Mercury is a chemical element normally found in nature. High concentrations can be toxic for the brain, especially for children, because their nervous systems are still developing.
Most mercury has been eliminated from the daily life of children. Although the exact levels of mercury toxicity in fish are not well known, the Food and Drug Administration advises pregnant women against eating large quantities of big fish such as tuna and swordfish, which are more prone to contain mercury.
Many of the parents I see express concern about mercury content in the vaccines we use. Their fears are based on the fact that thimerosal, a mercury derivative, was used many years ago to prevent contamination in vaccines. It was removed from most vaccines four or five years ago, not because it was detrimental but because it is no longer needed.