A large convoluted space, such as a multi-story concourse of an airport is isolated and a chemical or biological agent in the concourse is deactivated. A plurality of deactivation gas sources (100) introduce a deactivation gas at multiple points along the concourse. Fans (66, 106) circulate the deactivation gas, sensors (110) sense concentrations of the deactivation gas at numerous points around the concourse and exhaust fans (66) exhaust air, spent deactivation gas, and some deactivation gas. A control processor (84) controls the generators, the circulation fans, and the exhaust fans in accordance with the gas concentrations sensed by the sensors to increase and decrease deactivation gas concentration in selected subregions of the concourse by increasing or decreasing generation, increasing or decreasing exhausting, or altering flow patterns among subregions.
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