By Emily Atkin

When a power plant burns coal, it produces pollution: Sulfur dioxide, nitrogen oxide, mercury, particulate matter, carbon dioxide. As the plants emit into the air, more often than not the wind blows, and the mixture of harmful emissions travels across state lines.

Those states that create large amounts of pollution are called “upwind” states, and they are responsible for higher ozone levels, increased healthcare costs, and decreased air quality in their neighboring “downwind” states.

The issue of how to regulate it has long perplexed the EPA and the courts. Cross-state transport rules implemented in 1977 and tightened in 1990 were never able to effectively combat the complex problem. For example, it’s not just upwind states that bring pollution to downwind states. Some upwind states receive emissions from other upwind states which contribute to their own pollution problems. The EPA has thus deemed the cross-state pollution problem as a “dense, spaghetti-like matrix” of overlapping soot.

But in 2011, the EPA finalized its Cross-State Air Pollution Rule (CSAPR), which promised to “slash hundreds of thousands of tons of smokestack emissions that travel long distances through the air leading to soot and smog, threatening the health of hundreds of millions of Americans living downwind.” CSAPR, according to the EPA, would achieve up to $280 billion in annual health benefits by preventing up to 34,000 premature deaths, 15,000 nonfatal heart attacks, 19,000 cases of acute bronchitis, 400,000 cases of aggravated asthma, and 1.8 million sick days a year beginning in 2014.

In response, a coalition of fossil fuel companies, unions and those upwind states sued the EPA seeking to invalidate the rule, and they won. In August 2012, the conservative D.C. Circuit Court of Appeals ruled that the EPA exceeded its authority by making upwind states reduce more pollution than what federal air quality standards prescribe. In other words, they said the EPA’s rule makes upwind states cut too much pollution.

The case has now made its way to the U.S. Supreme Court, which will hear an extended 90-minute session of oral arguments on Tuesday. Here are 4 reasons why the Supreme Court very well might reverse the D.C. Circuit’s ruling — and why it matters.

Air Pollution Worsens Economic Inequality

In some downwind states like Delaware, Connecticut, and Maryland, more than 80 percent of the air pollution can come from outside the state, according to Howard Fox of Earthjustice, who is representing the American Lung Association in the case.

“There’s economic inequality of having a downwind business spend money to control their pollution when upwind businesses are able to pollute and not spend as much,” Fox told ThinkProgress. “That creates a competitive disadvantage.”

Delaware governor Jack Markell told Reuters that removing pollution in his state would cost between $10,000 and $40,000, but that it would only cost $200 to $500 per tonne in upwind states, “where even some basic control technologies have not been installed.”

Human Health Suffers As Air Pollution Kills Thousands

In a brief filed with the Supreme Court, a consortium of energy industry players including Entergy Corp. Luminant Energy, and United Mine Workers of America said the EPA’s cross-state pollution rules were a prime example of “overcontrol.” But briefs filed by both The American Thoracic Society (ATS) and the EPA disagreed, citing some jarring statistics about the real health effects of air pollution.

“When air pollution levels are high, deaths can occur immediately, or within months, by inducing heart attacks or strokes,” the ATS brief said. The EPA cited that one out of 20, or 5 percent, of deaths in the country can be attributed to lasting effects of air pollution. Children, senior citizens, pregnant women, and people with asthma, cardiovascular issues, and diabetes are the most at-risk, the brief said.

Even under current pollution rules, the ATS said, the country will see an estimated 2,550 to 6,560 more premature deaths due to air pollution than would occur under the new cross-state rules.

Pollution From Coal Plants Causes Climate Change

Though it does not seem to be a crucial part of the EPA’s case, reductions in emissions from coal and other fossil fuel plants will be crucial in the fight against climate change in the United States.

According the Union of Concerned Scientists, coal plants are the nation’s top source of carbon dioxide emissions, which are the primary cause of global warming. Utility coal plants in the United States emitted a total of 1.7 billion tons of CO2 in 2011, even with clean coal technologies.

The UCS also cites burning coal as a leading cause of smog, acid rain, and toxic air pollution.

This Is Why The U.S. Constitution Exists

“Inter-state air pollution is right at the core of why the federal government created the Constitution in the first place,” Tom Donnelly, counsel at the progressive think tank Constitutional Accountability Center (CAC), told ThinkProgress. CAC also filed a brief with the Supreme Court in this case, which outlines the Constitution’s support for the EPA’s authority.

“The text, history and structure of the Constitution all strongly support Congress’s power to enact laws that address genuinely national problems like instate air pollution,” the brief said, citing Resolution VI, which declares that Congress has authority “to legislate in all cases for the general interests of the union, and also in those to which the states are separately incompetent.” That resolution was translated into Article I, which affords the federal government the ability to provide national solutions to national problems.

“When Congress and the Executive Branch are acting to address such a problem [as pollution], their efforts are entitled to great deference from the judiciary,” Donnelly said. “This case has everything to do with making sure that the federal government has the flexibility it needs to address genuinely national problems.”