Air Change Per Hour (ACH) in Pharmaceutical Cleanrooms
Understand the air-change rate, calculate ACH from measured supply airflow, and use the result correctly within a cleanroom HVAC design and qualification strategy.
Quick answer: what is ACH?
Air changes per hour (ACH) is the volume of air supplied to a room in one hour divided by the room’s volume. In a pharmaceutical cleanroom, ACH is a useful HVAC design and performance indicator, but it does not by itself prove cleanliness, contamination control, or GMP compliance. Establish the needed airflow through risk assessment and demonstrate performance using relevant qualification tests, such as airflow volume, pressure differentials, airflow visualization, particle classification, and recovery testing.
Air change per hour (ACH), also called air changes per hour or air-change rate, describes how many room-volumes of air are delivered to a controlled space each hour. Pharmaceutical manufacturers use ACH when designing, commissioning, qualifying, and monitoring HVAC systems serving production rooms, laboratories, dispensing areas, and classified cleanrooms.
ACH matters because cleanroom air must dilute and remove airborne particles, support the intended airflow pattern, control temperature and humidity, and maintain appropriate pressure relationships. However, a high ACH value alone does not guarantee good contamination control. Supply-air distribution, return/exhaust locations, process particle generation, people and equipment, filter integrity, room pressure, and operating practices all influence performance. WHO guidance calls for the HVAC qualification parameters—including room air-change rates—to be determined using a risk-based approach and for system limits to be justified. WHO TRS 1010, Annex 8.
What does air change per hour mean?
ACH is a volumetric ratio. If the total air supplied to a room in one hour equals the room’s internal volume, the nominal air-change rate is 1 ACH. If the supplied volume equals 20 times the room volume in an hour, the room has a nominal rate of 20 ACH.
In imperial units, the same calculation is:
For multiple supply diffusers, add the measured airflow rates from all relevant supply outlets before dividing by room volume. Use a consistent set of units and document the source of each measurement.
How to calculate ACH in a pharmaceutical cleanroom
1Determine the room’s internal volume
Measure the clear internal length, width, and height, then calculate volume. Use the approved room dimensions or as-built drawings. Where the geometry is irregular, define how the volume is determined and retain the calculation.
2Measure supply airflow at the outlets
Measure the airflow from each relevant supply terminal using a calibrated instrument and an approved method. Capture the HVAC operating state, damper positions, fan condition, and whether measurements are taken at rest, during simulated operation, or in another defined condition.
3Sum the supply airflow
Add the airflow readings from all supply outlets serving the room. Keep units consistent; convert litres per second, cubic metres per minute, or cubic feet per minute before calculating.
4Divide total hourly supply by room volume
If the airflow is recorded in m³/h, divide directly by room volume in m³. If the flow is in m³/s, multiply by 3,600 first. If using cfm and ft³, multiply cfm by 60 before division.
5Compare with the approved design basis
Compare the calculated ACH with the design specification and approved operating range. Investigate out-of-range results, unexplained differences from prior trends, or results that coincide with airflow, pressure, or cleanliness concerns.
Worked example
Consider a cleanroom with internal dimensions of 8 m × 6 m × 3 m and measured supply airflow of 3,600 m³/h.
Total supply airflow = 3,600 m³/h
ACH = 3,600 ÷ 144 = 25 air changes per hour
The calculated result is 25 ACH. This arithmetic does not establish that 25 ACH is appropriate for the room. Suitability depends on the room’s design intent, air distribution, process load, classification target, pressure cascade, and qualification data.
Supply airflow, recirculated air, and fresh air
ACH usually refers to the total supply airflow delivered to the room, which can include recirculated air passing through the HVAC system. It is not automatically the same as the percentage of outdoor fresh air. A system may have a high recirculated supply-air rate and a much lower outdoor-air fraction. The fresh-air quantity is selected separately to address occupancy, building leakage, exhaust losses, pressure control, and applicable ventilation or environmental requirements.
Define the calculation basis in the protocol: which outlets count, whether terminal HEPA supply is included, how transfer air is handled, and whether the stated figure represents measured or design airflow. Do not add return-air and supply-air rates together for a single room unless the approved method explicitly calls for a particular system balance calculation; doing so can double-count the same air.
Why ACH matters in pharmaceutical cleanrooms
Air replacement contributes to removal or dilution of airborne particles generated by people, materials, and manufacturing activity.
Airflow supports control of temperature, humidity, and room conditions where these affect product, materials, or equipment.
