Once inside a BSL-4 lab, scientists breathe air filtered by a high-efficiency particulate air (HEPA) filter system. HEPA filters use ultra-fine fibers to remove microscopic particles 0.3 microns in size and smaller from the air with nearly 100 percent efficiency. (One micron is a millionth of a meter, about the length of a typical bacterium.) Larger particles are of course removed more easily. HEPA filters can trap any bacteria or viruses that may be in the air. Exhaust air from the building always passes through two HEPA filters. The air pressure across these filters is monitored, and if a filter becomes clogged and the pressure drops, an alarm sounds. The filters are inspected regularly, and they are sterilized before being replaced.
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Last Updated November 29, 2009