How much mercury should coal plants be allowed to emit? There’s been a years-long fight in Washington, D.C., over that very question—and who wins it may ultimately depend on whether Brett Kavanaugh is confirmed to the Supreme Court.

Mercury is a potent neurotoxin, and particularly dangerous to the brains of developing fetuses and young children. Thus, say Democrats, environmentalists, and public health professionals, its release into the atmosphere should be severely restricted. Back in 2011, the Obama administration attempted to do just that, proposing the first-ever federal limits on toxic heavy metal pollution from coal plants.

But Republicans, the coal industry, and President Donald Trump, citing the costs of regulatory compliance, argue for looser regulations. To that end, the Environmental Protection Agency soon will unveil a proposal to “dramatically weaken” and potentially repeal Obama’s Mercury and Air Toxics Standard (MATS), according to The New York Times.

A few things inevitably will happen when the Trump administration officially unveils its proposal to weaken MATS. Environmental groups will file a lawsuit arguing that the move violates the Clean Air Act. The lawsuit will be heard by the D.C. Circuit Court of Appeals—where Kavanaugh currently serves. Finally, that court’s decision will be appealed to the Supreme Court. If Kavanaugh is sitting on the Supreme Court come that time, it’s almost certain Trump’s proposal would be upheld because the justification for it echoes a dissenting opinion Kavanaugh wrote four years ago.

Trump’s new proposal, according to the Times, will do one very important thing: “repeal a 2011 finding made by the EPA that when the federal government regulates toxic pollution such as mercury from coal-fired power plants, it must also, when considering the cost to industry of that rule, take into account the additional health benefits of reducing other pollutants as a side effect of implementing the regulation.” Put another way, the EPA will no longer be required to count so-called “co-benefits” of reducing pollutants like soot and nitrogen oxide when it’s regulating mercury. It will only be required to count the benefits of reducing mercury.

The change is significant, because Obama’s mercury rule only brings in $6 million in health benefits from reducing mercury annually, but it brings in up to $90 billion per year in health benefits from reducing co-pollutants. Without that $80 billion, the EPA can’t legally justify the strong mercury rule—because it costs the coal industry a whopping $9.6 billion per year.

Kavanaugh expressed skepticism about the EPA’s practice of counting “co-benefits” in 2014, when the D.C. Circuit heard a case case seeking to invalidate MATS. A divided three-judge panel upheld the Obama-era rule, with Kavanaugh dissenting. He noted the “disputed” nature of the EPA’s estimated $90 billion in annual benefits from mercury regulation. The coal industry’s estimates, he wrote, “focus on the reduction in hazardous air pollutant emissions attributable to the regulations, which amount to only $4 to $6 million dollars each year. If those figures are right, the rule costs nearly $1,500 for every $1 of health and environmental benefit produced.”

That ruling was appealed to the Supreme Court, which in 2015 blocked the mercury rule and ordered the EPA to do a new cost analysis. The vote was 5-4, as Justice Anthony Kennedy sided with the court’s conservatives. During oral arguments of a later case over the MATS rule, Kavanaugh “cited comments by Chief Justice John Roberts that counting such benefits is ‘an end-run and bootstrapping and disproportionate,’” according to E&E News. (After conducting a new cost analysis, the Obama administration reinstated the mercury rule, and was sued yet again, but the case was delayed because of the Trump administration’s plan to rewrite the rule.)

Kavanaugh is generally skeptical of strong environmental regulation at the federal level. “He maintains a level of discomfort of anything he regards as a reach, authority-wise, from the EPA,” Brendan Collins, an environmental litigator who has argued before Kavanaugh, told me earlier this year. That makes him similar to the other four conservative justices currently sitting on the court. And Kavanaugh, if confirmed, would be filling the seat vacated by Anthony Kennedy, who consistently voted to uphold strong environmental regulations.

There’s a lot the public still doesn’t know about Kavanaugh. It’s unclear, for example, whether he’s guilty of the sexual assaults he’s been accused of, or whether he would vote to overturn the landmark abortion case Roe v. Wade. But it’s clear that his confirmation wouldn’t just shift the balance of the nation’s highest court; it would alter the contents of the atmosphere.