Mont. Code Ann. § 75-2-204

The department may by rule prohibit the construction, installation, alteration, operation, or use of a machine, equipment, device, or facility that it finds may directly or indirectly cause or contribute to air pollution or that is intended primarily to prevent or control the emission of air pollutants, unless the owner or operator obtains a permit under this part or has registered the source of air contaminants with the department if the source is in a category for which only registration is required by the rules adopted to implement this part.