How fast does head hair grow and how much hair is needed for hair drug testing?
Studies on this subject indicate that head hair grows on average approximately 1.3 cm (or 1/2 inch) per month. This growth rate varies slightly (estimated at .2 cm per month, meaning there is some ( 1 week) time variation possible. A standard hair drug test with GC/MS confirmation requires 40+ milligrams of hair or approximately 90 to 120 strands. The thickness of different types of head hair (thick brown vs. thinning gray) is the reason for this variation.
