Terra Universal is a world leader in the design and manufacture of ISO-rated cleanroom systems and components. Terra expertise stems from over 40 years of experience in designing, building, and manufacturing components for cleanroom systems and controlled environments alike. In this article, you'll learn about essential requirements for ISO-rated (ISO classified) cleanroom classes, ISO design requirements, and planning essentials for various ISO-rated cleanroom builds, furnishings, and supplies to achieve your desired cleanroom rating.

A Timeline of ISO Cleanroom Standards vs FS209E Cleanroom Classifications

Before global cleanroom classifications and standards were adopted by the International Standards Organization (ISO), the U.S. General Service Administration’s standards (known as FS209E) were applied virtually worldwide. However, as the need for international standards grew, the ISO established a technical committee and several working groups to delineate its own set of standards.

FS209E contains six classes, while the ISO 14644-1 classification system adds two cleaner standards and one dirtier standard (see chart below). The "cleanest" cleanroom in FS209E is referred to as Class 1; the "dirtiest" cleanroom is a class 100,000. ISO cleanroom classifications are rated according to how much particulate of specific sizes exist per cubic meter (see second chart). The "cleanest" cleanroom is a class 1 and the "dirtiest" a class 9. ISO class 3 is approximately equal to FS209E class 1, while ISO class 8 approximately equals FS209E class 100,000.

In November 2001, Federal Standard 209E was superseded by the new ISO 14644-1 international standards. References to FS209E are still used; the comparison chart below illustrates the relationship between the two standards.

Airborne Particulate Cleanliness Class Comparison:

ISO Class English Metric
ISO 1    
ISO 2    
ISO 3 1 M1.5
ISO 4 10 M2.5
ISO 5 100 M3.5
ISO 6 1,000 M4.5
ISO 7 10,000 M5.5
ISO 8 100,000 M6.5
ISO 9    

Airborne Particulate Cleanliness Classes (by cubic meter):

Number of Particles per Cubic Meter by Micrometer Size
Class 0.1 micron 0.2 micron 0.3 micron 0.5 micron 1 micron 5 microns
ISO1 10 2        
ISO2 100 24 10 4    
ISO3 1,000 237 102 35 8  
ISO4 10,000 2,370 1,020 352 83  
ISO5 100,000 23,700 10,200 3,520 832 29
ISO6 1,000,000 237,000 102,000 35,200 8,320 293
ISO7       352,000 83,200 2,930
ISO8       3,520,000 832,000 29,300
ISO9       35,200,000 8,320,000 293,000

In cleanrooms, particulate concentration changes over time — from the construction and installation of equipment to its operational status. ISO delineates three cleanroom classification standards: as-built, at-rest and operational. Read more about cleanrooms and gowning room design considerations/recommendations.

As instruments and equipment are introduced and particulates rise, an "as-built" cleanroom becomes an "at-rest" cleanroom. When people are added to the matrix, particulate levels rise still further in the "operational" cleanroom.

Find Answers: Every Question You'll Need to Answer Before Buying a Cleanroom

A Guide to Gowning Procedures

Required Testing (ISO 14644-2)

ISO 14644-2 describes the type and frequency of testing required to conform to certain standards. The following tables indicate mandatory and optional tests.

Schedule of Tests to Demonstrate Continuing Compliance
Test Parameter Class Maximum Time Interval Test Procedure
Particle Count Test <= ISO 5 6 Months ISO 14644-1 Annex A
> ISO 5 12 Months
Air Pressure Difference All Classes 12 Months ISO 14644-1 Annex B5
Airflow All Classes 12 Months ISO 14644-1 Annex B4

Optional Testing (ISO 14644-2)

Schedule of Additional Optional Tests
Test Parameter Class Maximum Time Interval Test Procedure
Installed Filter Leakage All Classes 24 Months ISO 14644-1 Annex B6
Containment Leakage All Classes 24 Months ISO 14644-1 Annex B4
Recovery All Classes 24 Months ISO 14644-1 Annex B13
Airflow Visualization All Classes 24 Months ISO 14644-1 Annex B7

In addition to ISO 14644-1 and ISO 14644-2, eight other cleanroom standards documents exist, as well as three specific to biocomtamination applications.

