WASHINGTON (AP) — There's new evidence that exposure to exhaust from diesel engines increases the risk of lung cancer.

Diesel exhaust has long been classified as a probable carcinogen. But the 20-year study from the National Cancer Institute took a closer look by tracking more than 12,000 workers in certain kinds of mines — facilities that mined for potash, lime and other nonmetals. They breathed varying levels of exhaust from diesel-powered equipment, levels higher than the general population encounters.

The most heavily exposed miners had three times the risk of death from lung cancer compared to workers with the lowest exposures, said the study released Friday by the Journal of the National Cancer Institute.

But even workers with lower exposures had a 50 percent increased risk, wrote lead author Debra Silverman, an NCI epidemiologist.

"Our findings are important not only for miners but also for the 1.4 million American workers and the 3 million European workers exposed to diesel exhaust, and for urban populations worldwide," Silverman wrote.

She pointed to some highly polluted cities in China, Mexico and Portugal that in past years have reported diesel exposure levels that over long periods could be comparable to those experienced by miners with lower exposures.

Litigation from some mining companies had delayed release of the study findings.

A separate industry group not involved in that litigation said Friday that the study looked back at mines using decades-old equipment, and there's far less pollution from diesel engines today.

"Diesel engine and equipment makers, fuel refiners and emissions control technology manufacturers have invested billions of dollars in research to develop and deploy technologies and strategies that reduce engine emissions, now ultimately to near zero levels to meet increasingly stringent clean air standards here in the United States and around the world," said Allen Schaeffer of the nonprofit Diesel Technology Forum.