Abstract: An apparatus for detecting and containing chemical or biological contaminants. The invention includes one or more optical contaminant detectors, capable of detecting chemical and biological agents. A containment assembly is installed in the duct work of a building, just downstream from the main intake. Air flows linearly through the containment assembly. The air first flows through a first damper, then through the contamination sensor or sensors, and then through a second damper. If a sensor senses a contaminant, a controller shuts off the HVAC system, while simultaneously closing the first and second dampers. The containment assembly is thereby hermetically sealed—trapping any contaminants inside. The sensors and second damper are spaced sufficiently far apart so that no contaminant will flow through the second damper before its closure. The controller can also be configured to alert authorized personnel as to the potential contamination.