A:Answer As someone mentioned the aair LITE can provide 4 air changes an hour (ACH) in a room that is 350 square feet and has 8 foot ceilings when running on it's highest fan speed. It's all based on volume of air. If your ceilings are taller or the room is larger, you will see a reduction in the ACH. Conversely, smaller rooms will have more ACH or your can reduce the fan speed to maintain 4 ACH. Depending on where you plan to use the air purifier, the desired number of air changes can vary. We never recommend less than 2 ACH in any situation, 4 ACH is ideal.