In a brief legal memo, the Trump EPA has dropped “once in, always in” (OIAI), a Clinton-era EPA policy that aimed to lock in reductions of hazardous air pollution from industrial sources.

Industry lawyers and Senate Republicans have long argued that eliminating OIAI will actually provide a stronger incentive for businesses to reduce emissions, since they can now more easily lower emissions and avoid the regulations that major pollution sources must endure.

However, environmental activists and lawyers are concerned with the abrupt change, saying that it may actually increase exposure to hazardous air pollution—especially among vulnerable populations, who live near major industrial polluters more often.

“They’re really going to be killing people,” said Hip-Hop Caucus vice president Mustafa Ali, the former environmental justice head at EPA, in an interview with Earther. “You’re going to have all types of public health problems.”

To see how OIAI worked, imagine a business that emits 11 tons of a given hazardous air pollutant (HAP) per year. Under EPA regulations, facilities that emit more than 10 tons of one HAP, or 25 tons of HAPs in total, are reclassified from area sources to major sources.

By law, major sources must retool their processes to get their emissions down to the lowest levels set by peers within the industry. These benchmarks are called the Maximum Achievable Control Technology, or MACT, standards.

By hewing to MACT standards, let’s say the company’s HAP emissions go down from 11 tons per year to three tons. According to OIAI, the company would have to abide by MACT standards permanently, locking in eight tons of annual emissions reductions.

Under the new EPA policy, however, the company could do just enough to reduce emissions from 11 tons to nine. By dropping below the 10-ton threshold, the company goes from being a major source to being an area source—thereby jettisoning the MACT requirement.

While going from 11 tons of emissions to nine is technically a reduction, it’s actually more pollution relative to what the facility could have achieved by complying with MACT standards. This phenomenon, called “backsliding,” is what OIAI aimed to prevent.

Environmental groups are poised to sue the EPA to block the policy change.

“This is among the most dangerous actions that the Trump EPA has taken yet against public health,” said John Walke, clean air director at the Natural Resources Defense Council (NRDC), in a statement. “NRDC will fight this terrible decision to unleash toxic pollutants with every available tool.”

Full list at National Geographic