Short answers, pulled from the story.
A micrometre (symbol: μm) is a unit of length equal to one millionth of a metre, or one thousandth of a millimetre. It is also called a micron. The width of a single human hair ranges from approximately 17 to 181 micrometres.
The micrometre is commonly used to measure wavelengths of infrared radiation, the sizes of biological cells and bacteria, and the diameter of wool fibres. It also describes everyday objects such as plastic wrap (10-12 μm thick) and paper (70-180 μm thick).
The SI revoked the term micron and the standalone symbol μ in 1967. The revocation was necessary because the standalone use of μ to mean micrometre conflicted with the SI's adoption of micro- as an official unit prefix in 1960, making the older usage ambiguous within the new system.
Micron and micrometre refer to the same unit of length, one millionth of a metre. Micron remains in informal use, especially in American English, to avoid confusion with the micrometer measuring instrument, which shares the same spelling as the unit in American usage.
A typical bacterium measures between 1 and 10 micrometres in length. A human red blood cell is 6 to 8 micrometres in diameter, and a strand of spider web silk is 3 to 8 micrometres wide.
Before desktop publishing, typists approximated the Greek letter mu by combining a slightly lowered slash with the letter u on mechanical typewriters. Early word processing software sometimes substituted just the letter u when the Greek character was unavailable.