The US Environmental Protection Agency next month will unveil plans to start weakening the economic justification for a rule limiting toxic mercury pollution from coal plants.

The EPA isn’t rescinding the standard as of yet but has finished deciding to reconsider the underlying analysis for the 2011 rule, according to the government’s newly published agenda.

Donald Trump’s administration is revoking major environmental protections enacted by the Barack Obama administration, arguing Obama rules overestimated health benefits and undercounted costs.

Conservatives say Obama’s EPA shouldn’t have counted health improvements from the mercury rule that would have come from eliminating other kinds of pollution from coal plants, including soot and nitrogen oxide linked to respiratory illnesses and early deaths.

Power companies have already spent the money to comply with the mercury standard, but the EPA wants to rework the cost-benefit analysis behind the regulation anyway. Air law experts say the move could set the agency up to roll back other pollution protections that depend on the mercury rule math. It could also throw settled other regulations into question, setting off a frenzy of lawsuits against the government and businesses, they say.

Obama’s EPA estimated the rule would result in $37bn to $90bn in annual health benefits but that only a small portion of that would be from reducing mercury levels. The agency expected that installing mercury controls would cost industry $9.6bn per year, so the benefits would outweigh the costs. Excluding those secondary benefits would imply the rule cost more than it was worth.

The supreme court in 2015 ruled the EPA broke the law in deciding to regulate mercury without first considering the costs. The EPA in 2016 responded to that with a new analysis. The agency now wants to reassess.

The EPA is still working on its required follow-up review of the risks from mercury and the technologies available to limit it and will issue a draft along with the proposal to re-evaluate the cost-benefit analysis in November.