Last updated
Cleanroom used for the production of microsystems. The yellow (red-green) lighting is necessary for photolithography, to prevent unwanted exposure of photoresist to light of shorter wavelengths. Clean room.jpg
Cleanroom used for the production of microsystems. The yellow (red-green) lighting is necessary for photolithography, to prevent unwanted exposure of photoresist to light of shorter wavelengths.
Cleanroom from outside Cleanroom outside.jpg
Cleanroom from outside
Entrance to a cleanroom with no air shower Cleanroom entrance.jpg
Entrance to a cleanroom with no air shower
Cleanroom for microelectronics manufacturing with fan filter units installed in the ceiling grid Cleanroom1.jpg
Cleanroom for microelectronics manufacturing with fan filter units installed in the ceiling grid
Cleanroom cabin for precision measuring tools Cleanroom-Cabin.JPG
Cleanroom cabin for precision measuring tools
Typical cleanroom head garment Cleanroom Garment2.JPG
Typical cleanroom head garment

A cleanroom or clean room is a facility ordinarily utilized as a part of specialized industrial production or scientific research, including the manufacture of pharmaceutical items and microprocessors. Cleanrooms are designed to maintain extremely low levels of particulates, such as dust, airborne organisms, or vaporized particles. Cleanrooms typically have an cleanliness level quantified by the number of particles per cubic meter at a predetermined molecule measure. The ambient outdoor air in a typical urban area contains 35,000,000 particles for each cubic meter in the size range 0.5 μm and bigger in measurement, equivalent to an ISO 9 cleanroom, while by comparison an ISO 1 cleanroom permits no particles in that size range and just 12 particles for each cubic meter of 0.3 μm and smaller.



The modern cleanroom was invented by American physicist Willis Whitfield. [1] As employee of the Sandia National Laboratories, Whitfield created the initial plans for the cleanroom in 1960. [1] Prior to Whitfield's invention, earlier cleanrooms often had problems with particles and unpredictable airflows. Whitfield designed his cleanroom with a constant, highly filtered air flow to flush out impurities. [1] Within a few years of its invention in the 1960s, Whitfield's modern cleanroom had generated more than US$50 billion in sales worldwide (approximately $398 billion today). [2] [3]

Physicist scientist who does research in physics

A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate causes of phenomena, and usually frame their understanding in mathematical terms. Physicists work across a wide range of research fields, spanning all length scales: from sub-atomic and particle physics, through biological physics, to cosmological length scales encompassing the universe as a whole. The field generally includes two types of physicists: experimental physicists who specialize in the observation of physical phenomena and the analysis of experiments, and theoretical physicists who specialize in mathematical modeling of physical systems to rationalize, explain and predict natural phenomena. Physicists can apply their knowledge towards solving practical problems or to developing new technologies.

Willis Whitfield was an American physicist and inventor of the modern cleanroom, a room with a low level of pollutants used in manufacturing or scientific research. His invention earned him the nickname, "Mr. Clean," from Time Magazine.

Sandia National Laboratories United States research lab

The Sandia National Laboratories (SNL), managed and operated by the National Technology and Engineering Solutions of Sandia, is one of three National Nuclear Security Administration research and development laboratories. In December 2016, it was announced that National Technology and Engineering Solutions of Sandia, under the direction of Honeywell International, will take over the management of Sandia National Laboratories starting on May 1, 2017.

The majority of the integrated circuit manufacturing facilities in Silicon Valley were made by three companies: MicroAire, PureAire, and Key Plastics. These competitors made laminar flow units, glove boxes, clean rooms and air showers, along with the chemical tanks and benches used in the 'Wet Process' building of integrated circuits. These three companies were the pioneers of the use of Teflon for airguns, chemical pumps, scrubbers, water guns, and other devices needed for the production of integrated circuits. William (Bill) C. McElroy Jr. worked as engineering manager, drafting room supervisor, QA/QC, and designer for all three companies and his designs added 45 original patents to the technology of the time. McElroy also wrote a four page article for MicroContamination Journal, wet processing training manuals, and equipment manuals for wet processing and clean rooms. [4]

Polytetrafluoroethylene polymer

Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The best known brand name of PTFE-based formulas is Teflon by Chemours. Chemours is a spin-off of DuPont, which originally discovered the compound in 1938. Another popular brand name of PTFE is Syncolon® by Synco Chemical Corporation.

