One of the first signs of heavy metal poisoning is depression. Lead poisoning is no doubt to blame for many cases of depression and for severity of symptoms. Depression sets in before the most drastic neurological and psychiatric effects, such as retardation and lowered IQ, appear. The brain has only a few ways of letting us know it is not working properly, and depression is one of the first flares sent up. Though lead levels in gas and paint are now famously regulated, other sources, like jet fuel, still pour lead into our environment.
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Hair analysis is a good way to screen for heavy metal exposure, despite its flaws in other areas. This test is affordable and non-invasive. Blood or urine tests would be appropriate to follow up on any results showing high levels of lead.
(Source: The Brain Chemistry Plan, by Michael Lesser, M.D., with Colleen Kapklein.)