"What impressed me was the consistency of the findings that ozone clearly ... does have an effect," Dr. Evelyn Talbott of the University of Pittsburgh, who worked on the study, said in a telephone interview.

"It's small, but when you talk about a small effect over 300 million people, it's a lot."

The report looks at ground-level ozone, a component of smog, as opposed to the ozone found in the high atmosphere, which protects the Earth from ultraviolet rays.

Ozone is a form of oxygen formed by the reaction of sunlight on air containing other pollutants such as hydrocarbons and nitrogen oxide. It is a powerful oxidizer, meaning it can damage cells in a process akin to rusting.

It is known to cause respiratory problems and worsen heart disease. Children and the elderly are at special risk.

The EPA asked the National Research Council, part of the advisory National Academies of Science, to analyse the link between ozone and early death.

ILL AT GREATER RISK

A committee appointed by the council found that deaths related to ozone exposure are more likely among people with pre-existing diseases and other factors that could increase their susceptibility. But they said premature deaths are not limited to people who are already within a few days of dying.

They looked at studies that linked deaths directly with variations in ozone levels, as well as animal studies that examined whether there was a biological explanation for ozone causing death.

"Do you see the disease on days when ozone is higher? And the answer is yes," Talbott said. "There does appear to be a dose response."

The committee looked at studies done in several cities across the United States as well as in Canada and Europe. They took into account differences in temperature and humidity that may affect the ozone level.

The effects on deaths are clear, Talbott said -- and the findings excluded serious illnesses and visits to the emergency room if the patient did not die.

"If you have a town that has got many old people ... then obviously this ozone thing is probably a bigger player," Talbott said. "It touches everybody but I think it touches the infirm and elderly (more)."

The EPA toughened standards for ozone pollution in March but outside experts complained its new requirements were more lax than the EPA's own scientists recommended.

The new standards are 75 parts per billion in ambient air in the United States. The previous standard was 80 parts per billion.

The EPA's Clean Air Scientific Advisory Committee recommended a standard of 60 to 70 parts per billion.