From the article I just posted :
HOW DOES WEED GET IN MY HAIR, ANYWAY?
When you smoke, vape, or eat marijuana, all you’re really doing is facilitating the release of the drug’s active compounds into your bloodstream. Once they’re in your veins, these cannabinoids ride the blood highway to the cannabinoid receptors on the body’s cells, where they bind and exert their effects. Along the way, some of the cannabinoids — metabolites of psychoactive THC are what most tests look for — enter the blood vessels that feed the cells of the scalp. There, at the interface between the vessel and the follicle cells known as the hair papilla, THC jumps the gap, entering the matrix, where actual hair growth occurs. Over the next couple of weeks, the hair from the root sprouts upward, carrying THC past the scalp, where it becomes fair game for testing for the next three months.
Active compounds from marijuana travel in the bloodstream to a strand of hair via the hair papilla in the hair follicle.Wikimedia
HOW LONG DOES IT STAY THERE?
The most commonly cited number for the amount of time marijuana can be detected in your hair is 90 days. That time frame is calculated on the basis of how long it takes for hair at the root to grow past the scalp, and it takes into account the part of the hair that drug testers sample.
According to Quest Diagnostics, one of the premier drug testing labs in the United States, it takes five to 10 days before hair at the root pushes out past the scalp and into the world. In a drug test, samples are usually taken from hair 1.5 inches from the scalp. Since hair grows at about 0.5 inches per month, then you have about 3 months, or 90 days, before you’re in the clear.