On Friday, the Trump administration's Environmental Protection Agency (EPA) announced that it did not think that current mercury emissions rules placed on coal-fired power plants were "appropriate and necessary," based on the agency's revised look at the costs and benefits of the rule.

The Mercury and Air Toxics Standards (MATS) rule has been in place for years, and energy companies that own coal-fired power plants are already in compliance. The rule places limits on the amount of mercury that a power plant can emit. Mercury emitted into the air can end up in soil and water, where it has "toxic effects on the nervous, digestive and immune systems, and on lungs, kidneys, skin and eyes," in addition to causing developmental defects in children and babies, according to the World Health Organization.

But the coal industry has blamed the MATS rules for killing the power plants it sells to. Indeed, when the rule went into full effect in 2015, 30 percent of coal plant closures were tied to the cost of compliance with the rule. Today, coal plant closures continue not because of the MATS rule but because coal can't compete with cheaper and relatively cleaner natural gas.

Consequently, the Trump administration's EPA is leaving the current MATS rule in place while undercutting the justification for the rule in such a way that it could preclude more stringent mercury standards in the future and could possibly set the stage for looser rules in the future.

In a press release, the EPA said that compliance with the rule costs power companies $7.4 to $9.6 billion annually, while benefits tallied only $4 to $6 million annually. In a draft of its proposed rule (PDF), Trump's EPA said that the Obama-era EPA had erred by quantifying the co-benefits of reducing other, non-mercury pollutants in its analysis. The EPA writes:

"Although an analysis of all benefits and costs in accordance with generally recognized benefit-cost analysis practices is appropriate for informing the public about the potential effects of any regulatory action... this does not mean that equal consideration of all benefits and costs, including co-benefits, is appropriate for the specific statutory appropriate and necessary finding called for under CAA section 112(n)(1)(A)."

EPA employees worked through the week last week despite the government shutdown, but the Friday MATS rule justification revision is likely to be the last action the EPA takes until government funding is restored.