How does a hair test work?  When people are subjected to a hair test for a drug screen the lab conducting the screen usually tells the subject that they are looking to go back and examine the hair from the last ninety day period.  This ninety day period requires a hair sample of approximately one and one half inches of hair.  The average person’s hair grows about one half inch per month.  Each half an inch reflects a thirty day period.  The sample hairs are taken from various parts of the scalp with each strand cut as close to the scalp as possible.  The labs usually require anywhere from fifty to a hundred strands of hair to get an accurate reading.

Body hair and head hair grow at a different rate; otherwise we would all be combing our arm hair and going to the hair dresser for eyebrow perms.  Body hair tends to grow at a much slower pace than the hair on the head and generally is less accurate to determine an exact time frame any toxins were ingested but will show that there were some toxins used, usually up to a year. Body hair only grows to a certain length and then the growth period stops.  Once the body hair stops growing, then it starts to harden and shrink.  A new hair starts to form underneath and then pushes the old hair out and the cycle starts all over again.  This is why we don’t have to braid the hair on our arms and legs.

It generally takes a few days from the time that you ingest the toxin until it shows up in your hair but once it is in the hair follicle it is detectable until it reaches the end of the follicle and is cut off of the head. Hair generally has to be at least one half of an inch to conduct a test properly.

Many people think that if they are in a room or car where someone has been smoking marijuana, that they will test positive on a hair test for THC.  This is inaccurate.  A hair test only detects the metabolite of the drug and therefore the drug must be consumed to be found present in the follicle.