Start from the element, not the episode.

What it is

A single proton with a single electron, and about 62% of all the atoms in a human being, though only around 10% of the mass. Almost none of it is loose. It is bonded into water, into every organic molecule you contain, and — most consequentially — into the weak, constantly breaking hydrogen bonds that hold the two strands of DNA together, fold proteins, and give water the anomalous properties life depends on. The free version, the bare proton, exists at a concentration of about forty nanomoles per litre in blood, and defending that number is one of the body's least negotiable jobs.

Why it matters

Hydrogen is where the wellness industry's favourite word lives. Alkaline water, alkaline diets, acid–base balance, ionisers, drops — all of them are claims about the concentration of one ion, and almost all of them are claims the body has already settled. But the element is not a dead end for the show, because two genuinely live research questions also sit here: molecular hydrogen as a selective antioxidant, which has small human trials and no large ones, and hydrogen breath testing, which is the commonest gut test in the world and is much weaker than it is presented as being.

Too little

Acidaemia below pH 7.35, and the causes are the whole of acute medicine: lactate from hypoperfusion, ketones from insulin deficiency, retained CO₂ from a failing respiratory pump, lost bicarbonate from the gut, retained acid from a failing kidney. Around pH 7.0 cardiac contractility falls and catecholamines stop working properly; survival below about 6.8 is rare.

Too much

Alkalaemia above 7.45, most often from vomiting, diuretics or hyperventilation. It shifts the dissociation curve so haemoglobin holds oxygen more tightly and delivers less, drops ionised calcium and causes the tetany and perioral tingling of a panic attack.

The window is 0.1 of a pH unit wide and it is defended by three systems on three timescales — the bicarbonate buffer in seconds, the lungs in minutes, the kidneys over days. No food moves it. This is the specific, checkable fact that the entire alkaline-diet category depends on people not knowing.

What it is

The third element and the lightest metal: an alkali metal so reactive it is stored under oil and so light it floats on water. It has no known essential role in human biology, which makes it unusual among the elements in this atlas. It is given as a salt — usually the carbonate — and the ion is what does the work. Lithium orotate, sold over the counter at roughly a hundredth of the prescription dose, is the same ion attached to a different counter-ion; the orotate is not the active part.

Why it matters

Lithium is the reference treatment for bipolar disorder and the only agent with randomised evidence that it reduces death by suicide, an effect that appears at least partly separable from its effect on mood. It also has the narrowest therapeutic index of any drug in routine psychiatric use: the level that treats and the level that harms are within a factor of two of each other, which is why it is one of the very few drugs whose blood concentration is measured as a matter of course.

Too little

There is no lithium deficiency disease. What there is instead is a body of ecological work — not trials — reporting lower suicide rates in areas whose drinking water carries more naturally occurring lithium, at concentrations thousands of times below a therapeutic dose. It is a genuinely strange finding, it has been replicated in several countries, and it remains observational: the places with lithium in the groundwater differ from the places without it in more ways than the lithium.

Too much

Coarse tremor, vomiting, ataxia, confusion, then seizures and renal failure. Toxicity is most often precipitated not by taking more but by losing less — dehydration, a new NSAID, an ACE inhibitor or a thiazide diuretic all reduce renal clearance and raise the level on an unchanged dose. Long-term use causes nephrogenic diabetes insipidus, hypothyroidism and, in a minority, progressive chronic kidney disease.

The one element here whose dose-response band is drawn in blood concentration rather than in daily intake, because that is how it is actually managed.

What it is

A metalloid that never occurs free in nature; in biology it is always borate or boric acid, a form that is well absorbed and readily excreted by the kidney. It is unambiguously essential to plants — which is why it is in food at all — and its essentiality in humans has been argued for forty years without being settled.

Why it matters

Boron's interest is that it does something measurable to the steroid hormones at supplement doses. Small human studies giving around 3 mg/day report rises in free testosterone and falls in sex-hormone-binding globulin and in inflammatory markers. Those studies are small and some are not controlled. It is a real signal from a mineral almost nobody measures, and it is also the kind of signal that supplement marketing converts into a claim the data will not carry.

Too little

No deficiency syndrome has been defined in humans, which is the honest way to say it. Depletion studies report changes in calcium and magnesium handling and in cognitive electrophysiology, but nobody gets ill from a low-boron diet in the way they get ill from a low-iron one.

