A micron, or micrometer, is defined as one-millionth of a meter. The symbol for micron is μ, an upside-down "h" symbol. A human hair is between 40 and 50 microns wide, whereas a dust mite is about 400 microns long.
Most objects in the micron scale need a microscope to be observed. Other small objects measured in microns include red blood cells that are between 6 to 10 microns wide and bacteria that typically measure between 5 and 20 microns in diameter.
Even smaller than microns are nanometers, or one-billionth of a meter and 1,000 times smaller than a micron. Viruses are between 30 and 40 nanometers. A strand of DNA is 2.5 nanometers wide. Atoms are between 0.1 and 0.3 nanometer wide, depending upon the element.