Abstract: A building protection system for a building having an inlet air duct with an inlet damper operable to regulate inlet air entering the building and passing through the inlet air duct, a return air duct with a return damper operable to regulate return air, and a fan system operable to circulate a mixture of the inlet air and the return air in the building, the building protection system including: (a) a chemical sensor system operable to detect one or more chemicals in the inlet air; (b) a chemical sensor system operable to detect the one or more chemicals in the return air; (c) a radiological sensor system operable to detect one or more radioactive material in the inlet air; (d) a radiological sensor system operable to detect the one or more radioactive materials in the return air; and (e) a control system in communication with the sensor systems, and a building control system, the control system being operable to signal the building control system to close the dampers and to turn off the fan system whenever o