at risk, regardless of their medications, simply "by breathing dirty air," said James Plumb, a community medicine specialist at Thomas Jefferson University Hospital, who spoke at the news conference.

Ozone high in the atmosphere shields the Earth from destructive ultraviolet rays. But ground-level ozone, the main component of smog, is harmful. It is formed when two categories of pollutants - nitrogen oxides, mainly from vehicles and power plants, and volatile organic compounds, from vehicles and various other sources - react in sunlight and summer heat.

Air quality here and across the country has improved dramatically but still causes problems for asthmatics and others with respiratory problems. Houses are built tighter, sealing in pollutants, and obesity can worsen breathing conditions.

"When ozone is high, you see a huge increase in emergency room visits," said pulmonologist Reynold Panettieri, director of the asthma program at the University of Pennsylvania Perelman School of Medicine, who was not involved with the new report.

Ozone induces airway inflammation, which asthma medications do not significantly help, he said. Patients, especially children and the elderly, may experience chest tightness, coughing and wheezing, during the day and waking them up at night. So doctors recommend that they stay indoors on high-ozone days, use air-conditioning, and exercise early or late, when it is cooler.

Over the last five or 10 years, research has found that inhaled ozone goes beyond the lungs to cause inflammation in and around blood vessels, becoming a factor in cardiovascular disease, said Panettieri. Although high ozone levels may trigger angina in some people with heart disease, he said, the main effect, unlike for asthma, is hidden and gradual.

"There is pretty compelling evidence that [ozone limits] should be moving downward," he said.

The current standard - the one that was exceeded locally on 29 days last year - was set by the Bush administration at a concentration of 75 parts per billion over eight hours. The EPA's scientific advisers had recommended 60 to 70 ppb. (The PennEnvironment report calculated the extra days based on a theoretical standard of 70 ppb.)

Environmental groups were furious at what they considered to be capitulation to industry, and sued. Industries also sued, arguing that meeting the tighter standard would be costly and unnecessary.

Obama's EPA administrator, Lisa P. Jackson, reopened the issue and made clear that she supported the tighter standard. When the White House overruled her on Sept. 2, environmental advocates were again furious, and surprised as well - and vowed to continue the legal action they had set aside.

The president has angered environmentalists on several issues recently, although another smog rule, limiting "cross-state pollution" from power plants, is scheduled to take effect next year.

Douglas Biden, president of the regional Electric Power Generation Association, referred to that rule Wednesday when asked to respond to the new environmental report urging tougher standards. "It's kind of like getting blood out of a stone," he said, noting that on hot summer days the existing standard still isn't being met.

Environmentalists are mounting a semi-orchestrated campaign. Shortly after PennEnvironment's City Hall news conference Wednesday, the New Jersey Sierra Club sent out a news release citing the same report, titled "Danger in the Air."

The day before, the National Resources Defense Council and other organizations released a separate report titled "U.S. Latinos and Air Pollution: A Call to Action."

Ozone was a key focus of that report, which quoted federal statistics estimating that nearly half of Hispanic Americans and Asian Americans live in counties that frequently violate current standards.

Contact staff writer Don Sapatkin at 215-854-2617 or