Air pumped into the building collects heat, moisture and impurities from inside the building and it must therefore be exhausted. The evaporative cooler doesn’t recirculate the inside air: if you close the building, the effectiveness of the machine will be diminished. Mechanical extraction or even just leaving some doors or windows open helps remove unhealthy air from the indoor environment by supplying 100% fresh, filtered air. For this reason, evaporative coolers are more healthy than conventional air conditioning.