hair is not a good indicator of mercury poisoning because people who have mercury poisoning retain the mercury in their brain, lymphatic system and organs and often do not excrete much through hair as a result. low amounts in hair can often reflect higher amounts of poisoning in the body. i suggest the book "it's all in your head" by hal huggins.
as far as the mercury under crowns, yes, many dentists put in under or leave it under crowns. there is no way to distinguish the different metals so the crown has to be pulled off and checked and replaced with a new one if there is mercury. i have heard that hal huggins says he has a test which can distinguish the metals without doing this, don't know if this is true, but you can write and ask at hugnet.com.
mercury does not hang out in blood or urine, so these tests are worthless and mean nothing. the best tests for mercury are the fractionated porphyrin fecal and urine for mercury. beyond that, the Melisa test is also good.