Too much

Acute boric acid poisoning is a real thing with a blue-green vomit and a characteristic 'boiled lobster' rash, and it comes from ant baits and old antiseptic powders, not from supplements. Chronic high intake causes nausea and, in animals, testicular toxicity — the opposite of the effect the supplement aisle sells it for.

An element whose upper limit was set from reproductive toxicity in animals and whose selling point is reproductive hormones in men.

What it is

A diatomic gas held together by a triple bond so strong that the atmosphere is chemically almost inert — which is why nitrogen fixation, biological or industrial, is one of the most consequential chemistries on earth. In the body nitrogen is never free: it is in amino acids, in nucleotides, in haem, and in the small signalling molecule that gave the show its interest in beetroot.

Why it matters

Two very different stories share this element. The first is protein: nitrogen balance is the classical way of asking whether someone is building or losing tissue, and it is the reason protein requirements are quoted the way they are. The second is nitric oxide — one nitrogen, one oxygen, a gas with a half-life of seconds that relaxes blood vessels, and the reason a beetroot has cardiovascular pharmacology.

Too little

Negative nitrogen balance is the body taking itself apart for amino acids — which it will do in starvation, in critical illness and, much more quietly, in older people eating too little protein to offset the anabolic resistance that comes with age.

Too much

There is no demonstrated harm from high protein intake in people with normal kidneys, and this is one of the more persistent pieces of received wisdom that the trials do not support. In established kidney disease the calculus genuinely changes. Separately, the nitrate side of this element has its own ceiling, and it is contested — see the note.

The nitrate–nitrite question is the live controversy: the same chemistry is a cardiovascular benefit in vegetables and a putative carcinogen in cured meat, and the difference is argued to be the vitamin C and polyphenols that travel with the vegetable.

What it is

Half the mass of the earth's crust, two thirds of the mass of a human, and the element that makes complex life energetically possible by sitting at the end of the electron transport chain and accepting what everything upstream hands it. It is also chemically a radical: molecular oxygen carries two unpaired electrons, which is why liquid oxygen sticks to a magnet and why the simple dot-and-cross structure taught in schools is wrong about it. That unpaired pair is the whole of its chemistry — the reason it burns things, the reason it rusts iron, and the reason aerobic life had to evolve an entire antioxidant apparatus to survive using it.

Why it matters

Oxygen is the atlas's case against the assumption that more of a good thing is better. It is a nutrient, a drug, a toxin and a signal, and medicine has been wrong about the dose in both directions inside living memory: an epidemic of infant blindness caused by too much, and a generation of withheld oxygen in breathless patients caused by a physiological theory that turned out to be mostly wrong. The current answer — a target range, not a tap — arrived only in the last decade.

Too little

Hypoxaemia, and the reason it is dangerous is that its symptoms are unreliable. Cyanosis needs about 5 g/dL of deoxygenated haemoglobin to be visible, so a profoundly anaemic patient can be critically hypoxaemic and not look blue at all. Chronic hypoxaemia drives pulmonary hypertension and polycythaemia; acute hypoxaemia kills brain first.

Too much

Two named toxicities, both described before 1900. The Paul Bert effect is central nervous system oxygen toxicity at high partial pressure — seizures, which is why recreational and technical divers care about it. The Lorrain Smith effect is pulmonary: prolonged high inspired oxygen damages the alveolar lining. Between the two sits the everyday version, which is not a named syndrome but a mortality signal.

IOTA pooled 25 randomised trials and 16,037 acutely ill adults and found that a liberal oxygen strategy INCREASED in-hospital mortality — relative risk 1.21 — without improving any other outcome, at high quality of evidence. The authors put the turn at an SpO₂ of about 94–96%. This is one of the cleanest reversals in acute medicine and it is still not fully reflected in practice.

What it is

The most electronegative element in existence — it pulls electrons harder than anything else, and as a consequence free fluorine gas reacts with almost everything. Nothing biological ever meets it in that form. What matters medically is the fluoride ion, F⁻, which is the placid, fully-reduced end state of all that reactivity.

Why it matters

Fluoride substitutes for hydroxide in the hydroxyapatite crystal of tooth enamel, producing fluorapatite, which dissolves at a lower pH than the original. That is the entire mechanism of cavity prevention, and it is a chemical fact rather than a contested one. What is contested is the size of the benefit at current exposures, and what else the ion does at higher ones.

Too little

More caries, though the margin has narrowed considerably since fluoride toothpaste became universal — which is the honest complication in the whole debate.

