ISPE Guide for Sterile Manufacturing Facilities (Volume 3, Published: January 1999), specifies a minimum of 20 air changes per hour is required in controlled areas. How many air changes depends on clean room design, cleaning schedule, personnel clothing and discipline, number of personnel inside room, etc.
Theoretically, if you are able to prove that with 20 air changes per hour, in both “at rest” and “in operation” conditions, you fulfill particulate and microbiology conditions, than can say that 20 changes is enough (in practice, this is not enough). More air changes give you more creditableness on system performance. If the air supply remains constant, the cleanroom is dirtier if there are more people in the cleanroom, more activity from people, cleanroom garments that are less effective in preventing dispersion of contamination, and more contamination coming from production machinery and processes.
In FDA Guidance for Industry; Sterile Drug Products Produced by Aseptic Processing — Current Good Manufacturing Practice, it is stated: “For Class 100,000 (ISO 8) supporting rooms, airflow sufficient to achieve at least 20 air changes per hour is typically acceptable. Significantly higher air change rates are normally needed for Class 10,000 and Class 100 areas.”
You can also find the equitation’s for calculating the required number of air changes, in function of all above mentioned parameters, and see, what the figures say.
My opinion, perform engineering study and measuring with available air changes. If the measurements’ shows that current air changes is not enough to maintain specified environmental conditions, You will need to make changes on Your system in order to improve it and increase the number of air changes.
Good luck LeKler