As the blood flows into the hair follicle the toxins, such as the THC metabolites are carried along with the blood flow.  As your hair grows the toxins follow along that trajectory but never leave the follicle until they are cut out.  As long as there are toxins in your blood there will be toxins flowing into your hair.  When your blood is finally clean then any new hair growth will be clean.  When you take a hair test the drug levels are measured by the drug molecules imbedded inside the hair follicle. Only the inside of the follicle is tested so the sample can not be contaminated by any external substances on the outside of the hair follicle.  The sample is thoroughly washed before it is tested.

The hair test measures the toxin levels of any drug or toxin in a unit measure called a picogram.  A picogram is one trillionth of a gram. That is a very small unit of measure but luckily the cutoff levels for certain drugs are any where from 1 pg/mg of hair for marijuana to 500 pg/mg of hair for cocaine.

The testing facility collects about one hundred milligrams of hair.  This is about ninety to one hundred strands of hair or about the thickness of the tip of a shoelace.  They cut as close to the scalp as they can as this is the timeline by which they base their analysis.  The hair in the human head grows approximately one half inch per month on average.  The testing facilities normally try to get a sample that is about one and a half inches long as this would be equivalent to ninety days of hair growth and ninety days of activity.

(To Be Continued)