ISO Document Title
ISO 14644-1 Classification of Air Cleanliness
ISO 14644-2 Cleanroom Testing for Compliance
ISO 14644-3 Methods for Evaluating and Measuring Cleanrooms and Associated Controlled Environments
ISO 14644-4 Cleanroom Design and Construction
ISO 14644-5 Cleanroom Operations
ISO 14644-6 Terms, Definitions and Units
ISO 14644-7 Enhanced Clean Devices
ISO 14644-8 Molecular Contamination
ISO 14644-9 Surface Cleanliness by Particle Concentration
ISO 14644-10 Surface Cleanliness by Chemical Concentration
ISO 14698-1 Biocontamination: Control General Principles
ISO 14698-2 Biocontamination: Evaluation and Interpretation of Data
ISO 14698-3 Biocontamination: Methodology for Measuring Efficiency of Cleaning Inert Surfaces

ISO and Federal Air Change Rates for Cleanrooms

A critical factor in cleanroom design is controlling air-change per hour (ACH), also known as the air-change rate, or ACR. This refers to the number of times each hour that filtered outside air replaces the existing volume in a building or chamber. In a normal home, an air-conditioner changes room air 0.5 to 2 times per hour. In a cleanroom, depending on classification and usage, air change occurs anywhere from 10 to more than 600 times an hour.

Cleanroom Air Change Rate Standards

ACR is a prime variable in determining ISO and Federal cleanliness standards. To meet optimal standards, ACR must be painstakingly measured and controlled. And there is some controversy. In an appendix to its ISO 14644-1 cleanliness standard, the International Standards Organization addressed applications for microelectronic facilities only. (ISO classes 6 to 8; Federal Standards 1,000, 10,000 and 100,000.) The appendix contained no ACR standards for pharmaceutical, healthcare or biotech applications, which may require higher ACR regulations.

What is the Best Method to Determine Ideal Air Change Rates?

According to current research, case studies and experiments, using an ACR range (rather than one set standard) is a better guideline for cleanliness classification. This is true because the optimal ACR varies from cleanroom to cleanroom, depending on factors such as internal equipment, staffing and operational purpose. Everything depends on the level of outside contaminants trying to enter the facility versus the level of contaminants being generated on the inside.

How Does Air Change Rate Affect Cleanroom Cleanliness?

The breadth of these ranges reflects how dramatically people and processes affect cleanliness. Low-end figures within each contamination class generally indicate air velocity and air change requirements for an as-built or at-rest facility—where no people are present and no contaminating processes under way. When there are people and processes producing contaminants, more air changes are required to maintain optimal cleanliness standards. For instance, some manufacturers insist on as many as 720 air changes per hour to meet Class 10 standards.

Determining the appropriate number of air changes for a particular application requires careful evaluation of factors such as the number of personnel, effectiveness of garbing protocol, frequency of access, and cleanliness of process equipment. Learn more about fan/filter units and see the models offered by Terra, such as room-side replaceable, ducted and energy-efficient.

Learn more about fan/filter units and see the models offered by Terra, such as room-side replaceable, ducted and energy-efficient.

How Does Cleanroom Air Change Rate Affect Cleanroom Energy Efficiency?

Rajan Jaisinghani, in his paper "Energy Efficient Low Operating Cost Cleanroom Airflow Design," presented at ESTECH 2003, recommended the following ranges based on FS209E classifications:

FS Cleanroom Class ISO Equivalent Class Air Change Rate
1 ISO 3 360-540
10 ISO 4 300-540
100 ISO 5 240-480
1,000 ISO 6 150-240
10,000 ISO 7 60-90
100,000 ISO 8 5-48

Jaisinghani’s recommendations concur with other recent studies of ACR, which criticize some existing air rate standards (developed in the 1990s) as being unscientific because they are based on fans and filters inferior to today’s models. So when these older standards are applied, the resulting ACR is often too high. In fact, some studies have found that reducing the ACR (and its attendant air turbulence) can result in a cleaner atmosphere.

