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An environmental group threatened to sue two of the nation's biggest rail owners Tuesday under a novel legal theory that would classify diesel exhaust as hazardous waste.

The Natural Resources Defense Council sent letters to Union Pacific Corp. and Burlington Northern Santa Fe Railway, saying it will file a lawsuit within 90 days under the Resource Conservation and Recovery Act, which regulates hazardous solid waste disposal.

The notice letter, which is required before proceeding with a lawsuit, cited problems at 16 rail yards across California, from Oakland to San Bernardino.

The conservation group argues that minute particles in diesel air pollution, which include lead, cadmium, arsenic and other toxic elements, are solid waste. If successful, such a suit could open the door for legal action against similar air pollution sources such as ports, airports or anywhere with a lot of diesel equipment, said David Pettit, a senior attorney with the council.

"I think the reason why other people haven't tried it is on first glance you would think that the emissions are a gas and RCRA doesn't apply to gases," Pettit said. "The fallacy with that is the exhaust has two components: one is a gas and the other component is a solid and those solids will kill you if you inhale enough of them."

Lena Kent, a spokeswoman for Fort Worth, Texas-based BNSF, did not immediately return a call and emails for comment.

Aaron M. Hunt, a spokesman for Omaha, Neb.-based Union Pacific, did not address the notice but said the company had made substantial progress in reducing emissions over the past several years.

"Union Pacific operations are regulated at the state and federal levels, and we are in compliance with those environmental requirements," he said in an email. "Union Pacific has voluntarily worked with state and federal regulators for more than a decade to substantially reduce locomotive and other emissions in and around California rail yards."

Millions of cargo containers loaded on trucks and trains travel by freeway and railway through Southern California then to the rest of the country. The West Coast ports are the nation's principal gateway for cargo container traffic from Asia, with the ports of Los Angeles and Long Beach handling about 40 percent of the nation's cargo.

The rail yards have long been blamed for health problems in communities along those transit corridors. The tiny particles contained in diesel exhaust can penetrate deep into the lungs, carrying a variety of toxins that have been linked to acute bronchitis, lung disease, heart attacks and other health problems.

Exposure to the pollution is especially dangerous for children whose lungs are still developing and the elderly, whose immune systems may be compromised.

State and local air quality regulators have struggled to regulate train pollution. A federal appeals court last year struck down a lawsuit involving a local air regulator that wanted to reduce pollution from idling locomotives. The court determined the agency was overstepping its authority because only the federal government is authorized to regulate interstate commerce.

Southern California air quality regulators recently announced a major study focusing on a San Bernardino rail yard that has been found to pose the greatest health risk of any rail yard in the state. The two-year study will cost an estimated $846,000, and researchers hope it will determine whether there is a higher asthma and fatal cancer rate in the surrounding community.

The Natural Resources Defense Council's notice of intent to sue was also sent by two environmental justice groups, East Yard Communities for Environmental Justice and the Center for Community Action and Environmental Justice, which have been working for years on rail pollution and say little has been done.

"It is time for the rail companies to be good neighbors and right the wrongs they have imposed on California communities," said Angelo Logan, executive director for East Yard Communities.

The letter recommends a series of remedies for the pollution, including the use of cleaner locomotives, electrifying rail lines in urban areas and reduced idling.

Environmental law experts say that while RCRA regulates solid hazardous waste, it could potentially be applied to diesel particulates.

Holly Doremus, an environmental law professor at the University of California at Berkeley, said there has been debate over what constitutes solid waste, but in this case there's no doubt the tiny toxic elements are waste.

She added that typically the Clean Air Act is supposed to apply to such particulate pollution but the restrictions haven't been particularly effective.

"It's not a slam dunk either way, and I think it's very creative by the NRDC to have found this possibility," she said.