Understanding the Role and Calculation of Air Changes per Hour in Cleanrooms


When customers decide to purchase an air conditioning unit, one key factor influencing their choice is often the tonnage—the unit’s cooling capacity measured in tons.

The size of an air conditioning unit is determined by the volume of the room it serves.  This tonnage signifies the amount of heat the unit can remove from the room in one hour, meaning:  a higher tonnage means the AC can cool more air.   

Similarly, in the pharmaceutical industry, it is important for the number of air changes/air exchange rates to be sufficient. The supply of air by Heating, Ventilation, and Air Conditioning (HVAC) systems in one hour equates to one air change per hour (ACH). Therefore, a room with 60 air changes per hour = 60 times the air supply on the room volume.In this blog, as a leading Pharmaceutical HVAC manufacturer, we will explain the importance of ACH in pharmaceutical cleanrooms, understand how it’s calculated, and highlight its critical role in ensuring product quality and regulatory compliance.

A Critical Cleanliness Control

ACH stands as a crucial airflow engineering method to measure the cleanliness level of the pharma environment. The air in the room is frequently replaced with fresh, filtered air in one hour. This constant renewal of clean air and removal of contaminants helps minimize the risk of product contamination, maintain product integrity, and provide a safe working environment for personnel. Consequently, ACH emerges as an indispensable clean air solution that influences and upholds cleanliness and controls airborne particles in cleanrooms.


The calculation of ACH in a cleanroom involves determining the volume of air within the room and the rate at which fresh air is supplied and exhausted. The formula for calculating ACH is as follows:

ACH = (Q * 60) / V

ACH is the air changes per hour
Q is the airflow rate in cubic feet per minute (CFM), and
V is the volume of the room in cubic feet.
By utilising this formula, the required airflow rate can be determined to achieve the desired air changes per hour in the cleanroom area.

Establishing the said ACPH rate involves considering the following key points:

1. Clean-Up or Recovery Time

Different classes of cleanrooms require varying durations for a room that falls out of its classification to return within the specified class. A typical guidance time period for clean-up or recovery is about 15–20 minutes. 

2. Airflow Directions

Specified airflow directions promote containment. The placement of supply and return or exhaust air grilles should facilitate appropriate airflow directions and avoid obstruction of equipment, utilities, containers or personnel.

3. Interleading Rooms

In scenarios involving interleading rooms, setting appropriate limits prevents overlapping in actual values. Failure to do so may result in a loss of pressure differential between areas and even reversal of airflow.

4. GMP (Good Manufacturing Practice) Guidelines for Pharmaceuticals

The Air Changes per Hour (ACH) requirements for cleanrooms in the pharmaceutical industry are often dictated by Good Manufacturing Practice (GMP) regulations, which can vary by region and country. These regulations typically specify the necessary ACH based on the specific type of operation being performed.

The industry and the class of cleanroom (ISO Class 1 to ISO Class 9) determines the required ACPH.

Factors influencing ACH

1. Room Volume

Larger cleanrooms require more air changes to achieve the same level of air quality compared to a smaller room.

2. Occupancy and Activity

Cleanrooms with higher occupancy and more vigorous activity may require higher ACH to maintain the indoor air quality. 

3. Type of Contaminants

Processes generating more contaminants may require more frequent air changes. For example, cleanrooms with high levels of volatile organic compounds (VOCs) may require more frequent air changes compared to those primarily concerned with particulate matter. 

4. HEPA/ULPA FiltersThe required ACH level greatly depends upon factors such as filtration efficiency, HVAC system design, and air distribution. High-efficiency filters reduce the required ACH and impact ACH calculations.

Air Change Calculation for Unidirectional and Non-Unidirectional Cleanrooms

1. Cleanrooms with Unidirectional Airflow

Unidirectional airflow patterns move horizontally or vertically across space in parallel movement, usually between 60 – 90 FPM. Airflow remains within 18 degrees of parallel with sufficient velocity to sweep away particles before affixing to surfaces. Thus, cleanrooms with unidirectional airflow patterns (ISO CLASS 1 – 5), determine air exchange rates by average air velocity. In some cases, ISO Class 6 cleanrooms may also require unidirectional air, and it is suggested to calculate air change with average airflow velocity

2. Cleanroom with Non-Unidirectional Airflow

Data points may not represent a normal distribution of the actual room conditions due to the interaction of air during exit and entry of the non-unidirectional cleanrooms. This airflow is used for cleanrooms that require a level of cleanliness between ISO class 9 and ISO class 6. Influences from turbulence, eddies, processes equipment, and pressure differentials may yield inaccurate air velocity readings in non-unidirectional cleanrooms. Hence, air exchange rates are determined by the air changes per hour (ACH) calculation method.

3. Mixed Cleanroom Airflow

In some cleanrooms, a combination of unidirectional and non-unidirectional airflow becomes essential. This specialised airflow design aims to establish laminar airflow around specific work zones or over delicate materials, safeguarding them from contaminants. High-priority areas are shielded through focussed laminar airflow hoods or laminar flow workbenches. Meanwhile, non-unidirectional airflow is used to ensure the circulation of air throughout the rest of the cleanroom in a randomised way. This air undergoes filtration through multiple filters to maintain contaminant levels within acceptable limits. The integration of both airflow types in a mixed cleanroom setup proves particularly beneficial for larger spaces where only specific workstations require laminar airflow.

Role of ACH in cleanrooms

A clean room is a controlled environment designed to maintain low levels of air contaminants such as dust, airborne microbes, aerosol particles, and chemical vapours. These controlled environments are vital in industries such as pharmaceutical manufacturing facilities, biotechnology, electronics, and healthcare, where even minimal contamination can lead to catastrophic consequences.

Thus, measuring ACH determines the level of air filtration and ventilation necessary to maintain the room’s cleanliness.  The higher the ACH, the more air changes occur, resulting in a cleaner environment of the cleanrooms. This is crucial in industries where stringent cleanliness standards need to be maintained to ensure the quality of products and processes.

Contamination Control: ACPH 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: Invisible particles in the air can compromise product quality and integrity. ACPH plays a vital role in continuously removing these particles by cycling fresh, filtered air. Higher ACPH values result in more effective particle removal, critical for industries like semiconductor manufacturing and pharmaceutical production.

Temperature and Humidity Control: Some cleanrooms require precise control over temperature and humidity. ACPH aids in evenly distributing conditioned air throughout the cleanroom, maintaining the desired environmental conditions.

Worker Safety: Cleanrooms may be utilised for hazardous processes or materials. Adequate ACPH helps protect workers by continually diluting and removing potentially harmful substances from the air.

Product Quality and Yield – In industries where product quality is paramount, 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.

In conclusion, the role and calculation of air changes per hour play a pivotal role in pharmaceutical surroundings by maintaining cleanliness, controlling contamination levels, protecting worker’s safety, and ensuring the overall quality of the environment.

Understanding the significance of ACH and implementing appropriate ventilation and filtration systems are essential for pharmaceutical projects to uphold the highest standards of cleanliness and quality in their operations. By prioritizing ACH calculations and monitoring, pharmaceutical engineering projects and manufacturing plants can create optimal environments for the safe and effective production of life-saving medications and treatments.

Contact Fabtech for expert assistance in cleanroom infrastructure, pharmaceuticals, and bio-pharma turnkey project solutions.