The recovery time (clean-up time) is also an important parameter to be determined. Once doors have been opened and people have been entering a room, the original conditions have been disturbed and, for a short while, before recovering, the room does not always correspond to the laid down parameters. It is important to know how long this period is. There are no regulations laid down as to how long this clean-up time should be. However, the generally accepted time to clean-up from one cleanroom classification to the next higher classification, should be less than 15 minutes.

It should also be remembered that a room is to be qualified “in operation” when it has a certain number of people in it. After qualification, the number of people in that room, as challenged during qualification, cannot be exceeded.

Temperature and humidity can also be important (comfort in clean areas, stability of effervescent products, etc.)