A filtration unit for use with an environmental control unit (ECU), usually a portable ECU, which includes at least two air flow paths wherein at least one of the air flow paths allows for use of air which has not passed through a nuclear, biological, and chemical (NBC) filter and at least one other does. The filtration unit may have at least three air paths, at least two of which provide NBC filtered air. This filtration unit can provide for longer expected operational life before filter replacement is required in both contaminated and non-contaminated environments. Further, the filtration unit may include remote sensing and control apparatus allowing for quick switching between operational modes depending on the need for filtered vs. unfiltered air and for a decreased reliance on personnel for maintenance and control, and a more simplified setup.