The choice which coal to buy is a complicated issue involving the BTU value, the ash content, and the transportation cost. Of primary concern since 1990 has also been the sulfur content of the coal. As a general rule, western coal is low ash and low sulfur but higher in transportation cost due to the distance from the market.

The 1990 Clean Air Act set a goal of reducing sulfur dioxide emissions and nitrous oxide emissions below the 1980 levels. Sulfur dioxide contributes to acid rain. Power plants have three ways to meet this requirement:

1. Use low sulfur coal from the west
2. Install pollution control equipment, e.g., smokestack scrubbers
3. Sulfur dioxide trading (see below)

Scrubbers are the most expensive option and low sulfur Wyoming coal is the least expensive. Sulfur dioxide trading allows power plants whose emissions are not up to the EPA limit to sell their available "pollution rights" to another power plant allowing them to raise their emissions that much above the EPA limit.