Air Changes Per Hour (ACPH)

What is ACPH?

Air Changes Per Hour (ACPH) simply tells you how many times all the air in a room gets replaced with clean, filtered air in one hour. So, if a room has an ACPH of 40, that means the air in that room is completely swapped out 40 times every hour.

This matters a lot in places where clean air is super important — like in hospitals, labs, factories that make electronics, or any cleanroom setup. Keeping the air fresh and moving helps reduce dust, germs, and other particles, making the space safer and healthier to work in.

ACPH = Air changes per hour

CFM= Cubic feet per minute

Example:

Let’s say:

• The room is 20 ft long, 15 ft wide, and 10 ft high
  → Room Volume = 20 × 15 × 10 = 3,000 cubic feet
• The HVAC system supplies 500 CFM

Now plug into the formula:

Guidelines

The required ACH can vary depending on the cleanliness class or classification of the cleanroom, which is determined by the industry and specific application. Here are some general guidelines for ACH in cleanrooms based on ISO standards:

• ISO 14644-1: This standard provides cleanliness classes for cleanrooms, and it is commonly used as a reference for ACH requirements. The ACH requirements vary depending on the class, with higher classes requiring more frequent air changes to maintain cleanliness.
• USP 797 and USP 800: These are guidelines provided by the United States Pharmacopeia for pharmaceutical compounding cleanrooms. USP 797 outlines ACH requirements for sterile compounding areas, and USP 800 specifies ACH requirements for hazardous drug compounding areas.
• GMP (Good Manufacturing Practice): ACH requirements for cleanrooms in the pharmaceutical industry are often determined by GMP regulations, which can vary by region and country. These regulations may specify ACH based on the type of operation being conducted.

The Role of ACPH

Air Changes Per Hour is of paramount importance in cleanrooms, where stringent environmental control is necessary to prevent contamination and maintain important levels of cleanliness. Here’s why ACPH is crucial in cleanroom environments:

• Contamination Control: Cleanrooms are designed to limit particulate and microbial contamination to protect sensitive processes and products. Air Changes Per Hour ensures a constant flow of clean, filtered air into the cleanroom, reducing the concentration of airborne contaminants. This frequent air exchange helps maintain the required cleanliness levels for various cleanroom classifications, as defined by ISO standards.
• Particle Removal: Particles in the air, even those invisible to the naked eye, can compromise the quality and integrity of products manufactured in cleanrooms. ACPH plays a vital role in removing these particles by continuously cycling in fresh, filtered air. Higher ACPH values result in more effective particle removal, which is crucial for industries like semiconductor manufacturing and pharmaceutical production.
• Temperature and Humidity Control: In addition to cleanliness, some cleanrooms require precise temperature and humidity control. ACPH assists in distributing conditioned air evenly throughout the cleanroom, helping maintain the desired environmental conditions.
• Worker Safety: Cleanrooms may be used for hazardous processes or materials, such as those involving toxic chemicals or biohazardous substances. Sufficient ACPH helps protect workers by continually diluting and removing potentially harmful substances from the air.
• Product Quality and Yield: In industries like semiconductor manufacturing and biotechnology, where product quality is critical, ACPH contributes to consistent production outcomes. A clean and stable environment with the right ACPH minimizes the risk of defects and ensures high product yields.

Factors Influencing ACPH

Several factors influence the calculation of ACPH:

• Room Size: Larger cleanrooms typically require more air changes to maintain uniformity in air quality.
• Type of Product and Industry: Industries such as biotechnology, semiconductor manufacturing, and pharmaceuticals demand elevated cleanliness standards, necessitating correspondingly higher ACPH values.
• Cleanliness Level: The industry and the class of cleanroom (ISO Class 1 to ISO Class 9) will determine the required ACPH.
• Processes: Certain processes generate more contaminants, necessitating higher ACPH to maintain desired cleanliness levels.
• HEPA/ULPA Filters: The efficiency of air filtration systems plays a significant role. High-efficiency filters can reduce the required ACPH.
• Personnel: The number of personnel working in the cleanroom can contribute to particle generation and influence ACPH calculations.

Air Changes Per Hour (ACPH) is a critical metric for maintaining the purity and precision of cleanrooms. It is a testament to the commitment of industries to meet stringent quality and safety standards. Understanding the specific ACPH requirements for your cleanroom is essential for ensuring the integrity of your processes and the reliability of your products. As technology continues to advance, the optimization of ACPH will remain central to the evolution of cleanroom technology, enhancing the capabilities of industries that rely on these controlled environments.

Planning to design a cleanroom with a well-balanced Air Changes Per Hour (ACPH) and require expert assistance, please do not hesitate to contact us. We are here to help you create a cleanroom environment that meets your specific needs and cleanliness requirements. Contact us now for a consultation!

FAQ

1. What is Air Changes Per Hour (ACPH) and why does it matter?
Air Changes Per Hour (ACPH) is a simple way to understand how often the air inside a room is completely replaced with clean, filtered air in one hour. For example, if a room has an ACPH of 20, it means all the air in that room is removed and replaced with fresh air 20 times every hour.

This is important in places where clean air is critical — like cleanrooms, pharmaceutical labs, hospitals, or electronics manufacturing. Having the right ACPH helps keep the air free of dust, germs, chemicals, and other particles. It also helps control temperature, humidity, and air pressure, all of which are essential for safety, product quality, and regulatory compliance.

2. How do I calculate ACPH for a room or facility?
To calculate ACPH, you need the room size and the airflow rate from your HVAC system.

Here’s the formula:
ACPH = (CFM × 60) ÷ Room Volume
Room volume is length × width × height (in feet)
CFM is the airflow rate (cubic feet per minute).

3. What ACPH level do I need for an ISO 7 or ISO 8 cleanroom?

Typical air change rates for cleanrooms vary by classification. For an ISO 7 cleanroom, the recommended ACPH usually falls between 40 and 60, while an ISO 8 cleanroom typically requires between 15 and 30 air changes per hour. The exact number depends on factors like how sensitive the process is, how many particles are generated, the number of people in the room, and the room’s design. Cleanrooms with stricter cleanliness levels—like ISO 5—need much higher air change rates to maintain their standards.

4. Does higher ACPH mean better cleanliness?

Not always. While increasing the number of air changes per hour does help remove dust and contaminants faster, it’s not the only thing that matters for cleanliness. Factors like how the air flows through the room, the quality of the filters, the pressure difference between rooms, and how the space is used all play a big role.

For example, if air flows in a way that stirs up particles instead of pushing them out, or if filters aren’t working well, just pumping in more air won’t help much. Also, running an HVAC system at very high ACPH can use a lot of energy, which isn’t always practical.

The key is to design the cleanroom with the right airflow patterns, good filters, and proper pressure controls, so the ACPH level you use actually improves air quality without wasting energy.

5. What’s the difference between ACPH and CFM (Cubic Feet per Minute)?

CFM tells you how much air is moving through a system every minute. Think of it as how fast the air is flowing. ACPH tells you how many times the whole air in a room gets swapped out in one hour.

To figure out ACPH, you need to know both the airflow (CFM) and the size of the room. Without both, you can’t say how often the air is changing.

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