Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
Drug test
However, there is one major difference between most drugs and alcohol metabolites in the way in which they enter into the hair: on the one hand like other drugs FAEEs enter into the hair via the keratinocytes, the cells responsible for hair growth. These cells form the hair in the root and then grow through the skin surface taking any substances with them. On the other hand, the sebaceous glands produce FAEEs in the scalp and these migrate together with the sebum along the hair shaft (Auwärter et al., 2001, Pragst et al., 2004). So these glands lubricate not only the part of the hair that is just growing at 0.3 mm per day on the skin surface, but also the more mature hair growth, providing it with a protective layer of fat.