Clearing The Air
In maintaining clean rooms, engineers try to keep out particles 0.5 micron and larger. A key device is the high-efficiency particulate air (HEPA) filter, which traps at least 99.97 percent of particulates 0.3 micron wide. (It does even better with bigger particulates.) Other measures, however, must be included, because additional particulates are constantly being created—walking, for instance, generates about five million of them per minute. For ultralow penetration air (ULPA) filters, the target is 0.12 micron with 99.999 percent efficiency.
Number of particles less than 0.5 micron wide suspended in a cubic foot of air in:
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Office building: 500,000 to 1 million
Room in a U.S. home: 350,000
Hospital operating room: 1,000 to 10,000
NASA clean room: 100
Computer-chipmaking clean room: 10 to 1,000
Biohazard Level 4 lab: 10
Typical size, in microns:
Bacterium: 1 to 10
Virus: 0.02 to 0.45
Compiled by Barry E. DiGregorio
SOURCES: NASA; Coastwide Laboratories; The Quarantine and Certification of Martian Samples (National Academies Press, 2002); Wikipedia.org, orders of magnitude (length)
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