Some time ago, someone pointed me at a “natural health” site which expressed shock that “Big Pharma” was putting “toxic copper” into baby formula. Those poor babies! Now the copper was there, all right, but only because copper is an essential mineral. Indeed, copper is present in human breast milk, at a concentration of about 0.36 milligrams per litre, and inadequate copper intake has terrible consequences, especially in premature babies. The copper was necessary. The key idea here, which the diagram below is intended to capture, is sola dosis facit venenum (“the dose makes the poison”).
Many essential vitamins and minerals, like copper, transition from a “no effect” dose (blue) to a beneficial dose (green) to a toxic dose (red). In the upper three bars of the diagram, the black dot indicates the recommended daily intake (which we should ingest), and the white bar marks the recommended upper limit, which we should not exceed (disclaimer: this diagram may contain inadvertent errors; please take your medical advice from official sources).
Something similar happens with medicines, like paracetamol (acetaminophen). Small amounts do nothing for your headache; in adults, one or two tablets (0.5–1 gram) safely ease mild pain; but exceeding the dosage indicated on the packet can cause liver failure and death.
For toxic heavy metals like mercury, cadmium, lead, or silver, there is no beneficial level – the transition is from a “no effect” dose (blue) to progressively greater harm, up to and including death. In the lower four bars of the diagram, the white dot indicates the daily intake of the average person (which generally seems to have no observable effect), and the white bar marks the recommended upper limit.
When people are exposed to levels above the white bar, health authorities start to get worried. For example, shark meat can contain 1 mg of mercury per kg or more. Australian authorities recommend that if shark meat is eaten by pregnant women or children, it should be limited to 1 serve per fortnight (with no other fish eaten that fortnight). But even there, it is the dose that makes the poison.