"les salles en surpression par rapport à la pression atmosphérique pour éviter que divers polluants (poussières, bactéries...) puissent entrer que l'on retrouve principalement dans l'industrie pharmaceutique et électronique " (from: http://fr.wikipedia.org/wiki/Salle_blanche)

"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/or ultra-low penetration air (ULPA) filters to remove internally generated contaminants.
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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." (from: http://en.wikipedia.org/wiki/Cleanroom)

"Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently if there is any leak from the positively pressured system it will egress into the surrounding environment.
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This kind of positive pressure also used on operating theaters and in vitro fertilisation (IVF) labs. To maintain positive pressure the room should be sealed to prevent egress of air.

Hospitals may have positive pressure rooms for patients with compromised immune systems. Air will flow out of the room instead of in, so that any airborne microorganisms (e.g., bacteria) that may infect the patient are kept away." (from: http://en.wikipedia.org/wiki/Positive_pressure)