Certain essential metals and metal ions are required for the action of the human body, generally by their action on permitting the activity of enzymes and allowing specific carrier molecules to work adequately (e.g. haemoglobin).   However, some are toxic.

There have been specific worries in ASD concerning mercury in that it is not essential and is noted to be associated with mental illness when present in the  body in excess.  The inoculation of thimerosal (also known as thiomersal, and mercurothiolate) as a disinfectant in vaccines has been pointed at as a cause of excess mercury.  However it has been difficult in actually demonstrating reliably this excess, and the levels shown are below that found as causing mental illness in adults.  Attempts to remove it using standard medical procedures have been claimed as producing an improved mental condition in ASD.   Thimerosal breaks down to ethylmercury and the 25micrograms per vaccine dose (largely in the past as most is not now used), is excreted faster than the methyl mercury daily dose of 0.47micrograms/kg/day as being safe.  Hence a 5kg child would have expected to be safe in less than 10 days per vaccine.    Much of the data is published not in scientific journals but as part of WHO, CDC, UK (NHS) and Australian official databases, which should be viewed to get a background to mercury toxicity effects.  However, these documents do not explain the apparent advantages shown by some cases when mercury is removed from the body or the lack of high levels of mercury