Pharmacy Cleanroom Differential Pressurization: Cascading Airflow Design
Controlling room pressure is only one of many important aspects of the pharmacy cleanroom facility design. When creating separate rooms of different class, differential pressurization through a cascading effect enhances the performance keeping contamination from reaching the critical process environment. But how does cascading airflow work exactly?
Clean to Less Clean
A successful pressure cascade occurs when cleanroom environment ensures that the flow of contamination is from clean to less clean. They are created by using airflow to create pressure differentials between any rooms or zones, which ensure that contamination flows from the cleanest zone down.
Positive Pressure Differential (Non-Hazardous Cleanroom Suite)
United States Pharmacopeia (USP) General Chapter <797> Pharmaceutical Compounding - Sterile Preparations states: “Continuous differential positive pressure is required to minimize airflow from an area with lower air-quality classification to an area of higher air-quality classification. In a cleanroom suite, a minimum differential positive pressure of 0.020-inch water column is required between each ISO classified area (e.g., between the buffer room and ante-room). The pressure differential between the ante-room and the unclassified area must not be less than 0.020-inch water column.”
In the case of USP <797> for non-hazardous compounding where product protection is the end goal, a positive pressure cascade is applied from areas of critical importance to those of less critical importance. The below diagram demonstrates the positive pressure cascade showing the flow of contamination from the ISO 7 Buffer Room to the less clean ISO 8 Ante Room down to the least clean area, the unclassified space.
Positive pressure will make the air flow out of the room instead of in. What this means is that the air in the clean room will have a tendency to leak out of the room, instead of in, thus preventing unfiltered air or air particulates from entering.
Negative Pressure Differential (Hazardous Cleanroom Suite)
In cases where personnel protection is the end goal, a negative pressure differential is maintained to ensure no contamination generated within the cleanroom is lost to the surrounding environment.
USP Chapter <800> Hazardous Drugs -Handling in Healthcare Setting states that areas handling NIOSH listed materials/products are required to have negative pressure from surrounding areas to contain HDs and minimize risk of exposure. These cleanrooms are to have a negative pressure between 0.01 and 0.03 inches of water column relative to all adjacent areas.
The below diagram demonstrates the flow of air when an ISO 7 positive pressured Ante Room and an ISO 7 negative pressured Buffer Room are present and how contamination is prevented from entering suite through the positive pressure Ante Room, and the air that might be contaminated by hazardous products pulls the air back towards the Buffer Room, keeping possible hazardous contaminants from escaping the room.
Full Picture: Non-Hazardous and Hazardous Cleanroom Suite
Given the likely chance your operations contain both hazardous and non-hazardous drug compounding, a cleanroom setup might contain a shared ISO 7 positive pressured Ante Room that has access to an ISO 7 positive pressured Buffer Room and an ISO 7 negative pressure Buffer Room, the cascading airflow would look something like this:
Conclusion
In order to achieve any of the 3 examples, the HVAC needs to be designed to control the room pressure by some means and proper placement of low wall air returns to ensure proper calculation of supply air that will be returned in the room to make a positive pressured zone, or placement of low wall air exhausts for air need to be exhausted out of the room to create a negative pressured zone. See last month’s article on HVAC Design for USP <797> and <800> Cleanrooms.
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