Introduction: How Many Air Changes per Hour Does a Cleanroom Need?
If you are planning a new cleanroom or optimizing an existing one, one of the first questions you will face is:
“How many air changes per hour (ACH) does my cleanroom need?”
The answer is not as simple as pulling a number from a chart. Instead, the correct ACH depends on several factors, including the cleanroom’s ISO classification, its industry application, and whether you measure the space “at rest” or “in operation.”
In North America, the most important references include ISO 14644, IEST recommended practices, FDA guidance, and USP <797>/<800> standards. Therefore, understanding how these standards align—and how to apply them in practice—is essential for anyone designing or managing a cleanroom.
Why Cleanroom Air Changes per Hour (ACH) Matter
Air changes per hour measure how often the total volume of air in a cleanroom is replaced with HEPA- or ULPA-filtered supply air. A higher ACH improves contamination control, shortens recovery time, and ensures that the cleanroom maintains its ISO classification even during operation.
However, more airflow is not always better. Excessive ACH increases energy use and operating costs without providing proportional benefits. As a result, the best approach is risk-based: you should select an ACH that matches the contamination risk and the application requirements.
In addition, ACH interacts with other design elements such as pressure differentials, localized airflow, and operator discipline. When these factors align, the cleanroom achieves both compliance and efficiency.
How Standards Guide ACH Selection
ISO 14644 Cleanroom Airflow Guidance
ISO 14644 provides the global foundation for cleanroom classification. The standard sets particle concentration limits for each ISO class but does not dictate a fixed ACH. Instead, it allows engineers to determine airflow rates that achieve the required particle counts. This flexibility means ISO 14644 relies on engineering judgment combined with validation testing.
FDA Guidance and USP <797>/<800> ACH Requirements
For pharmaceutical and biotech applications, more specific guidance comes from the FDA’s Guidance on Aseptic Processing (2004) and USP <797>/<800> standards:
- ISO 5 environments (laminar flow hoods or isolators): typically 240–600 ACH via unidirectional airflow
- ISO 7 cleanrooms (buffer areas): generally ≥ 30 ACH
- ISO 8 areas (anterooms): often ≥ 20 ACH
- Hazardous drug compounding (USP <800>): negative-pressure rooms must maintain ≥ 30 ACH
These recommendations offer practical starting points but still require confirmation through testing.
IEST Recommended Cleanroom ACH Ranges
In microelectronics and similar industries, the Institute of Environmental Sciences and Technology (IEST) provides well-accepted benchmarks:
- ISO 3–4 areas: 400–600 ACH
- ISO 5 zones: 240–480 ACH
Here, the emphasis is on rapid recovery times (often ≤ 15 minutes) and careful unidirectional airflow design.
Typical ACH by Cleanroom Classification
Although every project should undergo validation, the following ranges are commonly applied in North America:
| ISO Class | Typical ACH Range | Industry Examples |
|---|---|---|
| ISO 5 | 240–600 | Aseptic filling, semiconductor tools |
| ISO 7 | 30–60 | Buffer rooms, device assembly |
| ISO 8 | 10–20 | Ante-rooms, support areas |
These values should serve as guidelines, not absolutes. Final design decisions must consider room size, personnel density, and contamination risks.
At Rest vs Operational Cleanroom ACH
One critical consideration is the difference between “at rest” and “in operation” states. A room may meet ISO 7 particle limits when no operators are present, yet fail once staff and equipment enter.
Therefore, many engineers design toward the operational state, ensuring the cleanroom stays compliant under real working conditions. To confirm this, you should perform validation tests such as particle counts and recovery time measurements.
Beyond ACH: Other Cleanroom Design Considerations
While ACH plays a key role, it should not be the only factor in cleanroom design. You can often achieve better results and greater efficiency by:
- Using localized unidirectional airflow (LAF) at critical work points
- Improving gowning protocols and operator discipline
- Optimizing pressure differentials between cleanroom zones
By taking this holistic approach, you create a cleanroom that maintains compliance while balancing both performance and cost.
Conclusion: Choosing the Right ACH for Your Cleanroom
Selecting the right air changes per hour requires balancing ISO 14644 requirements with guidance from the FDA, USP, and IEST. The right ACH is not universal. Instead, it depends on your process, your risk profile, and your regulatory environment.
Start with recognized standards, validate your cleanroom under both at rest and in operation conditions, and adjust airflow accordingly. If you are planning a cleanroom project or evaluating your current facility, consider working with a design team experienced in these standards.
That way, your ACH will not only meet compliance on paper but also deliver reliable performance in practice.