according to erowid.org : a hair test for marijuana can detect up to 90 days. i always thought it was longer, but they are saying 90 days. also, the period of detection for a single use of marijuana to a habitual user are the same.
the ever questionable wiki has this: look for the chart http://en.wikipedia.org/wiki/Drug_test from wiki: Hair drug screen Hair testing is quite accurate and can go back 6 months or longer, showing any controlled substances used in a sort of timeline. As hair grows out, any drugs used are encased in the hair shaft, so the longer the hair, the longer back in the individual's drug history the lab can detect. Most legitimate testing facilities, however, only use hair within about 3-5 cm of the scalp, and discard the rest. This limits the detection history to about 90 days, depending upon the rate of growth of the individual's hair. Some people attempt to circumvent this through shaving their heads. In the absence of the required amount of hair on the scalp, body hair can be used as an acceptable substitute. Additionally, for pre-employment hair testing, the inability to obtain a sample may be grounds for not hiring the individual.
with hair testing, i´ve often wondered about religious reasons and how that would hold up. for instance, if you´re rasta and dont cut your hair or shave your beard then you obviously wouldnt be lettin some fools cut some of your hair for a drug test. for the body hair, you could make the same argument that you dont cut ANY of your hair. so if you are legitimately rasta (i´m not, btw, this is all hypothetical) then they couldnt deny you the job based on not providing a hair sample. but then i´m sure they´d just do a UA or something.
