Nanotechnology refers mainly to engineered particles that range from 1–100 nanometers in size. A nanometer (nm) is one-billionth of a meter, equivalent to one-millionth of a millimeter, and one-thousandth of a micron. For comparison, a piece of copy paper is 100 microns thick — in nanometers this is 100,000nm thick. A human hair is about 75 microns in diameter or 75,000nm. Also, a red blood cell is 5 microns wide, this is 5,000nm.