When lead was discovered in the Flint water supply it made headlines worldwide, reflecting the devastating consequences its consumption can have. Although lead is among the most common heavy metal pollutants, it's far from the only one. Many heavy metals have devastating effects, even in tiny quantities. Other heavy metals, however, are harmless to eat, while still others are necessary for human life.
Dr Sam Jones has created a video for the American Chemical Society introducing why different elements that we lump together in the category heavy metals have such varied biological effects. In the process Jones eats gold like some megalamaniacal king of ancient times, but in this case it's for science – or at least science communication.
As Jones notes, some heavy metals are so vital for human health we take supplements when we don't get enough in our diet. Iron and zinc are particularly likely to be in multivitamin pills. Other heavy metals are biologically harmless, aside perhaps to the wallet. Meanwhile, as a linked video shows, lead exposure possibly caused the decline of the Roman Empire. It's also the most likely reason for the explosion in crime the western world experienced from the 1960s to 1990s. Meanwhile the effects of mercury poisoning are traumatic to even hear described.
The problem, as Jones explains, is not in the metals' weight, but in their individual chemistry. The binding sites for calcium in neurons actually show a higher affinity for lead than calcium, causing them to absorb lead instead of calcium and disrupt communication between brain cells. The toxicity of mercury depends on its phase and what it is bonded to. It's never good for you, but in pure liquid form it largely passes through. On the other hand, methylmercury is so easily absorbed a few drops can cause death.
We encounter most heavy metals so rarely that our knowledge of their effects is still evolving, As a 2014 Interdisciplinary Toxicology paper notes; “Metal toxicity depends upon the absorbed dose, the route of exposure and duration of exposure, i.e. acute or chronic.”
Even the definition of metal, let alone heavy metal, varies depending on which field of science you come from. The toxicology paper uses a chemist's definition; “Metals are substances with high electrical conductivity, malleability, and luster, which voluntarily lose their electrons to form cations.”. Heavy metals are then those at least five times as dense as water (roughly the average density of the Earth). To an astronomer, however, any element other than hydrogen and helium is a metal, something that would surely never lead to an interdisciplinary communication failure.