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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.
The Earthâs atmosphere is filled with trillions of tiny dust particles. Most of these particles, or particulates, are too small to be seen with the human eye. The smallest particulates are by coincidence the same length as the wavelength of blue light. As the light from our Sun shines into the atmosphere, most of the colors are able to reach the Earthâs surface uninterrupted. However, because blue light has a wavelength that is the same size as the particulates in the air, this light is scattered in every direction.