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon 639-1 ısoo

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed of discrete electronic components. The IC's mass production capability, reliability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.


Cleanrooms can be very large. Entire manufacturing facilities can be contained within a cleanroom with factory floors covering thousands of square meters. They are used extensively in semiconductor manufacturing, biotechnology, the life sciences, and other fields that are very sensitive to environmental contamination. There are also modular cleanrooms. [5]

Biotechnology Use of living systems and organisms to develop or make useful products

Biotechnology is the broad area of biology involving living systems and organisms to develop or make products, or "any technological application that uses biological systems, living organisms, or derivatives thereof, to make or modify products or processes for specific use". Depending on the tools and applications, it often overlaps with the (related) fields of molecular biology, bio-engineering, biomedical engineering, biomanufacturing, molecular engineering, etc.

The air entering a cleanroom from outside is filtered to exclude dust, and the air inside is constantly recirculated through high-efficiency particulate air (HEPA) and/or ultra-low particulate air (ULPA) filters to remove internally generated contaminants.

HEPA A type of air filter to remove particulates

High efficiency particulate air (HEPA), originally called high-efficiency particulate absorber but also sometimes called high-efficiency particulate arresting or high-efficiency particulate arrestance, is a type of air filter. Filters meeting the HEPA standard have many applications, including use in clean rooms for IC fabrication, medical facilities, automobiles, aircraft and homes. The filter must satisfy certain standards of efficiency such as those set by the United States Department of Energy (DOE).

Staff enter and leave through airlocks (sometimes including an air shower stage), and wear protective clothing such as hoods, face masks, gloves, boots, and coveralls.

Airlock device which permits the passage of people and objects between a pressure vessel and its surroundings

An airlock is a device which permits the passage of people and objects between a pressure vessel and its surroundings while minimizing the change of pressure in the vessel and loss of air from it. The lock consists of a small chamber with two airtight doors in series which do not open simultaneously.

Air shower (room)

Air showers are specialized enclosed antechambers which are incorporated as entryways of cleanrooms and other controlled environments to reduce particle contamination. Air showers utilize high-pressure, HEPA- or ULPA-filtered air to remove dust, fibrous lint and other contaminants from personnel or object surfaces. The forceful "cleansing" of surfaces prior to entering clean environments reduces the number of airborne particulates introduced.

Equipment inside the cleanroom is designed to generate minimal air contamination. Only special mops and buckets are used. Cleanroom furniture is designed to produce a minimum of particles and is easy to clean.

Mop mass or bundle of coarse strings or yarn attached to a stick

A mop is a mass or bundle of coarse strings or yarn, etc., or a piece of cloth, sponge, or other absorbent material, attached to a pole or stick. It is used to soak up liquid, for cleaning floors and other surfaces, to mop up dust, or for other cleaning purposes. The word is attested in English as early as 1496, but new refinements and variations of mop designs have been introduced, from time to time. For example, American inventor Jacob Howe received U.S. patent #241 for a mop holder in 1837 and Thomas W. Stewart in 1893. At the 1968 Miss America protest, protestors symbolically threw a number of feminine products into a "Freedom Trash Can", which included mops.

Bucket container

A bucket is typically a watertight, vertical cylinder or truncated cone or square, with an open top and a flat bottom, attached to a semicircular carrying handle called the bail.

Common materials such as paper, pencils, and fabrics made from natural fibers are often excluded, and alternatives used. Cleanrooms are not sterile (i.e., free of uncontrolled microbes); [6] only airborne particles are controlled. Particle levels are usually tested using a particle counter and microorganisms detected and counted through environmental monitoring methods. [7] [8] Polymer tools used in cleanrooms must be carefully determined to be chemically compatible with cleanroom processing fluids [9] as well as ensured to generate a low level of particle generation. [10]

Some cleanrooms are kept at a positive pressure so if any leaks occur, air leaks out of the chamber instead of unfiltered air coming in.

