Judy, This is intentionally mentioned BECAUSE in a scenario where there will be NO humans within the environment for an extended period of time, there is no need to "destruct the ozone" and "reengergize with fresh O2. I recently had a meeting with a friend/client of mine who presented me with this scenario with his doctors' office suite which is closed from noon Saturday until 8 am Monday morning, and through EM, we were told this. this is a BIG deal for MY client, as it makes taping off rooms, HVAC systems, etc., etc., not required. and the fact is, that circulating our ozone mixture THROUGH HVAC systems has obvious health benefits. It makes the AsepticSure MUCH more user friendly in these situations.