A Best Available Control Technology (BACT) is For any specific source, the currently available technology producing the greatest reduction of Air Pollutant emissions,taking into account energy, environmental, economic, and other costs.

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Other /More definition:
A Best Available Control Technology (BACT) is the most stringent technology available for controlling emissions; major sources are required to use BACT, unless it can be demonstrated that it is not feasible for energy, environmental, or economic reasons.