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“Heavy metals found in baby food.” “Heavy metal contamination in the river.” The phrase appears constantly in the news, usually as a warning. But ask five scientists to define it and you’ll get five different answers. IUPAC — the body that standardizes chemical terms — published a report in 2002 calling “heavy metals” a meaningless term that should be avoided. Here’s why, and what people generally mean by it anyway.
The competing definitions
Over the years “heavy metal” has been defined by:
- Density — for example, metals denser than 5 g/cm³. That would include iron, copper and zinc (essential nutrients) and gold (harmless).
- Atomic weight — metals above a certain mass, such as that of sodium or calcium.
- Atomic number — metals above a certain position in the table.
- Toxicity — metals and metalloids that are poisonous even at low levels.
None of these lines up neatly with the others, and the toxicity definition sometimes includes arsenic (a metalloid) and even selenium (a nonmetal), neither of which is a heavy metal by any physical measure.
What the news usually means
In environmental and health contexts, “heavy metals” almost always refers to a short list of toxic metals and metalloids:
| Element | Main sources of exposure | Main health effects |
|---|---|---|
| Lead | Old paint and pipes, contaminated soil, some imported goods | Nervous system damage, especially in children; lowered IQ |
| Mercury | Fish (as methylmercury), gold mining, broken thermometers | Nervous system and kidney damage; harmful to developing babies |
| Cadmium | Cigarette smoke, some foods grown in contaminated soil, batteries | Kidney damage, bone weakening |
| Arsenic | Groundwater in some regions, rice, industrial pollution | Skin lesions, cancers, cardiovascular disease |
| Chromium (as chromium(VI)) | Industrial pollution | Cancer when inhaled |
| Thallium | Rare; historical rat poisons | Severe poisoning, hair loss |
The World Health Organization lists lead, mercury, cadmium and arsenic among its top ten chemicals of major public health concern.
Why they’re toxic
These metals interfere with the body’s chemistry in a few ways:
- Imitating essential elements. Lead can take the place of calcium and zinc in enzymes and bones; cadmium can substitute for zinc. The body absorbs them, but they don’t work the same way.
- Binding to sulfur. Mercury, lead and cadmium bind strongly to the sulfur atoms in proteins and enzymes, disabling them.
- Accumulating. Many are excreted slowly, so they build up over years. Mercury also biomagnifies up food chains: large predatory fish like swordfish and some tuna contain much more than small fish.
The form matters as much as the element
Toxicity depends enormously on the chemical form:
- Chromium: chromium(III) is a trace nutrient; chromium(VI) is a carcinogen.
- Mercury: liquid metallic mercury is poorly absorbed if swallowed (its vapour is the danger), while methylmercury in fish is absorbed very efficiently.
- Barium: barium sulfate is swallowed safely for X-rays; soluble barium chloride is poisonous. See solubility rules.
- Arsenic: inorganic arsenic compounds are highly toxic; the arsenobetaine found in seafood passes through the body largely harmlessly.
This is one of the main reasons chemists dislike the term “heavy metal”: it lumps together elements — and forms of elements — that behave very differently.
Essential “heavy” metals
Several metals that are dense enough to count as heavy by the density definition are essential for life in small amounts: iron (in haemoglobin), zinc, copper, cobalt (in vitamin B₁₂), manganese and molybdenum. The difference between nutrient and poison is often the dose — see elements in the human body.
Better terms
Scientists increasingly use more precise phrases instead: “toxic metals”, “potentially toxic elements”, or simply naming the specific element and compound. When you see “heavy metals” in a headline, it’s worth asking: which element, in what form, and at what level?
Quick answers
Is iron a heavy metal? By density, yes; in the toxic sense, usually not — it’s an essential nutrient, though too much can be harmful.
Is arsenic a heavy metal? It’s a metalloid, but it’s usually included in “heavy metal” lists because of its toxicity.
What’s the heaviest metal? By density, osmium. See the heaviest element.
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