This was demonstrated in a study conducted by Pacific Gas and Electric (San Francisco) and the Lawrence Berkeley National Laboratory (Berkeley). The study measured air change rates in several ISO Class-5 cleanrooms and came to the conclusion that there is "no consistent design strategy for air change rate, even for cleanrooms of the same cleanliness classification."

Critical Design Implications for Air Change Rates in Cleanrooms

ACR rates have critical design implications, especially when considering desired cleanliness, fan size and lower energy costs. The PG&E/Berkeley study caused many designers to reduce fan sizes. In short, a lower ACR often resulted in cleaner air.

The study revealed three abiding principles:

  • Lower air change rates result in smaller fans, which reduce both initial investment and construction cost.
  • Fan power is proportional to the cube of air change rates or airflow. A 30-percent reduction in air change rate results in a power reduction of approximately 66 percent.
  • By minimizing turbulence, lower airflow may improve cleanliness.

ISO 5 Cleanroom Air Change Rates

The study focused on Class-5 cleanrooms, concluding that an ACR range of from 250 to 700 air changes per hour is standard, but that "actual operating ACRs ranged from 90 to 625." It added that all of these optimized cleanrooms were certified and performing at ISO Class-5 conditions with these lower ACRs. Finally, the study concluded that rarely does a Class-5 facility require an ACR of more than 300.

The study also found that the "[b]est practice for ACRs is to design new facilities at the lower end of the recommended ACR range," with variable speed drives (VSDs) built in so that air flow adjustments can be made under actual operating conditions. Control can be performed manually or automatically. See the Smart modular cleanroom control systems offered by Terra.

In his report "An examination of ACRs: An opportunity to reduce energy and construction costs," Peter Rumsey, PE, CEM, essentially concurred with the PG&E-commissioned study by Berkeley. Rumsey issued a caveat, then brushed it aside by citing research subsequent to Berkeley’s: "Air cleanliness is a critical component of any cleanroom, far outweighing energy saving priorities. Designers and operators need evidence from others who have tried similar strategies in order to address the perceived risks of lowering air change rates."

Rumsey then went on to cite studies done by International Sematech (Austin, Texas); the Massachusetts Institute of Technology (Cambridge, Mass.); Intel (Santa Clara, Calif.); and Sandia National Laboratories (Albuquerque, N.M.), which echoed the Berkeley study.

In summary, current research and thinking on air change rates indicate that some existing standards are too high and can be lowered while still meeting all ACR criteria. Read Terra's blog post "What Makes a Room Suitable for Aseptic Proceses?"

Federal and ISO Ceiling Fan Coverage Specifications

Achieving the optimal air change rate requires proper ceiling fan coverage. The cleanest modular cleanroom incorporates filter/fan units (FFUs) in every 2’ x 4’ (610 mm x 1219 mm) ceiling bay. This near-100% coverage provides a laminar flow of filtered air to quickly remove contaminants from the room, thus meeting FS209E standards for Class 10 and ISO Class 1 standards.

How Do I Calculate FFU Coverage?

Such coverage, especially in a large cleanroom, can lead to higher energy consumption, thus increasing costs for both initial construction and ongoing operation. In most cases, a smaller percentage of ceiling coverage produces adequate cleanliness. Read more about FFU placement and performance efficiency; see the FFU-coverage formula below to help calculate the quantity of necessary ceiling modules.

Federal and ISO CleanroomAirflow Velocity Standards

In addition to ACR and ceiling coverage, the third factor integral to maintaining cleanliness is fan-generated air speed. Again, higher airflow velocity results in a "cleaner" cleanroom. The term "ventilation efficiency" refers to the speed of filtered air passing through the cleanroom in addition to the number of air changes per hour (ACH or ACR).

An earlier chart showed a range of recommended air change rates (ACRs) for different classes of cleanrooms. Ranges are given because as-built and at-rest facilities require a smaller ACR than an operational cleanroom, where both people and equipment are actively engaged. Non-operational cleanrooms are found in the lower range; operational cleanrooms higher.