Air supply and extract balance helps maintain room pressure relationships and intended air direction between spaces.
ACH is only one system parameter. Airflow visualization may reveal stagnant zones or an undesirable path even when the total room airflow appears adequate. Similarly, a room can meet an ACH target while failing particle classification, pressure, filter-integrity, or recovery requirements. WHO guidance lists ACH alongside other performance parameters such as supply and return/exhaust air quantity, room pressures, airflow pattern tests, unidirectional airflow velocity, filter penetration, particle counts, and microbiological counts. WHO HVAC guidance.
Is there a standard ACH requirement for each cleanroom grade?
There is no single ACH number that should be applied automatically to every room based only on a grade label. Regulatory expectations focus on a suitable, qualified environment and system performance for the intended operation. The EU GMP Annex 1 qualification framework identifies tests such as airflow volume and velocity, pressure difference, airflow direction/visualization, filter integrity, temperature and humidity, recovery, and contamination measurements, where relevant. It does not assign a universal ACH value to every grade. EU GMP Annex 1, sections 4.23–4.27.
Older technical references and engineering documents may show indicative ACH ranges, including ranges sometimes quoted for non-sterile facilities. These can help with early design discussion, but they should not be copied as a current GMP requirement without checking the source, scope, product/process risk, and facility-specific performance evidence. Current WHO HVAC guidance for non-sterile pharmaceutical products requires qualification parameters to be risk-assessed and design conditions, operating ranges, alert limits, and action limits to be defined and justified. WHO TRS 1010, Annex 8.
| Room / operation context | ACH design question | What else must be demonstrated? |
|---|---|---|
| Non-sterile processing room | Can airflow manage the expected heat, moisture, dust, occupancy, and product-protection needs? | Containment or pressure direction, dust extraction, airflow distribution, temperature/humidity, and cross-contamination controls. |
| Classified sterile-product background | Does the air supply and circulation support the specified at-rest and operational environmental state? | Classification, filter integrity, pressure differentials, airflow visualization, recovery, and environmental monitoring. |
| Unidirectional airflow work zone | Is local airflow velocity and direction appropriate across the critical work area? | Velocity uniformity, visualization under representative conditions, HEPA integrity, and protection of exposed product. |
| Containment or hazardous-material room | Can airflow support containment without compromising product or personnel protection? | Pressure direction, exhaust treatment, containment performance, alarms, and occupational/environmental safeguards. |
ACH versus air changes in unidirectional airflow areas
For non-unidirectional (mixed or turbulent) rooms, ACH is commonly used as a room-level ventilation descriptor. In unidirectional airflow zones—such as critical filling or open-product protection areas—local velocity, flow uniformity, and airflow pattern are often more meaningful than a whole-room ACH figure. A large room-volume calculation can mask poor local protection at the point where sterile product is exposed.
Use ACH to describe the overall room air supply where appropriate, and separately specify and test the local airflow attributes required by the process. ISO 14644-3 provides cleanroom performance test methods for both unidirectional and non-unidirectional airflow configurations and for as-built, at-rest, and operational occupancy states. ISO 14644-3:2019.
ACH in HVAC qualification and routine control
ACH should be connected to the HVAC design basis, qualification protocol, control system, and lifecycle maintenance strategy. The user requirement specification and design documents should describe the intended operating range and relevant process conditions. Qualification then verifies the installed system can deliver the defined airflow and maintain the required environment.
- Design qualification: document why the selected airflow rate and distribution concept are suitable for room function and risk.
- Installation and operational qualification: verify system installation, instrument calibration, fan and damper functions, air volumes, alarms, and control sequences.
- Performance qualification: demonstrate the required room conditions under representative at-rest and operational states, as defined by the qualification strategy.
- Routine monitoring: trend supply airflow, room pressures, filter condition, particle data, temperature/humidity, and alarms as relevant.
- Change control: assess the impact of filter changes, balancing, room layout changes, new equipment, altered exhaust, or a revised production process.
WHO recommends that HVAC systems be qualified, that qualification scope be risk-based, and that the HVAC qualification master plan define testing procedures and protocols. It also says that HEPA filter replacement should be followed by installed filter leakage testing. WHO TRS 1010, Annex 8.
Common ACH calculation and interpretation errors
- Using design airflow instead of current measured airflow without stating the basis.
- Using the wrong room dimensions or omitting a ceiling void/room volume convention without justification.
- Mixing airflow units or forgetting to convert minutes to hours.
- Adding supply and return airflow together and double-counting the same air stream.
