I personally do not trust them. How close to the scalp the sample is taken will influence the reading as the hair closest to the scalp is the most recent record. The big problem is contaminmation from external sources. Surfers for example often show high levels of heavy metals the hair picks up from the water. And if someone works in a smoking environment or lives in a smoking household then the hair can be easily contaminated with cadmium from second hand smoke. Shampoos and hair dyes can also yield false results. Head and Shoulders contains selenium, and hair dyes can contain lead acetate.