Before Scott Pruitt was sworn in as the chief of the US Environmental Protection Agency under president Donald Trump, he was at odds with the organization. He vowed to dismantle several of the EPA’s current regulations limiting pollution, even going so far as suing the federal agency multiple times as Oklahoma’s attorney general.

Today, Oct. 10, Pruitt followed through on his pledge to roll back one of the Obama administration’s most important pieces of environmental legislation: the EPA chief signed a proposed rule that would reverse the 2015 Clean Power Plan meant to reduce carbon and other greenhouse gas emissions from power plants by about 870 million tons by 2030.

The Clean Power Plan gives states a number of options on ways to meet emissions targets, ranging from investment into clean energy like wind power, improving overall energy efficiency, and shifting away from coal power. Pruitt claims that the demands places on states to meet emission reduction goals goes beyond EPA’s legal power.

Although some states had already started investing more in alternative electricity, areas where coal mining is still fruitful, like West Virginia, Kentucky, and Pennsylvania, will now no longer have to meet emissions goals. The policy reversal is the biggest blow to the environment from the Trump administration since the president backed out from the Paris Climate Agreement.

The move doesn’t just put the environment at risk; air pollution in any form takes a costly toll on human health. Breathing in compounds like sulphur dioxide found in smog can lead to asthma, heart disease, premature births, and low birth weights. Back when the Clean Power Plan was put into effect, the EPA released a report (pdf) estimating the health impacts of the law. The agency found that the cost of treating pollution-related conditions—in addition to the cost of 300,000 days of missed school or work—adds up quickly. By reducing emissions, according to the EPA, the Clean Power Plan would save between $14 billion and $34 billion in health costs and prevent over 3,000 early deaths every year after 2030.

Over a dozen national groups of doctors, including the American Lung Association, the Allergy and Asthma Network, the American College of Physicians, and the National Medical Association, agree that Pruitt’s plan to reverse the Clean Power Plan is a bad, expensive idea. “The longer our nation’s leaders postpone action to clean up carbon pollution and other greenhouse gases, the more severe the health costs will be,” representatives said in a statement.

The new rule is expected to be published today in the Federal Register (a draft of the proposed rule was obtained by the Washington Post and other news organizations last week), but does not go into effect immediately; the comment period for the rule could go on for months. There’s no clear timeline, and plenty of environmental groups and Democrats are likely to challenge the rule, but the comment period is likely to end up a formality; Pruitt’s track record suggests he isn’t likely to accept any amendments to his rule that would pose a threat to fossil fuel industries.