It looks to me that there is a great variety of opinions on the natural half life of mercury in the brain: Some people like cutler say it's 20 years, some more mainstream research says it might be as low as 90 days (with studies on monkeys....). And also there is a very interesting theory that believes that this is genetical, ie. some people have different genes leading to some different protein in the brain (apo-E) that vary a lot how much mercury you can excrete naturally from the brain.

You have 2 genes for these proteins: each gene says if the proteins can carry out 0,1,2 atoms of mercury: hence:

with probability 1/9 you cant carry out mercury out of the brain
with probability 2/9 you can carry out 1 atom (every 2 proteins)
with probability 3/9 you can carry out 2 atom (every 2 proteins)
with probability 2/9 you can carry out 3 atom (every 2 proteins)
with probability 1/9 you cant carry out mercury out of the brain.

Maybe the probability of the genes are not equal so these probabilities are wrong, but you get the idea.

If you are unlucky you might have big problems removing mercury from the brain <img src="/ubbthreads/images/graemlins/frown.gif" alt="" />