U.S. President Barack Obama is to announce Wednesday a regulation aimed at decreasing ozone emissions from power plants and factories. The rule comes through the Environmental Protection Agency as a way to improve not only air quality but also public health, as smog can cause or aggravate heart disease and asthma, among other ills, the New York Times reported.

In announcing the rule, the EPA is meeting its Dec. 1 deadline set by a federal judge earlier this year after environmental activists sued the Obama administration for not imposing limits on ground-level ozone sooner. "It’s a big task for the president, an opportunity to fix a past mistake,” Paul Billings, senior vice president of the American Lung Association, told The Guardian. “I think he is feeling emboldened to make some legacy moves here.”

The regulation is likely to cap the highest ozone levels allowed at 65 to 70 parts per billion. Right now, the threshold is 75 parts per billion -- set by the Bush administration in 2008, the New York Times reported. Here are five things to know about the regulation expected to come out later Wednesday:

1. This issue has come up before. The EPA was going to release the rule three years ago, but it was unpopular with Republicans concerned about its impact on local government and industries. Obama, looking ahead to his re-election, pulled it at the last minute. He cited "the importance of reducing regulatory burdens and regulatory uncertainty, particularly as our economy continues to recover" in a 2011 statement.

Health and environmental groups didn't like that decision. They sued the administration, saying it was a politically motivated choice that put Americans at risk. The court ordered that the EPA take action by Dec. 1. 

2. It may be the most expensive proposal ever. Politico called the EPA plan "the costliest regulation of all time." The U.S. would lose about $3 trillion and 3 million jobs a year through 2040, according to the National Association of Manufacturers, a business lobby. Meeting the new standards in California alone would cost between $800 million and $1.6 billion, National Journal reported.

The GOP is already speaking out on the anticipated EPA rule. South Dakota Sen. John Thune tried to legally postpone the action, saying in a statement it would devastate American jobs and energy prices. "I expect there to be strong, bipartisan opposition to what will be the most expensive EPA regulation in history," Thune said.

3. Smog is really bad for you. The World Health Organization labeled ozone emissions a major public health concern. About 7 million people died prematurely in 2012 as a result of air pollution, according to its website. It hurts people's respiratory and cardiac systems and can cause cancer. “Ozone is not only killing people but causing tens of millions of people to get sick every day,” National Association of Clean Air Agencies Executive Director William Becker told the New York Times. 

4. But it's getting better. The past few decades have seen considerable improvements in air quality, and that's one of the reasons the EPA proposal was being criticized. “Air quality will continue to improve under the existing standards,” the American Petroleum Institute's Howard Feldman told the New York Times. “The current review of health studies has not identified compelling evidence for more stringent standards, and current standards are protective of public health.”

5. Activists are pushing for even lower levels. The EPA was scheduled to seek public comment on a proposal that would limit ozone emissions even further, from 75 parts per billion to 60. That's what the European Union's threshold is, the Guardian reported, and it's what a scientific panel advised the Bush administration to implement in 2008.