<unk> s can be very large. Entire manufacturing facilities can be contained within a<unk>  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<unk>  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<unk>  is designed to generate minimal air contamination. There are even specialised mops and buckets.<unk>  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.<unk> s 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<unk> s 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<unk>  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<unk> s 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<unk>  usually requires wearing a<unk>  suit.
In cheaper<unk> s, in which the standards of air contamination are less rigorous, the entrance to the<unk>  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<unk> s, but use some<unk>  practices together to maintain their cleanliness requirements.
Cleanroom
Ventouse
Nail 
Unitaid
Microbiology template
Injury cover
Firefighter
Tanning booth