Human hair is 10^22 atoms thick. Also known as Rule Of Thumb: Any little yet observable amount of substance contains about one mole of atoms. This approximation is within a few orders of magnitude of being correct.
The number of atoms in 12 grams of Carbon-12 is defined as a mole. It measures roughly 6.02 x 1023. A medium-length strand of hair could weigh as little as a fraction of a gram. So 10^22 is a better estimate in this case.