There is no market for clean air. It is a free good. Yet, when a coal plant uses it and releases pollution, their consumption of clean air affects us all. Because they face no costs for using up the clean air, they will pollute at higher levels than society wants. Virtually all economists agree that these types of situations warrant government intervention and the federal government has set pollution standards for many years.
President Trump is now arguing that the states should each set their own pollution standards. I fear the interests of the coal industry (and other polluting industries) will dominate in the major coal-producing states and they will adopt lax standards. I live next door to West Virginia. Their air becomes my air. Therefore, as the winds blow across our country, the pollution standards set in one state will affect the people in other states. These are not decisions that only affect those living in a state, but rather they affect us all and, therefore, should be made at the federal level.