I guess andy means that some show poisoning towards it, others tolerate the stuff, but damage has to be happening on some level becuase the nature of mercury is damaging to us human beings, it just comes down to tolerance and what levels some peole can handle before they exhibit toxic symptoms. Some only take a very small amount, others can handle much more, but regardless, mercury is going to be doing something, even if the damage, is slow and silent and the person isn't sensitive to the mercury.