The world's most prestigious cancer research group on Tuesday classified diesel engine exhaust as carcinogenic to humans and concluded that exposure is associated with increased risk of lung cancer.

The International Agency for Research on Cancer - part of the World Health Organization - made the announcement at a meeting in France, finding, in part, "that diesel exhaust is a cause of lung cancer, and also noted a positive association with an increased risk of bladder cancer. The Working Group concluded that gasoline exhaust was possibly carcinogenic to humans."

The California Air Resources Board came to the same conclusion in 1998, finding that particulates associated with diesel emissions from trains, trucks, tractors and construction equipment pose a hazard to public health. That decision was met with an outcry from fuel producers and other industry groups who argued that regulating diesel emissions would cripple California's economy.

When the state's Scientific Review Panel made that determination 14 years ago, it was the first official agency to connect diesel to cancer, according to John Froines, who chairs the review panel, which identifies toxic air contaminants for the state.

Froines, a professor of public health at UCLA, said that Tuesday's announcement will be difficult to refute because the International Agency for Research on Cancer is known to be conservative and careful about its pronouncements.

"The implications of this decision are immense," he said. "That agency is the most prestigious scientific organization in the world. If it says diesel is a  human lung carcinogen, you can be sure they are undoubtedly correct.

"What happens next?" Froines speculated. "In the United States, one would hope that the Environmental Protection Agency would adopt the same stance, and that other states would follow IARC's lead. You would hope that it would be taken up by other countries."