The air exchange rate is telling you how quickly an air purifier can clean the air within the desired space. For example, if your IDEAL air purifier states it's ideal for a room size of 800 sq. ft. with an Air Exchange Rate of 13 minutes. This means it only takes 13 minutes to clean the air in an 800 sq. ft. room. If the machine ran for just under 1 hour it would have cleaned that air in that 800 sq. ft. room 4-times.