Some cleanroom HVAC systems control the humidity to such low levels that extra equipment like air ionizers are required to prevent electrostatic discharge problems.

Low-level cleanrooms may only require special shoes, with completely smooth soles that do not track in dust or dirt. However, for safety reasons, shoe soles must not create slipping hazards. Access to a cleanroom is usually restricted to those wearing a cleanroom suit.

In cleanrooms in which the standards of air contamination are less rigorous, the entrance to the cleanroom may not have an air shower. An anteroom (known as a "gray room") is used to put on clean-room clothing.

Some manufacturing facilities do not use fully realized cleanrooms, but use some practices or technologies typical of cleanrooms to meet their contamination requirements.

In hospitals, theatres are similar to cleanrooms for surgical patients' operations with incisions to prevent any infections for the patient.

Air flow principles

Turbulenter Reinraum.png
Air flow pattern for "Turbulent Cleanroom"
Laminar Flow Reinraum.png
Air flow pattern for "Laminar Flow Cleanroom"

Cleanrooms maintain particulate-free air through the use of either HEPA or ULPA filters employing laminar or turbulent air flow principles. Laminar, or unidirectional, air flow systems direct filtered air downward or in horizontal direction in a constant stream towards filters located on walls near the cleanroom floor or through raised perforated floor panels to be recirculated. Laminar air flow systems are typically employed across 80% of a cleanroom ceiling to maintain constant air processing. Stainless steel or other non shedding materials are used to construct laminar air flow filters and hoods to prevent excess particles entering the air. Turbulent, or non unidirectional, air flow uses both laminar air flow hoods and nonspecific velocity filters to keep air in a cleanroom in constant motion, although not all in the same direction. The rough air seeks to trap particles that may be in the air and drive them towards the floor, where they enter filters and leave the cleanroom environment. US FDA and EU have laid down guidelines and limit for microbial contamination which is very stringent to ensure freedom from microbial contamination in pharmaceutical products. [11] [12]

Personnel contamination of cleanrooms

The greatest threat to cleanroom contamination comes from the users themselves. [13] In the healthcare and pharmaceutical sectors, control of microorganisms is important, especially microorganisms likely to be deposited into the air stream from skin shedding. Studying cleanroom microflora is of importance for microbiologists and quality control personnel to assess changes in trends. Shifts in the types of microflora may indicate deviations from the “norm” such as resistant strains or problems with cleaning practices.

In assessing cleanroom microorganisms, the typical flora are primarily those associated with human skin (Gram-positive cocci), although microorganisms from other sources such as the environment (Gram-positive rods) and water (Gram-negative rods) are also detected, although in lower number. Common bacterial genera include Micrococcus, Staphylococcus, Corynebacterium, and Bacillus, and fungal genera include Aspergillus and Pencillium. [8]

Cleanroom classification and standardization

Cleanrooms are classified according to the number and size of particles permitted per volume of air. Large numbers like "class 100" or "class 1000" refer to FED-STD-209E, and denote the number of particles of size 0.5 µm or larger permitted per cubic foot of air. The standard also allows interpolation; for example SNOLAB is maintained as a class 2000 cleanroom.

A discrete, light-scattering airborne particle counter is used to determine the concentration of airborne particles, equal to and larger than the specified sizes, at designated sampling locations.

Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 µm or larger permitted per m3 of air. So, for example, an ISO class 5 cleanroom has at most 105 particles/m3.

Both FS 209E and ISO 14644-1 assume log-log relationships between particle size and particle concentration. For that reason, zero particle concentration does not exist. Some classes do not require testing some particle sizes, because the concentration is too low or too high to be practical to test for, but such blanks should not be read as zero.

Because 1 m3 is about 35 ft3, the two standards are mostly equivalent when measuring 0.5 µm particles, although the testing standards differ. Ordinary room air is around class 1,000,000 or ISO 9. [14]

ISO 14644-1 and ISO 14698

ISO 14644-1 and ISO 14698 are non-governmental standards developed by the International Organization for Standardization (ISO). [15] The former applies to clean rooms in general (see table below); the latter to cleanrooms where biocontamination may be an issue.

