A floor-space saving NBC filtration system for collective protection shelters with NBC filter unit and means for changing the NBC filtration system state between a ventilation mode, effective during normal times or conventional hazard, and a NBC protection mode, effective during NBC hazard. The NBC filtration system is contained in a space out of a comfortable reach of the shelter occupants thus saving usable floor-space. The means for changing the NBC filtration system state is operated from a location within the comfortable reach of the shelter occupants. The NBC filter unit inlet and outlet are located at close proximity to each other, a pre-filter outlet is located in proximity to the NBC filter unit inlet and outlet with a changeover valve, serving as means for changing the NBC filtration system state, located there between. Resulting in simple operation and space saving by the changeover valve replacing prior art space consuming flexible pipes.