- Assuming ACH equals fresh-air changes, when most supply air may be recirculated.
- Treating a high ACH result as proof of particle classification, sterility assurance, or effective air distribution.
- Applying a generic ACH range without considering occupancy, particle generation, airflow path, process protection, or containment needs.
- Failing to assess changes in fans, filters, dampers, exhaust equipment, or room use that can alter actual ventilation.
ACH calculation and qualification checklist
- Room purpose, cleanliness state, and protection/containment objective are documented.
- Room volume is traceable to approved dimensions or drawings.
- Supply airflow is measured using calibrated equipment and a defined method.
- All supply outlets included in the calculation are listed and summed consistently.
- Units, conversion factors, airflow operating state, and formula are recorded.
- ACH design target and operating range have a documented technical and risk-based rationale.
- ACH is assessed with pressure, airflow pattern, filter integrity, classification, and recovery tests as applicable.
- Out-of-range findings, system changes, and relevant trends are managed through the quality system.
Key takeaways
- ACH is the hourly supply airflow divided by room volume; it describes a ventilation rate, not the full cleanroom performance.
- Calculate using measured, representative supply airflow and clearly documented room dimensions, units, and operating state.
- There is no universal GMP ACH value suitable for every pharmaceutical room or cleanroom grade.
- ACH must be considered alongside airflow direction and visualization, pressure differentials, filter integrity, classification, and recovery.
- WHO guidance supports a lifecycle, risk-based approach and requires the design basis, operating range, and limits to be defined and justified.
Frequently asked questions
What is ACH in a pharmaceutical cleanroom?
ACH means air changes per hour. It is calculated by dividing the total air supplied to a room per hour by the room’s internal volume. It is one HVAC performance metric used to describe room ventilation.
What is the formula for calculating ACH?
ACH = supply airflow rate per hour ÷ room volume. In SI units, divide m³/h by m³. If airflow is measured in m³/s, multiply it by 3,600 before dividing by room volume in m³.
Is there a GMP-mandated ACH for each cleanroom grade?
No single ACH target is universally mandated for every pharmaceutical cleanroom grade. The airflow rate should be justified for the room’s use and demonstrated through applicable qualification tests and ongoing controls.
Are 20 or 30 ACH mandatory for pharmaceutical rooms?
Numbers such as 20 or 30 ACH are often repeated as design rules of thumb, but they are not automatic universal GMP requirements. Use a site-specific engineering and risk rationale, then verify actual performance.
Does ACH mean the amount of fresh outdoor air?
Not necessarily. ACH usually describes total supply air delivered to the room, which may include recirculated filtered air. The outdoor-air fraction is a separate HVAC design parameter.
Does a high ACH guarantee a cleanroom meets its particle classification?
No. Classification depends on measured airborne particle concentration under defined occupancy conditions. Distribution, contamination sources, HEPA performance, room design, cleaning, and operations matter as well as total airflow volume.
Is ACH an appropriate metric for unidirectional airflow zones?
ACH can describe whole-room ventilation, but local airflow velocity, uniformity, and visualization are usually more relevant for evaluating unidirectional airflow zones that protect exposed product.
How often should ACH be measured?
Set measurement frequency in the HVAC qualification and monitoring strategy based on risk, system performance, room criticality, applicable regulations, and change history. Reassess after significant HVAC or room changes.
Should return air be added to supply air when calculating ACH?
Usually no. The common room ACH calculation uses the supply airflow delivered to the room. Adding return air may double-count the same air. Follow the facility’s approved calculation basis and state it in the report.
What should be done if the measured ACH is outside its approved range?
Document and investigate the result, assess the impact on room conditions and any production performed, identify the cause, and determine corrective actions. Requalification may be needed after repair or system adjustment.
Related pharmaceutical validation topics
Explore related guidance on cleanroom HVAC, equipment qualification, data integrity, and GMP systems:
- World Health Organization, WHO GMP guidelines for HVAC systems for non-sterile pharmaceutical products, TRS 1010, Annex 8 (2018).
- European Commission, EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products (2022).
- ISO, ISO 14644-3:2019, Cleanrooms and associated controlled environments—Part 3: Test methods.
- U.S. FDA, Air Velocity Meters: inspection technical guide with a room ACH calculation method.
This article is educational. Apply the current regulations and controlled procedures applicable to the site, product, and jurisdiction. The facility’s Quality Unit and qualified engineering personnel should approve HVAC specifications, limits, qualification scope, and room disposition.
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