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A group of environmentally minded Utah doctors plans to bring legal action against local television personalities who, the doctors say, have made illegal modifications to diesel vehicles' pollution control systems.

Utah Physicians for a Healthy Environment announced Wednesday morning that it has notified four men known collectively as the Diesel Brothers that the environmental organization plans to sue if they do not stop removing pollution controls from trucks serviced at their Wasatch Front businesses.

Brian Moench, president of Utah Physicians, said his organization has been aware since 2007 that some auto shops specialize in defeating automobile emissions systems, he said.

"We got a number of complaints from citizens about 'coal rollers,' " Moench said, " — some of these people who do this to enjoy the notoriety of putting a big black cloud of smoke over other motorists and pedestrians."

Moench said Utah Physicians has discovered online videos in which the Diesel Brothers — also featured on a Discovery Channel reality TV show of the same name — demonstrate what appear to be modified pollution control systems. The group, he said, also found videos in which the Diesel Brothers appear to demonstrate how diesel vehicles can be modified to bypass their emissions controls, and to advertise repair kits intended to facilitate these modifications.

Attempts to reach any of the Diesel Brothers for comment Wednesday were unsuccessful. In a December 2015 Facebook post, one of the Diesel Brothers said their videos were not about "rolling coal" — i.e., disabling emissions systems to produce a visible discharge of diesel smoke — and were not intended to irritate environmental regulators.

Moench said the Davis County Health Department later tested a vehicle modified by the Diesel Brothers and confirmed that it released significantly more pollution than an unmodified vehicle.

In a statement, Dennis Keith, manager of the Davis County Health Department's Air Quality Bureau, confirmed that the department had tested a modified diesel vehicle, but was unaware of any intended lawsuit. The allegations made by Utah Physicians, he said, "are very concerning to the Davis County Health Department."

"Tampered vehicles can have a very harmful effect on our air," he said in the statement. "These tampered vehicles are illegal and should not pass a Davis County emission test."

However, Keith said, the businesses named in the lawsuit weren't permitted to conduct emissions tests, and therefore fell outside the jurisdiction of the health department.

Diesel emissions include hazardous air pollutants — a subset of chemicals characterized by the Environmental Protection Agency as harmful to human health in relatively small quantities — that are associated with cancer, birth defects and neurological effects, Moench said. Additionally, he said, diesel vehicles are disproportionately large sources of more common pollutants such as nitrogen oxide and particles.

Moench said that even without modifications to bypass federally mandated emissions standards, diesel vehicles may be responsible for two-thirds of the particulate matter associated with transportation emissions in a given airshed. The majority of the soot released by diesel vehicles would be categorized as PM 2.5, the small particles notorious for building up under Utah's wintertime inversions, he said.

Utah Physicians is still working on an estimate of how many vehicles may have been modified by the Diesel Brothers. Moench said he believes the Diesel Brothers have been modifying diesel emissions systems at their Utah shops for about three years.

He said Utah Physicians seeks a cessation of the Diesel Brothers' activities, but would also like to see them issue some sort of recall on their work.

"We feel like they have a responsibility to undo what they have done," Moench said, "and that they should take back all the vehicles that they have modified and make them legal again."

Twitter: @EmaPen