On April 14, the Environmental Protection Agency (EPA) released a statement announcing the establishment of an electronic mail box that coal and oil companies may use to request exemptions from the Clean Air Act. The statement provided a legal basis for this action:
The Clean Air Act allows the President to exempt stationary sources of air pollution from compliance with any standard or limitation under section 112 for up to two years if the technology to implement the standard is not available and it is in the national security interests of the United States to do so.
EPA
The polluting companies will have until April 21 to ask the Trump administration for the exemption. Trump himself will decide on whether or not the requests are approved. Exemptions can last for up to two years and be renewed.
The Environmental Defense Fund filed a Freedom of Information Act request for submissions to the electronic mailbox.

