NSR—the program imposing onerous permitting requirements on the construction of new sources and "major modification" projects at existing sources—requires industrial sources of air emissions to determine whether the projects they propose will increase those emissions. EPA adopted regulations in 2002 to provide a new structure for those critical emission calculations, which specifies that sources must calculate the "sum of the differences" between a baseline and a future projection for each existing emission unit. That language is particularly important for individual projects that may cause emissions to go down at one unit but up at another.
Prior EPA guidance had read the word "difference" to have only one meaning: "increase." Any decreases that a project may cause were ignored. EPA's past policy of ignoring emission decreases has discouraged many projects that could actually reduce emissions and improve air quality. After all, if only the increases count, even projects that cause significant decreases in the emissions of a pollutant overall could still trigger expensive and time-consuming permitting requirements if the project might also cause some increases of that pollutant.
Under the leadership of Administrator Pruitt and Assistant Administrator Wehrum, EPA has now re-read the 2002 regulations in the way they were likely intended, so that the word "difference" can now mean a positive number (increase) or a negative number (decrease). That reading is not only grammatically and mathematically correct, it is also good policy. If a project affects more than one unit (the only type of project addressed in the new guidance), and the emissions from one unit go up while the emissions of another unit go down, it is the "sum of those differences" that has the potential to impact the environment. Therefore, it is also the "sum of those differences" that should govern whether a permit is needed to comply with the Clean Air Act, which only requires permitting for "increases [in] the amount of any air pollutant emitted by such source."
Counting only the increases, as required in past EPA guidance, would result in false increases and unnecessary permitting requirements. EPA's newest guidance, which is intended to avoid unnecessary permitting actions, can be found here: Project Emission Accounting Guidance.
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