I was told some toxins and poisons can only be tested for in the root of hair. Is this true?
This is not generally true, but if your exposure to toxins was only recent, then the toxins will only show up at the roots, which is the most recent growth of your hair.
The hair is like a time-line. Your tips are the oldest growth, and your roots are the most recent growth, having just come out of your scalp at the head. So, you can actually locate a specific time and date on the strand of hair.
Hair grows approximately 1/2 inch each month, so if your hair is 3 inches long, you can determine the age of that place on your hair strand.
If you were not exposed to a specific toxin 3 months ago (let’s say cadmium), but recently were exposed, then the cadmium will not show up on the tip of your hair, but will register on the hair just growing out of the scalp.