A previously unknown class of pollutants, formed in combustion ranging from cigarettes to power plants and diesel engines, was announced at the American Chemical Society's national meeting.

Whether they are harmful to human health is unknown, although the chemist who discovered the particles said their existence might help explain why some nonsmokers get lung cancers and other diseases often associated with tobacco use. He is already working to develop a new cigarette filter that may remove more of the harmful substances.

H. Barry Dellinger, a combustion chemist and professor at Louisiana State University in Baton Rouge, said yesterday at the Convention Center that the particles can last for "hours, days and in some cases indefinitely," and can be transported long distances.

Because of their longevity, he termed them "persistent free radicals."

Scientists have long been familiar with "free radicals" - atoms, molecules or portions of molecules that are unstable, making them highly reactive.

They can damage cells in the body. But diets heavy on fruits and vegetables containing "anti-oxidants" are credited with preventing the damage.

Most atmospheric free radicals exist for less than a second. What's different and distressing about the substances Dellinger found, he said, is that they persist.

The process begins when combustion in an industrial smokestack, automotive tailpipe or even a household chimney releases fine particles of unburned material, such as soot.

As the gases begin to cool, altered versions of chemicals in the exhaust - the persistent free radicals - form and attach themselves to the soot particles.

A substance's persistent free radicals may be more toxic than the substance itself, Dellinger said.

Bill Suk, acting deputy director of the National Institute of Environmental Health Sciences, said the research was "significant" because "it takes the monitoring and the exposure of products of incomplete combustion to yet another level of potential impact on people's health."

Whether it actually affects people's health, however, remains unknown, he said.

The federal Environmental Protection Agency, which has devised national standards for particle pollution, estimates that more than 15,000 premature deaths per year can be attributed to fine particle pollution.

It can exacerbate conditions such as asthma, chronic bronchitis and lung disease.

Adding persistent free radicals to the mix of already known particulates could help scientists answer some questions.

"If you have an understanding of how these are formed and then can figure out how they interact in a biological way, then you can have a basic understanding of how diseases progress and are formed," Suk said.

"Our job is to figure out and determine how to link the exposure with the disease."

Dellinger, who is the Patrick F. Taylor Chair of Environmental Chemistry at LSU, agreed that epidemeologic studies "are called for at this point."

"I'm a chemist," he said. "We're working on determining just what types of radicals, the breadth of how many radicals could be formed, what sources could be responsible."