We all know mercury as a silver metal that is liquid at room temperature and often found in old thermometers. Mercury is found in many forms in our environment and is toxic in the methylated form. See this NIH site for details on sources of exposure.

Mercury is a neurotoxin that disrupts normal blood brain barrier function. It is also known to bind to and damage cellular organelles (functional parts) and cause neuronal cell death. Mercury is stored preferentially in fat cells. (your brain is mostly made of fat) Neuronal cell death is a dangerous process that if left unchecked can lead to dementia. Think of the phrase, "mad as a hatter". Historically, "Hatters" were people that made hats from felt. They were exposed to mercury in the production process and over time developed mercury induced dementia. 

Humans are being exposed to methyl mercury by consuming certain fish. Methyl mercury is concentrated in the fatty flesh of large fish like shark, mackerel, tuna, swordfish, tilefish and other high food chain fish. These large fish have high concentrations of mercury because they consume smaller fish that obtain mercury from the ocean. Read this article for more information.

We do not clear methyl mercury from our systems effectively. Therefore, we must reduce our intake and limit exposure. In general, I recommend a diet with wild caught salmon, trout, sardines, herring and other small fish. Incidentally, these small fish have excellent omega three fatty acid content. 

(Thiomerisol, a mercurial additive in vaccines, is no longer present in any vaccines that we use in clinical pediatrics. To my knowledge, the only source of methyl mercury in vaccines is in multi dose influenza vaccines that are used by some big box retailers and the government. The studies to date have not shown a link between thiomerisol vaccine use and autism, however, why use it if we have other options.)

I love fish and recommend it,