The Biden administration is moving to tighten the limits on how much mercury and other toxins coal-fired electric plants can release, setting up another potential conflict between Republican state officials and the federal government over fossil fuels.

The Environmental Protection Agency announced last week that it would update its rules on mercury and other air toxins for the first time in a decade. The limits, first put in place by the Obama administration, are widely credited with reducing the amount of mercury contamination, which can affect wildlife and humans.

People often ingest the chemical by eating fish. It can harm kidneys, the nervous system and immune systems in humans, particularly children.

“Today’s proposal will support and strengthen EPA’s Mercury and Air Toxics Standards, which have delivered a 90% reduction in mercury emissions from power plants over pre-standard levels, ensuring historic protections for communities across the nation, especially for our children and our vulnerable populations,” said EPA Administrator Michael Regan in a statement. 

The new rules would result in a 70% reduction in mercury emissions from power plants that use lignite, or “brown coal.” They would also seek to cut soot, dust and other small particles containing certain other pollutants by two-thirds.

“This proposal provides regulatory certainty that allows states, grid operators and power companies to make investments and planning decisions, while preserving the industry’s ability to deliver reliable and affordable electricity,” the EPA statement asserted.

Previous rules governing mercury and other airborne pollutants by the Obama administration sparked partisan outcry and yearslong legal fights that reached the U.S. Supreme Court. Republican attorneys general played a crucial role in those challenges, arguing that the EPA had not properly weighed the potential costs of the regulations.

Since then, other high court decisions have reined in the authority of the EPA on other air pollution matters. That gives the mercury standards outsized importance, because they can potentially lead to changes like the closure of coal-powered plants that can ultimately reduce greenhouse gas pollution.

A prominent Republican on Capitol Hill slammed the EPA for trying to clamp down further on those kinds of pollution.

“The Biden administration continues to wage war on coal and affordable, reliable energy by issuing unnecessary regulations intended to drive down electricity production from our nation’s baseload power resources,” said U.S. Sen. Shelley Moore Capito of West Virginia, the top Republican on the Senate Environment and Public Works Committee, in a statement.

“The Biden administration has again put politics over sound policy. With one job-killing regulation after another, the EPA continues to threaten the livelihood of those in West Virginia and other energy-producing communities across the country,” she added.

The Edison Electric Institute, which represents private electric utilities, took a less combative approach.

Emily Fisher, EEI’s general counsel and executive vice president of clean energy, said the electric industry “fully and successfully implemented” the previous mercury standards for years, “resulting in dramatically reduced mercury and related emissions.”

“While we still are reviewing the details of the [the new EPA proposal], we look forward to continuing to work with Administrator Regan and his team as they finalize the rule to ensure that implementation of the standard is consistent with our industry's ongoing clean energy transformation,” Fisher said in an emailed statement.

Utilities have all complied with the Obama-era rule. They’ve done that by installing scrubbers, switching to a fuel source other than coal or by shutting down coal-fired plants completely. In fact, advocates say the mercury rule did more than any other federal regulation to move U.S. utilities away from coal plants.

The Biden administration’s new proposal could lead to similar actions. The EPA estimates that it will reduce about 500 MW of coal-generated electricity, which is the equivalent of about one power plant or less than 1% of the country’s current coal capacity. But industry sources say that underestimates the potential impact.

The rule’s impact goes beyond mercury. It also attempts to limit pollution of nickel, arsenic and lead by calling for a two-thirds reduction in the amount of filterable particulate matter that contains those chemicals. The EPA is also proposing a requirement that power plant owners monitor their emissions continuously.

The proposal, said Earthjustice, an environmental justice advocacy group, “recognizes that an overwhelming majority of plants are already achieving even lower emission levels than the proposed limits require. This shows that far greater reductions in power plants’ toxic emissions are achievable.”

“This pollution,” added Jim Pew, the group’s director of federal clean air practice, “disproportionately harms lower-income households and communities of color. Continuous emissions monitoring is a big step in the right direction, and we hope EPA will adopt the same requirements for other polluting industries.”