Cleanroom
A cleanroom is an environment, typically used in manufacturing or scientific research, that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. More accurately, a cleanroom has a controlled level of contamination that is specified by the number of particles per cubic meter at a specified particle size. To give perspective, the ambient air outside in a typical urban environment might contain as many as 35,000,000 particles per cubic meter, 0.5 m and larger in diameter, corresponding to an ISO 9 cleanroom.

# Overview
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.
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 ultra low penetration air (ULPA) filters to remove internally generated contaminants.
Staff enter and leave through airlocks (sometimes including an air shower stage), and wear protective clothing such as hats, face masks, gloves, boots and cover-alls.
Equipment inside the cleanroom is designed to generate minimal air contamination. There are even specialised mops and buckets. Cleanroom furniture is also designed to produce a low amount of particles and to be easy to clean.
Common materials such as paper, pencils, and fabrics made from natural fibers are often excluded; however, alternatives are available. Cleanrooms are not sterile (i.e., free of uncontrolled microbes) and more attention is given to airborne particles. Particle levels are usually tested using a particle counter.
Some cleanrooms are kept at a positive pressure so that if there are any leaks, air leaks out of the chamber instead of unfiltered air coming in.
Some cleanroom HVAC systems control the humidity to relatively low levels, such that extra precautions are necessary to prevent ESD electrostatic discharge problems. These ESD controls ("ionizers") are also used in rooms where ESD sensitive products are produced or handled.
Low-level cleanrooms may only require special shoes, ones with completely smooth soles that do not track in dust or dirt. However, shoe bottoms must not create slipping hazards (safety always takes precedence). Entering a cleanroom usually requires wearing a cleanroom suit.
In cheaper cleanrooms, in which the standards of air contamination are less rigorous, the entrance to the cleanroom may not have an air shower. There is an anteroom, in which the special suits must be put on, but then a person can walk in directly to the room (as seen in the photograph on the right).
Some manufacturing facilities do not use fully classified cleanrooms, but use some cleanroom practices together to maintain their cleanliness requirements.

# Cleanroom classifications
The following is adapted from Rockwell Automation and from FILT AIR
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 US FED STD 209E, and denote the number of particles of size 0.5 m or larger permitted per cubic foot of air. The standard also allows interpolation, so it is possible to describe e.g. "class 2000".
Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 m or larger permitted per cubic metre of air. So, for example, an ISO class 5 clean room has at most 105 = 100,000 particles per m3.
Note that both FS 209E and ISO 14644-1 are based on assumed log-log relationships between particle size and particle concentration. For that reason, there are no "zero" particle concentrations listed. The table locations without entries are N/A ("not applicable") combinations of particle sizes and cleanliness classes. They should not be read as zero.
Because 1 m3 is approximately 35 ft3, the two standards are mostly equivalent when measuring 0.5 m particles, although the testing standards differ.  Ordinary room air is approximately class 1,000,000 or ISO 9.

## US FED STD 209E cleanroom standards
NOTE: US FED STD 209E was officially cancelled by the General Services Administration of the US Department of Commerce November 29, 2001, but is still widely used.

## ISO 14644-1 cleanroom standards

## BS 5295 cleanroom standards
BS 5295 Class 1 also requires that the greatest particle present in any sample does not exceed 5 m.