ISO 14644-1 defines the maximum concentration of particles per class and per particle size with the following formula [16]

Where is the maximum concentration of particles in a volume of 1m of airborne particles that are equal to, or larger, than the considered particle size which is rounded to the nearest whole number, using no more than three significant figures, is the ISO class number, is the size of the particle in m and 0.1 is a constant expressed in m. The result for standard particle sizes is expressed in the following table.

ClassMaximum particles/m3 aFED STD 209E
≥0.1 µm≥0.2 µm≥0.3 µm≥0.5 µm≥1 µm≥5 µm
ISO 110bdddde
ISO 210024b10bdde
ISO 31,00023710235bdeClass 1
ISO 410,0002,3701,02035283beClass 10
ISO 5100,00023,70010,2003,520832d,e,fClass 100
ISO 61,000,000237,000102,00035,2008,320293Class 1,000
ISO 7ccc352,00083,2002,930Class 10,000
ISO 8ccc3,520,000832,00029,300Class 100,000
ISO 9ccc35,200,0008,320,000293,000Room air
a All concentrations in the table are cumulative, e.g. for ISO Class 5, the 10 200 particles shown at 0,3 μm include all particles equal to and greater than this size.

b These concentrations will lead to large air sample volumes for classification. Sequential sampling procedure may be applied; see Annex D.
c Concentration limits are not applicable in this region of the table due to very high particle concentration.
d Sampling and statistical limitations for particles in low concentrations make classification inappropriate.
e Sample collection limitations for both particles in low concentrations and sizes greater than 1 μm make classification at this particle size inappropriate, due to potential particle losses in the sampling system.
f In order to specify this particle size in association with ISO Class 5, the macroparticle descriptor M may be adapted and used in conjunction with at least one other particle size. (See C.7.)


US FED-STD-209E was a United States federal standard. It was officially cancelled by the General Services Administration on November 29, 2001, [17] [18] but is still widely used. [19]

ClassMaximum particles/ft3ISO
≥0.1 µm≥0.2 µm≥0.3 µm≥0.5 µm≥5 µm
1357.5310.007ISO 3
103507530100.07ISO 4
1003,5007503001000.7ISO 5
1,00035,0007,50030001,0007ISO 6
10,000350,00075,00030,00010,00070ISO 7
100,0003.5×106750,000300,000100,000830ISO 8

EU GMP classification

EU GMP guidelines are more stringent than others, requiring cleanrooms to meet particle counts at operation (during manufacturing process) and at rest (when manufacturing process is not carried out, but room AHU is on).

ClassMaximum particles/m3 [20]
At RestIn Operation
0.5 µm5 µm0.5 µm5 µm
Grade A3,520203,52020
Grade B3,52029352,0002,900
Grade C352,0002,9003,520,00029,000
Grade D3,520,00029,000Not definedNot defined

BS 5295

BS 5295 is a British Standard.

ClassMaximum particles/m3
≥0.5 µm≥1 µm≥5 µm≥10 µm≥25 µm
Class 13,000 000
Class 2300,000 2,00030 
Class 3 1,000,00020,0004,000300
Class 4  200,00040,0004,000

BS 5295 Class 1 also requires that the greatest particle present in any sample can not exceed 5 μm. [21] BS 5295 has been superseded, withdrawn since the year 2007 and replaced with "BS EN ISO 14644-6:2007". [22]

See also

Related Research Articles

Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine substances from liquids or air. It is used in science labs to remove solids from liquids. This can be used to remove sand from water.

Microfiltration is a type of physical filtration process where a contaminated fluid is passed through a special pore-sized membrane to separate microorganisms and suspended particles from process liquid. It is commonly used in conjunction with various other separation processes such as ultrafiltration and reverse osmosis to provide a product stream which is free of undesired contaminants.

Sand filter sand

Sand filters are used as a step in the water treatment process of water purification.

