In order to keep the energy footprint of a newly planned cleanroom (grade C and D) as low as possible, we want to recirculate a certain amount of air over HEPA filters. The question specifically concerns the amount of air that can be recirculated in a cleanroom. What percentage is usually used here for fresh air/recirculated air? What is the advised minimum fresh air exchange rate in a cleanroom.

For cleanrooms classified as Grade C and D under EU GMP, there is no fixed percentage for the ratio of fresh air to recirculated air. The fresh air requirement primarily depends on the number of occupants, as cleanroom garments reduce heat dissipation.

Typically, 70–100 m³/h of fresh air per person is recommended to manage CO₂, humidity, and thermal loads while ensuring comfort and compliance with indoor air quality.

Since the air is mostly recirculated through HEPA filters to maintain cleanliness, the recirculation rate is usually over 90%. Fresh air is mainly used for ventilation, metabolic load compensation, and maintaining overpressure — it is not defined by percentage, but calculated based on occupancy and internal loads.