MANHATTAN (CN) – Hours after his inauguration, President Donald Trump issued a directive that led the Environmental Protection Agency to pull back new regulations aimed at limiting the discharge of toxic mercury. Now the Natural Resources Defense Council claims in a federal action that the maneuver should be vacated for violations of agency procedure.

When the White House turned back the clock on Jan. 20, according to the complaint, the final rule had already been posted for a day and scheduled for publication in the Federal Register

Signed by the EPA administrator over a month beforehand, the rule at issue would prevent dental offices from discharging 5.1 tons of mercury into public sewers.

“Dental offices are the main source of mercury discharges to municipal sewage treatment plants,” the complaint states, noting that mercury makes up about 50 percent of the amalgams used in fillings.

The NRDC noted that the rest of the amalgam is made up of some “combination of silver, tin, copper, zinc, and small amounts of indium and palladium,” and indeed the final mercury rule, as it was known, would have also blocked the annual discharge of 5.3 tons more of other toxic metals.

Among other reasons, according to the complaint, mercury in the water is dangerous because interactions with bacteria can produce methylmercury, a highly toxic form substance that bioaccumulates in fish and shellfish.

The EPA’s rule would have required dental offices to capture waste mercury via amalgam separator, so that the element could be recycled rather than flushed. Regulators estimated that implementation of the new rule would reduce the nationwide discharge of dental mercury to surface water from 1,003 pounds to 11 pounds.

With the cost of implementation estimated at between $59 million and $61 million, the American Dental Association even called the final rule as a “fair and reasonable approach to the management of dental amalgam waste.”