Laminar flow cabinet laboratory equipment

A laminar flow cabinet or laminar flow closet or tissue culture hood is a carefully enclosed bench designed to prevent contamination of semiconductor wafers, biological samples, or any particle sensitive materials. Air is drawn through a HEPA filter and blown in a very smooth, laminar flow towards the user. Due to the direction of air flow, the sample is protected from the user but the user is not protected from the sample. The cabinet is usually made of stainless steel with no gaps or joints where spores might collect.

A particle counter is an instrument that detects and counts physical particles.

First fix and second fix are terms used in the UK and Irish housebuilding and commercial building construction industry.

Barrier isolator is a general term that includes two types of devices: isolators and restricted access barriers (RABS). Both are devices that provide a physical and aerodynamic barrier between the external clean room environment and a work process. The isolator design is the more dependable of the two barrier design choices, as it prevents contamination hazards by achieving a more comprehensive separation of the processing environment from the surrounding facility. Nonetheless, both Isolator and RABS designs are contemporary approaches developed over the last 35 years and a great advancement over designs of the 1950s-70s that were far more prone to microbial contamination problems.

Pharmaceutical Microbiology is an applied branch of Microbiology. It involves the study of microorganisms associated with the manufacture of pharmaceuticals e.g. minimizing the number of microorganisms in a process environment, excluding microorganisms and microbial biproducts like exotoxin and endotoxin from water and other starting materials, and ensuring the finished pharmaceutical product is sterile. Other aspects of pharmaceutical microbiology include the research and development of anti-infective agents, the use of microorganisms to detect mutagenic and carcinogenic activity in prospective drugs, and the use of microorganisms in the manufacture of pharmaceutical products like insulin and human growth hormone.

The Institute of Environmental Sciences and Technology (IEST) is a non-profit, technical society. Information on ISO 14644 and ISO 14698 standards can be found through this organization.

ISO 14644 Standards were first formed from the US Federal Standard 209E Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones. The need for a single standard for cleanroom classification and testing was long felt. After ANSI and IEST petitioned to ISO for new standards, the first document of ISO 14644 was published in 1999, ISO 14644-1.

Contamination control activities aiming to control the existence, growth and proliferation of contamination

Contamination control is the generic term for all activities aiming to control the existence, growth and proliferation of contamination in certain areas. Contamination control may refer to the atmosphere as well as to surfaces, to particulate matter as well as to microbes and to contamination prevention as well as to decontamination.

Cleanliness suitability describes the suitability of operating materials and ventilation and air conditioning components for use in cleanrooms where the air cleanliness and other parameters are controlled by way of technical regulations. Tests are carried out to determine this.

Cleanroom suitability describes the suitability of a machine, operating utility, material, etc. for use in a cleanroom, where air cleanliness and other parameters are controlled by way of technical regulations in accordance with ISO 14644.

The shelf life of a product can be extended either by adding artificial preservatives or by taking hygienic measures during the manufacturing process. As the consumer trend today is towards preservative-free foods with a long shelf-life, industry is being forced to rethink its manufacturing methods. Instead of adding preservative agents, increased hygienic precautions can be taken during production. A clean and hygienic manufacturing environment is an essential prerequisite in order to keep contamination-related reject rates low. The utilization of surfaces in the manufacturing environment with antibacterial properties can significantly reduce contamination risks.

Ultrapure water is water that has been purified to uncommonly stringent specifications. Ultrapure water is a commonly used term in the semiconductor industry to emphasize the fact that the water is treated to the highest levels of purity for all contaminant types, including: organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile, reactive and inert; hydrophilic and hydrophobic; and dissolved gases.

Clean Air Delivery Rate

Clean Air Delivery Rate (CADR) is a figure of merit that is the cubic feet per minute (CFM) of air that has had all the particles of a given size distribution removed. For air filters that have air flowing through them, it is the fraction of particles that have been removed from the air times the CFM air flow rate through the device. More precisely, it is the CFM of air in a 1,008-cubic-foot (28.5 m3) room that has had all the particles of a given size distribution removed from the air, over and above the rate at which the particles are naturally falling out of the air. Different filters have different abilities to remove different particle distributions, so three CADR's for a given device are typically measured: smoke, pollen, and dust. By combining the amount of airflow and particle removal efficiency, consumers are less likely to be misled by a high efficiency filter that is filtering a small amount of air, or by a high volume of air that is not being filtered very well.

