The measure of clean room cleanliness is the number of air pollutants that can be found in an air sample. Many clean rooms are not intended to be completely sterile environments.
More typically, professionals concern themselves with the amount of dust floating in the air or that can be kicked up from the floor when stepped on. This dust is microscopic and can sometimes only be measured by specialized machinery. There are some cases in which sterility is a requirement; many medical research operations and testing procedures must be conducted in a sterile environment. Pharmacological studies, infectious disease testing and other highly sensitive medical research-related procedures often require the complete evacuation of any microbes that may be present in their surrounding environments. Clean room cleanliness is determined based on the presence of contaminants over 0.5 micrometers in size. A human hair is usually around 100 micrometers wide, and humans shed dead skin cells at a rapid rate of 100,000 particles per minute while standing still. For these reasons, very little skin or hair, if any, is allowed to be exposed in clean room settings; they are both usually covered by protective clothing. Also, most clean rooms are equipped with some kind of air quality control system, many of which involve air filters and fans that remove air from within the clean room and pump filtered air in.
Clean room designs range in terms of size, material and their intended standard of cleanliness. Clean rooms used by major manufacturers of microelectronics, pharmaceutical products or circuitry can be as large as warehouses; these large clean rooms are sometimes referred to as "ballrooms." Other clean rooms can be as small as household ovens; these are used in small scale medical testing procedures as well as specialized, limited or exclusive-run manufacturing operations. They can also be used for repairs and for small experiments. Permanent clean rooms are usually built into the structures of the buildings that house them, as are the machines that regulate their conditions. However, some permanent clean rooms can be enclosed by glass or clear plastic walls. Soft walls, which are most commonly used in clean rooms with lower decontamination standards, are often made of flexible acrylic materials or other plastics. All of the materials used in the construction of clean rooms must be carefully chosen for their non-fibrous, non static electricity-generating and non-corrosive qualities. Any material that could contribute to air contamination must be avoided during the construction of clean rooms. Even stainless steel can corrode when exposed to bleach, which is sometimes used as a disinfectant.
There are two main standards systems against which the cleanliness of a clean room is measured. US FED STD 209E is the system of standards set by the United States government; clean room models are assigned a class number based on the number of 0.5 micrometer-sized particles found in every cubic foot of atmosphere within the clean room. The class numbers correspond with the particle per cubic foot number. Class 1 clean rooms can be expected to contain one 0.5 micrometer-sized particle per square foot, Class 100 can be expected to contain 100 particles, and so on. If regular, unfiltered air were assigned a classification, it would be Class 1,000,000. The International Organization for Standardization also assigns standards to clean rooms based on their anticipated contaminant levels, though the ISO standards are assigned on a scale of three to eight, with three being assigned to the most effective clean room systems. The British Standard system is also recognized by some industries; their system measures contaminants in cubic meters instead of feet.
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