WASHINGTON — Children in the Salt Lake area are breathing some of the dirtiest summer air in the nation, according to a study released Monday by the watchdog group Environmental Defense.

The report ranked Salt Lake City's air quality the 37th worst in the nation. It based the ranking on a formula that multiplied the number of bad summer air days by the number of children in the area.

Los Angeles ranked the worst, followed by Riverside-San Bernardino, Calif., New York City and Philadelphia.

Laura Vernon, spokeswoman for the Department of Environmental Quality, was surprised with Salt Lake City's ranking. The Wasatch Front did not have a single voluntary no-drive day this year, the first time that's happened since 1999, she said. Last year, the state had 17 days when air quality was deemed bad enough to ask those who could to stay off the road to avoid contributing to pollution.

"We think overall, the air quality in Utah is good," Vernon said. "We may on occasion have a few bad days."

John Balbus, director of the health program at Environmental Defense, said there is room for improvement.

"Fighting for clean air in this country means fighting for the millions of kids that struggle to breathe because of pollution," he said. "The good news is that the country can curb the triggers of asthma and ease the burden of other health consequences from air pollution."

The report found that 34,374 children in the Salt Lake area have asthma that is aggravated by dirty air. The Utah Department of Health has a different number — 36,000 children with asthma in the entire state of Utah, said Mindy Williams, asthma program grant coordinator.

Environmental Defense used the report to call for a reduction in harmful pollution from power plant smokestacks "instead of weakening long-standing clean air protections."

There's no question air pollution creates health problems, especially for those who already have respiratory problems or heart defects, Williams said.

"We do know that air quality can affect health, particularly in children."

Children feel the effects of bad air especially keenly because they "breathe more air per pound than adults," Williams said. There are several reasons. For one thing, they often breathe through their mouths, which don't filter air like breathing through the nose does. They are also more likely to be active and outside, and "continue to play even if they have problems breathing."

Studies have shown that people who have sensitive airways are more likely to have health problems at lower levels of pollution. Those who don't have breathing problems are able to tolerate higher levels of pollution.

The Department of Environmental Quality maintains an air quality index, which measures pollutants in the air and assigns a number based on what it finds. In summer, ozone tends to be higher, while particulate matter dominates in winter.

The air quality index runs from zero to 500. Williams said whenever it rises above 101, people who are sensitive to bad air should stay inside and avoid outdoor exertion, regardless of age. When it reaches above 150, children's physical activities should be moved inside and all adults should avoid strenuous physical activity as well.

Williams also suggests that since ozone tends to peak around 4 or 5 p.m. — a time when many people are most active, including children playing after school — pollution can be a particular concern. They suggest school officials move events like football practice to the early morning instead for that very reason, Williams said. The mild summer may account for some of Utah's better air quality days, Vernon said. Last year, when pollution was more severe, it was unusually hot for an extended period of time.

"I also think that people are paying more attention to their everyday activities and doing what they can to reduce that," she said.

According to the study, 8 percent of summer days in the Salt Lake area were "dangerous." Based on the formula, that equals 648,118 dangerous summer days per year over the past three years. That, the report stated, is equivalent to exposing 110 school buses filled with children to bad air for the entire summer.

E-mail: spang@desnews.com; lois@desnews.com