Sierra Club v. Envtl. Prot. Agency

Pillard, Circuit Judge:

Industrial boilers are heavy-duty furnaces used to generate steam and other useful heat for a wide range of applications, such as milling paper and manufacturing car parts. These boilers reach and sustain extremely high temperatures, relying on varying combinations of fuels and combustion techniques to do so. But all share a common environmental risk: Without adequate controls in place, they send into the air large quantities of toxic pollutants that endanger public health.

To mitigate such dangers, the Environmental Protection Agency (EPA or Agency) issued rules under…