Option 1 flush out - Path 1

A couple more details, the first floor is a contiguous library space with two stack-induced cross ventilation chases with operable windows at the top. The second floor is a series of 6 science labs, each with a room-based exhaust fan. Couldn't we simply use the stack-effect (possibly temporarily fitted with exhausting fans) to power ventilate the lower level, and open the windows and turn on the exhaust fans in the classroom/labs to get the volume we need? The volume doesn't need to go through the mechanical system per se does it? This credit is to simply flush out pollutants in the environment itself, with or without the mechanical system itself? Anyway, those are my questions. Thanks!