Stand alone ambient Gas Phase Filter unit perfect for fire houses and bus garages features a four-inch pleated, industrial pre-filter which catches the larger particles, extending the life of the second stage, Merv 11 or optional Merv 15, multi-pocket bag filter. The final stage of filtration is (2) 2″ thick, 50/50 blend activated charcoal & potassium permanganate, after filter. Our disposable filters are easy to service and provide for simple, low-cost maintenance.