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The smallest particle visible to the naked eye is 40 microns. To give you some perspective, a piece of hair's width can range from 40 to 120 microns.
That means the room can have no more. than 35,200 particles greater than .5 micron in size per cubic meter. These are particles that are not visible to the human eye. (As a. comparison, a particle of cigarette smoke is between .5 and 2 micron in size. The end of a piece of human hair is about 60 to 100 microns.