The Environmental Protection Agency is trying to protect a loophole that allows vastly more pollution than is typically permitted from heavy-duty commercial vehicles — all in the name of helping out a Trump supporter.

But the university behind the study the EPA is using to justify the exemption has disavowed it. And two former EPA administrators, a Democrat and a Republican, are urging the agency to close the loophole.

“EPA would be basing a rulemaking—which could have such profound negative health impacts on the American people—on a flawed analysis,” wrote former Administrators Carol Browner and Christine Todd Whitman in a letter this month.

At issue are glider kits, new trucks that come without an engine or transmission so they “glide.” This allows you to customize the powertrain — the engine, transmission, and drive axle — often with refurbished hardware, thereby saving around 25 percent of the cost of a new, complete truck.

“This process creates a reliable, more fuel efficient truck that requires less maintenance, yields less downtime and has the safety features and amenities owners have come to expect in trucks on the road today,” according to Fitzgerald Glider Kits, the largest assembler of glider kits in the United States.

Glider trucks were originally used as a stopgap in trucking fleets to salvage motors and other components from trucks that were otherwise worn out or damaged, but they quickly caught on as a way to sidestep new emissions rules and save money.

The mix-and-match trucks end up polluting 40 to 55 times more than new trucks, releasing compounds like soot and nitrogen dioxide that cause smog and hurt breathing. Since gliders contain refurbished engines, they aren’t held to the same pollution control standards as new trucks with new motors.

A glider truck builder often buys the rest of the truck from a manufacturer like Peterbilt and installs a rebuilt engine, many that are decades old and predate modern pollution controls. Gliders can retail for $130,000, while a new truck can run between $140,000 and $180,000.

And as Camille von Kaenel reported for E&E News, they are becoming a more significant source of pollution:

If their emissions are unchecked, gliders, which constitute 5 percent of all heavy-duty vehicles on the road, would generate by 2025 a third of the truck fleet’s emissions of nitrogen oxides, gases that contribute to smog and acid rain.

The Obama administration tried to close this loophole, introducing new standards for glider trucks and capping their production at 300 per year per manufacturer starting in January this year.

But now Trump’s EPA is trying to repeal these new standards, and there are politics at play.

As Eric Lipton pointed out in the New York Times, Fitzgerald, the glider kit company, hosted a campaign event for President Trump. The company also met directly with EPA Administrator Scott Pruitt and donated $225,000 to Rep. Diane Black’s (R-TN) gubernatorial campaign:

Ms. Black introduced legislation in 2015 to protect the loophole when it was first in line to be eliminated by a stricter diesel emissions rule under the Obama administration. That bill failed, but after the election of Mr. Trump, she turned to Mr. Pruitt to carve out an exemption to the new rule — scheduled to take effect last month — and presented him with the study from Tennessee Tech.

The Tennessee Tech study, funded by Fitzgerald, found that glider trucks pollute as much as or even less than new trucks, contradicting the EPA’s own assessment. But after multiple media outlets pointed out the conflict of interest in the study, the university’s president Philip Oldham on Monday sent a letter to Pruitt to inform him that an investigation into possible research misconduct was underway.

“We request that you withhold any use or reference to said study pending the conclusion of our internal investigations,” he wrote.

Manufacturers like Volvo have written to the EPA to oppose this exemption, arguing that new truck manufacturers have worked hard to comply with stricter pollution controls. Glider kits, they say, sometimes sell for as much as a brand new truck, which means that the main benefit comes from not paying for pollution controls.

“Clearly these glider customers are not ‘unable to afford a new truck,’” according to Volvo’s comments to the EPA. Truck fleet owners like UPS and even other glider truck assemblers have asked to close the loophole.

The EPA is still weighing a decision, but a dozen states have sued to block the exemption for glider trucks. “Simply put, gliders are a pollution menace that, unless properly regulated, threaten to undermine the entire national program to reduce harmful emissions from heavy duty vehicles and engines,” according to a legal document filed by the attorneys general of California, New York, Connecticut, Illinois, Maryland, Massachusetts, New Mexico, North Carolina, Oregon, Pennsylvania, Vermont, and Washington.