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According to the United States Federal Standard, the cleanliness is defined on the basis of 0.5 μm particle in diameter. That means, a cleanroom of class 1000, has a controlled environment with less than 1000 particles per ft3 of the 0.5 μm particle size.
In the figure below, around 0.5 μm particles are smog, oil mist, cigarette smoke, etc. These are in sight only when they condense. In the cleanroom, the subjects are so small particles that they cannot be visible without using microscope.
HEPA filter used in cleanroom can remove particles larger than 0.3 μm in diameter with more than 99.97% collection efficiency. ULPA filter, now being used in super cleanroom for semiconductor industries, has more than 99.999% efficiency for 0.1 μm particle and is able to maintain the supply air free from particles.