Fan filter unit

A fan filter unit (FFU) is a type of motorized air filtering equipment. It is used to supply purified air to cleanrooms, laboratories, medical facilities or microenvironments by removing harmful airborne particles from recirculating air. The units are installed within the system's ceiling or floor grid. Large cleanrooms require a proportionally large number of FFUs, which in some cases may range from several hundred to several thousand. Units often contain their own pre-filter, HEPA filter and internally controllable fan air distribution.


  1. 1 2 3 Yardley, William (2012-12-04). "Willis Whitfield, Clean Room Inventor, Dies at 92". The New York Times . Retrieved 2013-06-22.
  2. "Sandia physicist, cleanroom inventor dies at 92". KWES . Associated Press. 2012-11-26. Retrieved 2012-12-03.
  3. "Willis Whitfield - Father of the Cleanroom" (PDF). Cleanroom online. September 2015. Retrieved 2016-05-18.
  4. William (Bill) C. McElroy Jr., MicroAire Engineering Manager and acting VP; Kay Plastics Engineering Manager; PureAire Drafting Room Manager
  5. What is a modular cleanroom?
  6. In NASA’s Sterile Areas, Plenty of Robust Bacteria New York Times, 9. October 2007
  7. Sandle, T (November 2012). "Application of quality risk management to set viable environmental monitoring frequencies in biotechnology processing and support areas". PDA J Pharm Sci Technol . 66 (6): 560–79. doi:10.5731/pdajpst.2012.00891.
  8. 1 2 Sandle, T (November 2011). "A review of cleanroom microflora: types, trends, and patterns". PDA J Pharm Sci Technol . 65 (4): 392–403. doi:10.5731/pdajpst.2011.00765.
  9. Heikkinen, Ismo T.S.; Kauppinen, Christoffer; Liu, Zhengjun; Asikainen, Sanja M.; Spoljaric, Steven; Seppälä, Jukka V.; Savin, Hele (October 2018). "Chemical compatibility of fused filament fabrication-based 3-D printed components with solutions commonly used in semiconductor wet processing". Additive Manufacturing. 23: 99–107. doi:10.1016/j.addma.2018.07.015. ISSN   2214-8604.
  10. Pasanen, T.P.; von Gastrow, G.; Heikkinen, I.T.S.; Vähänissi, V.; Savin, H.; Pearce, J.M. (January 2019). "Compatibility of 3-D printed devices in cleanroom environments for semiconductor processing". Materials Science in Semiconductor Processing. 89: 59–67. doi:10.1016/j.mssp.2018.08.027. ISSN   1369-8001.
  11. Limits for Microbial load for clean room as per US FDA and EU Guidelines for pharmaceutical products
  12. Cleanroom Air Flow Principles
  13. "Cleanroom and Controlled Environment Attire - ANSI Blog". The ANSI Blog. 2015-07-15. Retrieved 2018-11-24.
  14. Cleanroom Classification / Particle Count / FS209E / ISO TC209 /
  15. "ISO 14644-1:2015 - Cleanrooms and associated controlled environments -- Part 1: Classification of air cleanliness by particle concentration". ISO. Retrieved 2016-09-12.
  16. W. Whyte (17 October 2001). Cleanroom Technology: Fundamentals of Design, Testing and Operation. John Wiley & Sons. ISBN   978-0-471-86842-2.
  17. Cancellation of FED-STD-209E - Institute of Environmental Sciences and Technology
  18. "Archived copy" (PDF). Archived from the original (PDF) on 2008-05-28. Retrieved 2008-04-17., page 148
  19. "NUFAB SAFETY & PROTOCOL" (PDF). Retrieved 24 February 2016.
  20. Understanding Cleanroom Classifications
  21. Market Venture Philippines Inc. web site (2006-04-19). "What is a Clean Room?". Archived from the original on 2012-08-28. Retrieved 2007-06-02.
  22. "BS 5295-0:1989 - Environmental cleanliness in enclosed spaces. General introduction, terms and definitions for clean rooms and clean air devices". 2016. Retrieved 2016-04-18.