The rule in question is commonly known as the "good neighbor" rule.
At on its most basic level, it keeps states from letting their pollution travel elsewhere.
But some states argue the E.P.A. is going too far.
The Environmental Protection Agency issued a rule in 2011 requiring 28 up-wind states to reduce the pollution that's carried to other nearby states, down-wind.
"This is about people's basic health and a level playing field," said Delaware Governor Jack Markell.
Markell is among those hoping the Supreme Court will uphold the E.P.A.regulations.
Markell says the pollution Delaware inherits from other states prevent his state from meeting pollution standards.
On Monday, he and seven other states asked the E.P.A. to go even further than the current rule under debate and require up-wind states to make the same efforts limiting pollution as their down-wind neighbors.
"We are not asking any state to make improvements we have not made ourselves," Markell said.
But at least 15 other states, such as Kentucky and West Virginia argue the E.P.A. standards prevents states from creating their own solutions to pollution.
"The states opposing the E.P.A. are saying, no you can only regulate as needed, to meet the air quality standards," said former assistant E.P.A.Administrator Jeff Holmstead.
Now the Supreme Court will decide how far it too far when it comes to E.P.A. policies designed to reduce pollution.
Because of challenges to the rule it was never implemented and it was ultimately nullified by a federal appeals court, but the Supreme Court could reverse that decision.
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