What is the normal level of THC in a hair follicle test?

As far as hair test are concerned they are able to see as far back as 90 days. All drug test are based on meeting a criteria (ex. the total concentration of THC must be above a certain level to insure no false positives). However, with hair, as opposed to urine there is very little chance of false positives, since they clean outside of the hair first (the only possible source of a false positive in hair test). The amount you smoke may have some affect on whether or not it's detectable, but this is not likely. If you smoked once, and only once in your life it is likely to show up on a hair test if you smoked within the last 90 days. They can read levels and sometimes can tell when you used last, or around when (this is not always reliable though because everyone's hair grows at different rates and some drugs are able to move up the shaft easier than others. Marijuana does not move up the shaft easily). Basically, there is no precise level of marijuana that it takes to fail a hair drug test, or any drug test for that matter. But, hair test are very reliable and can detect trace amounts. Therefore, I would have to say ANY marijuana usage in the 90 days before a hair test would be traceable.