Combining all three factors—ACR, ceiling coverage and airflow velocity—results in the following table:

Class ISO 146144-1 (Federal Standard 209E) Average Airflow Velocity
m/s (ft/min)
Air Changes Per Hour Ceiling Coverage
ISO 8 (Class 100,000) 0.005 – 0.041 (1 – 8) 5 – 48 5 – 15%
ISO 7 (Class 10,000) 0.051 – 0.076 (10 -15) 60 – 90 15 – 20%
ISO 6 (Class 1,000) 0.127 – 0.203 (25 – 40) 150 – 240 25 – 40%
ISO 5 (Class 100) 0.203 – 0.406 (40 – 80) 240 – 480 35 – 70%
ISO 4 (Class 10) 0.254 – 0.457 (50 – 90) 300 – 540 50 – 90%
ISO 3 (Class 1) 0.305 – 0.457 (60 – 90) 360 – 540 60 – 100%
ISO 1 – 2 0.305 – 0.508 (60 – 100) 360 – 600 80 – 100%

Calculating Number of Cleanroom FFUs

Before deciding on the appropriate velocity and air changes for your application, Terra Universal recommends careful evaluation of factors such as number of personnel, effectiveness of garbing protocol, access frequency and cleanliness of process equipment.

Once the required air change figure is established, the number of required FFUs can be determined using this formula: No. of FFUs = (Air Changes/Hour ÷60) x (Cubic ft. in room÷ 650*)

*CFM output of a loaded FFU

Increased Difficulty: Technical Cleanroom FFU and Airflow Calculations

Minimum Cleanroom Air Change Rate for Class 100 Cleanroom Standards

Meeting Class 100 standards using the low-end air change recommendation (240/hour) inside a 12’ x 12’ x 7’ (3302 mm x 3302 mm x 2134 mm) cleanroom, with 1008 cu. ft. of volume, requires 6 FFUs. To meet the same standard using the high-end air change recommendation (480/hour) requires 12 FFUs.

Positive Pressure Cleanroom Design

Cleanrooms are designed to maintain positive pressure, preventing "unclean" (contaminated) air from flowing inside and less-clean air from flowing into clean areas. The idea is to ensure that filtered air always flows from cleanest to less-clean spaces. In a multi-chambered cleanroom, for instance, the cleanest room is kept at the highest pressure. Pressure levels are set so that the cleanest air flows into spaces with less-clean air. Thus, multiple pressure levels may need to be maintained.

Read More: Isolation vs Containment Design for Cleanrooms

A differential air pressure of 0.03 to 0.05 inches water gauge is recommended between spaces. In order to minimize disruptions to these cascading pressures when doors are opened, air locks are often specified between rooms of differing ISO cleanliness levels. Automated fan controls simplify pressure balancing by allowing fan speed adjustments at a centralized console panel. Why is pressure differential important and how is it measured? Read Terra's "Under Pressure in Critical Environments" blog for answers.

Gain Deeper Insights: Under Pressure in Critical Environments

Laminar and Turbulent Air Flow in Cleanrooms

Unidirectional cleanrooms, such as ISO 5 (Class 100) and cleaner facilities rely on unidirectional, or laminar, airflow. Laminar airflow means that filtered air is uniformly supplied in one direction (at a fixed velocity) in parallel streams, usually vertically. Air is generally recirculated from the base of the walls back up to the filtering system.

Non unidirectional cleanrooms, such as ISO 6 (Class 1,000) and above, generally utilize a non-unidirectional, or turbulent, airflow. This means the air is not regulated for direction and speed. The advantage of laminar over turbulent airflow is that it provides a uniform environment and prevents air pockets where contaminants might congregate. See more about the importance of airflow uniformity.

Terra cleanroom walls are made of durable, easy-to-clean, ISO-grade materials, as specified in FDA guidance documents. Combined with your cGMP processes, they will help you comply with operational requirements.