The Environmental Protection Agency is about to set in motion a review of one of the most significant — and expensive — air-pollution regulations the agency has ever issued.

The big picture: The rule, which sets first-ever federal limits on mercury and other toxic air pollution from coal-fired power plants, was issued in 2012 and virtually all operating coal plants are complying with it now.

The bottom line: Given that compliance, whatever EPA does probably won’t make a difference in President Trump’s goal to revive coal. But the agency's move could make it easier for currently-operating coal plants to comply with it and also the way future regulations are written.

Driving the news: The agency is going to soon send for review a draft proposal to the White House questioning the finding that the EPA needed to set the standards at all, according to Bloomberg Environment, which first reported the news Wednesday. An EPA spokeswoman also told Axios the agency plans to address questions raised by a Supreme Court ruling in 2015 finding that Obama’s EPA didn’t consider costs appropriately when writing the rule.

Flashback: This rule has a long and litigious history going back decades, but let’s focus on just what occurred under President Obama.

When Obama’s EPA issued the rule, it found it would cost power companies an annual $9.6 billion, making it one of the EPA’s most expensive regulations ever.

The agency said the public-health benefits of between $37 billion and $90 billion per year outweighed those costs.

The EPA arrived at those large benefit numbers by including indirect pollution co-benefits other than direct reduction in mercury pollution, a move Trump’s EPA is likely to review.

What’s next: The EPA will issue a draft proposal within the next few months, which will go through a public comment period before it becomes final after that.