Auditoriums present unique air quality concerns because of their intermittent occupancy. When fully occupied, they may hold 150 people per 1,000 square feet of space. When unoccupied, HVAC operation is required to maintain temperature. ASHRAE 62-1989 recommends an outside air introduction of 15 CFM per person for these applications.
Since the primary challenge of the air filtration is during occupied periods, having unoccupied periods with little challenge except re-circulation to maintain temperature, the change-out period on the final filters and the carbon is extended. Outside air introduction can be kept to bare minimums during unoccupied periods, saving energy expenditures.