Question about hair tests

First off, I am not taking one but I have a question because I am curious. The only thing I have seen around about the hair tests is that they can detect thc for a really long time. Can you just not smoke for a while and shave your head and body a few times and then pass or is all new hair growth infected for a long time. I was just wondering.

Hair test only go back 90 days. To the best of my knowledge once you stop smoking and then shave your hair the new growth would be clean as long as you stayed clean.