Hair testing does not have uniform sensitivity in relation to all drugs tested for. As example, cocaine is easy to detect in hair because it is incorporated into the hair. To put it basically, cocaine likes hair and attaches to it.
Not so with THC and even less so with THC-COOH (COOH is what they test for) both metabolites of cannabis. Neither metabolite of cannabis likes hair and simply doesn't get into and attach to hair, compared with other drugs. THC, and especially the main metabolite THC-COOH, have a very low incorporation rate (ICR) into hair. (see Musshoff & Madea
As example, there is a 3600-fold difference between the ICR of cocaine and that of THC-COOH in hair... Unlike for other illicit drugs, hair analysis lacks
The sensitivity to act as a detector for cannabinoids.. (Musshoff, pp 161-62)
Marilyn Huestis, National Institutes of Health, published a very thorough study on this subject. (Huestis, 2007
The chance that you would have COOH in your system is very, very low.
There is a wider issue involved. Marijuana is the most tested for substance in the US. People have the idea that drugs somehow get into the hair via circulation all at the same rate of incorporation and stay there for a long period. Not true.
Those paying for a hair drug test, far more expensive than urine, should be advised that only heavy marijuana use is likely to show up and that urine assays are far more effective. False negatives
are the rule rather than the exception when assaying low to moderate smokers via hair. (See Huestis, above) This leads to a false sense of security in parents and schools where hair testing is widely used. Marijuana use is not being detected until use is frequent and heavy (if hair test is sole method). These facts are not properly disclosed by the commercial testing labs.