(CNN) -- Exhaust from diesel engines can cause cancer, a prominent global cancer group that's part of the World Health Organization said Tuesday.
While the International Agency For Research On Cancer (IARC) has no power to set or enforce rules, many governments look to it for guidance and the decision could put pressure on those governments to introduce stricter limits on emissions, especially to protect workers who are exposed to diesel exhaust while on the job.
The IARC has for more than two decades classified diesel engine exhaust as a "probable" carcinogen -- a cancer-causing agent -- but until recently there was no clear evidence linking it to higher cancer rates.
This winter, however, two studies were published based on research involving more than 12,000 mine workers done by the National Cancer Institute and the National Institute for Occupational Safety and Health, known as the Diesel Exhaust in Miners Study, or DEMS.
Together, the two new papers found an increase in lung cancer rates among workers exposed to diesel exhaust underground, with greater exposure linked to steadily higher cancer rates. In workers with the highest exposure, deaths from lung cancer tripled in one study, and increased five-fold in the other.
It's unclear whether the decision will affect rules in the United States. Since 2008, the Mine Safety and Health Administration has enforced a limit of 160 micrograms of total carbon per cubic meter for workers in U.S. mines. "Total carbon" is used as a marker for diesel fumes.
Luke Popovich, a spokesman for the National Mining Association, said the IARC decision has "little bearing" on current conditions. Popovich said the recent studies aren't relevant, because they looked at worker exposures from the 1950s until the 1990s, when older and dirtier equipment was in use.
"It therefore does not reflect the technology changes that have been made since then," he said.
Industry groups and clean-air advocates alike say recent improvements are dramatic. The Diesel Technology Forum and the National Resources Defense Council both estimate that diesel emissions, including dangerous particles, are reduced by 99% in newer engines.
However, older engines are still in widespread use, especially in poor countries. Dr. Christopher Wild, the IARC director, said the United States and Europe already have "stringent" guidelines on diesel fumes, but that there is "relatively little information about diesel exhaust in developing countries." Other scientists warn that cancer takes years or even decades to develop, meaning that people exposed to fumes in the past are still at risk.
Mining groups have criticized the DEMS research ever since its launch in 1992. Last year, scientists funded by the industry-backed Mining Awareness Research Group (MARG) wrote a paper criticizing the DEMS study, saying its methods for estimating exposure were "imprecise" and "unreliable."
Henry Chajet, an attorney with Patton Boggs, a law firm that represents the mining awareness group, responded to the IARC decision by saying, "MARG members are deeply concerned about the study and its conclusions." He said researchers had no way to reliably estimate exposure, that the results are "confusing" and that researchers haven't shared all their data.
Chajet is an aggressive critic. Earlier this year, he sent letters to the Lancet and other medical journals, urging them to hold off publishing papers on the DEMS study. "We respectfully request that you and your counsel carefully consider any intent to publish these papers, as well as the impact and consequences of any such publication or distribution," Chajet wrote in his letter to the Lancet.
The IARC didn't specify a level at which diesel fumes are harmful, but data from DEMS suggests that cancer risks go up even at relatively modest levels -- the equivalent of air pollution in some major cities, including London, Mexico City and the Bronx section of New York City.
The IARC does not make policy recommendations, but Dr. Kurt Straif, the head of the classification program, warns that the danger may not be limited to mine workers. "We have learned from other carcinogens, such as radon, that initial studies showing a risk in heavily exposed occupational groups were followed by positive findings for the general population. Therefore actions to reduce exposures should encompass workers and the general population."
In a polluted city, the cancer risk may be low for a given individual, "but if you've got a ubiquitous exposure such as diesel engine exhaust, then it could make an awful lot of people ill," warns Lesley Rushton, a biostatistician at Imperial College London. Earlier this year, Rushton wrote an editorial in the Journal of the National Cancer Insitute, urging "stringent" standards for both occupational and environmental exposure.
Rushton also said the health risk isn't limited to cancer. For example, people with asthma had reduced lung function after simply walking for two hours down a London street heavily trafficked by diesel-powered cars and buses.