Dive Brief:

• The Environmental Protection Agency on Friday moved to reconsider the rationale behind the Mercury and Air Toxics Standards (MATS), an Obama-era regulation that set limits on mercury and other harmful air pollutants from coal- and oil-fired power plants.

• The EPA's move leaves the emission limits in place, but rescinds the agency's initial finding that the rule was necessary. The Obama EPA made that call based in part on expected reductions from pollutants not explicitly covered by the regulation, which the agency now says is inappropriate.

• Utilities have largely complied with the 2012 rule and lobbied to keep the existing standards in place, but altering the EPA's rationale for which regulations are "appropriate and necessary" could put limits on future environmental rules or open the door to rescinding MATS at a later date.

Dive Insight:

The EPA's latest move doesn't weaken any pollution standards immediately, but is the latest in a string of actions aimed at rolling back environmental rules for the coal sector and health protections for the public.

EPA has previously moved to rescind Obama-era rules on carbon pollution from new and existing power plants, as well as regulations on the disposal of coal ash, a toxic byproduct of burning coal, and methane emissions from oil and gas drillers.

The proposed changes to the MATS rule would create a higher threshold for future regulations by narrowing the range of benefits the agency can consider when devising new rules.

Finalized in 2012, MATS set federal standards for a variety of emissions from coal-fired power plants linked to health and developmental problems in humans, like mercury and dioxins.

For utilities, complying with the rule meant installing expensive pollution scrubbers on existing coal generators. The agency justified those costs in part by counting "co-benefits," like reductions in particulate matter and other pollutants covered by separate EPA regulations.

Industry groups argued that analyzing co-benefits was inappropriate, but it remained central to the agency's calculus throughout the Obama administration. Consideration of co-benefits was a key element in the EPA's decision to keep the core of the MATS rule in place after the Supreme Court ordered the agency to review its cost-benefit analysis in 2015.

EPA expected the rule to cost $9.6 billion annually, but in a supplemental finding after the ruling said that the health benefits were worth the price tag.

The EPA's latest move would remove consideration of those co-benefits and find that the MATS rule is not "appropriate and necessary" based on the costs to industry.

If finalized, that would be a major change to how the EPA determines which regulations are necessary — typically a call made on the basis of expected public health benefits.

"The primary consideration in this appropriate and necessary decision would appear to be cost," said Miles Keogh, executive director of the National Association of Clean Air Agencies. "That has future ramifications on what you can consider. You can't regulate something if it costs too much."

Supporters of the EPA's move say it would not prevent the agency from examining co-benefits in the future — only correct an overreach in using them to justify the MATS rule.

"EPA is not saying that it can't consider [particulate] co-benefits when it makes regulatory decisions," Jeff Holmstead, a former EPA air official from the George W. Bush administration, said in an emailed statement. "They're just saying that, in this case, where virtually all the benefits are 'co-benefits' of reducing a pollutant that is supposed to be regulated under other Clean Air Act programs, we can't use these co-benefits to justify a regulation that is only supposed to be about hazardous air pollutants."

That interpretation is likely to garner legal challenges from environmental groups if EPA retains it after a 60-day comment period on the MATS changes. If it survives, clean air officials worry it could mean less protective air regulations in the future.

"[Co-benefit] impacts are real," Keogh said. "They're not the primary pollutant of concern, they're not the directly regulated pollutant, but that doesn't mean an appropriate and necessary determination about them isn't worth doing or the harms they inflict aren't real."