The safe level of ozone is when all the generated ozone is being consumed. This only applies to continuous ozone use in occupied environments - not for shock treating. However, this is difficult to attain because it becomes a balancing act. Initially, the machine's output is set high to get rid of the problem odor as quickly as possible. As this is being accomplished, less ozone is required for the diminishing odor, etc., thereby leaving some residual ozone in the air. If the machine output is not turned down, then more residual ozone will remain. If a strong smell of ozone is noticed, then there is more ozone present than is required. Simply turn the rheostat (output level control) down. With ozone, more is not considered better.
Note: It is not necessary that ozone be detected by humans for it to be effective. Ozone can work even when humans are not able to smell it.