Too much

Dental fluorosis first, as mottling of the enamel; skeletal fluorosis at sustained high exposure. The cognitive question sits above the guideline value, not at it, and the distinction between those two statements is the investigation.

Unlike the other elements here, fluoride's dose is delivered by policy rather than by diet.

What it is

An alkali metal that exists in the body entirely as the ion, and almost entirely outside the cells. The gradient it holds across every cell membrane is maintained by the sodium–potassium pump, which consumes something on the order of a fifth to a quarter of resting energy expenditure. Sodium is not optional and it is not a contaminant: it is the electrical substrate of every nerve impulse you have ever had.

Why it matters

Sodium intake raises blood pressure, blood pressure raises cardiovascular risk, and reducing sodium at population scale reduces events. All three of those statements have good support. What remains genuinely contested is the shape of the curve at the low end — whether very low intakes are also harmful — and the answer matters, because public-health targets are set below what most of the world eats.

Too little

True hyponatraemia is usually a water problem rather than a salt problem — too much free water rather than too little sodium — and it is one of the commonest electrolyte abnormalities in hospital. Corrected too fast it causes osmotic demyelination, which is a permanent neurological injury caused by the treatment.

Too much

Higher blood pressure, and with it stroke and heart failure. The effect is larger in people who are older, hypertensive, or salt-sensitive, and salt sensitivity is itself not routinely measurable outside a research setting.

The J-curve argument is live: several observational cohorts report higher mortality at the lowest sodium intakes. Whether that is real physiology or reverse causation and measurement error in spot-urine estimates is the crux, and the single best rebuttal is that the randomised evidence points the other way.

What it is

An alkaline earth metal, always Mg²⁺ in the body, and the fourth most abundant cation in a person. More than half of it is locked in bone and most of the rest is inside cells — which means the serum magnesium on a routine panel is sampling roughly one per cent of the total.

Why it matters

ATP is not biologically active as ATP. It is active as Mg-ATP: the magnesium ion neutralises the phosphate charges so the molecule can be handled by an enzyme at all. Since several hundred enzymes use ATP, magnesium is a silent requirement across essentially all of metabolism, plus a direct role in nerve conduction and muscle contraction.

Too little

Muscle cramps, arrhythmia, tremor, and at severity, seizures. Chronic marginal deficiency is common and largely invisible, because the test does not detect it.

Too much

Diarrhoea first — which is why the upper limit applies to supplements rather than to food, and why magnesium salts double as laxatives. Dangerous hypermagnesaemia is essentially a renal-failure phenomenon.

The upper limit here is unusual: it governs the supplement, not the diet, because the gut self-limits.

What it is

A light metal so reactive that it is never found free, and so abundant that it is in essentially all food and water at trace levels. It has no known function in any organism. Absorption from the gut is well under one per cent, and the kidney clears what gets in — which makes renal function the single variable that decides whether aluminium matters to a given person.

Why it matters

Aluminium is the case study for how a toxicology question should be asked. There is one unambiguous human neurotoxic syndrome, it was caused by medicine, and it was solved. There is a second, far more discussed association with Alzheimer's disease that has been argued for fifty years without resolution. Treating the second as though it had the evidence of the first is the error; so is dismissing it because the first was fixed.

Too little

There is no low end. Aluminium is not essential, so the band above is a toxicity scale rather than a nutrient one — which is why it has no 'too little' zone at all.

Too much

Dialysis encephalopathy: a progressive dementia with speech arrest, myoclonus and seizures in patients dialysed against aluminium-containing water, together with a painful osteomalacic bone disease and a microcytic anaemia. It was identified in 1976, traced to the water and to aluminium-containing phosphate binders, and largely abolished by treating the dialysate. That is what a solved toxicology problem looks like.

The Alzheimer's question is the live one and it should be stated honestly: aluminium is found in plaques, it is neurotoxic at high exposure, and the epidemiology linking ordinary environmental exposure to sporadic Alzheimer's disease remains inconsistent and confounded.

What it is

The second most abundant element in the earth's crust and, in water and food, present as orthosilicic acid — the only form that is meaningfully absorbed. Most dietary silicon arrives in beer, whole grains and drinking water. Horsetail and bamboo extracts, the botanical sources sold as silica supplements, contain it largely as insoluble phytoliths, which is a materially different proposition from the soluble form.

Why it matters

Silicon is not established as essential in humans, but the observational bone data are more consistent than most trace-element data: higher dietary silicon associates with higher bone mineral density in several cohorts, particularly in men and premenopausal women. The hair-skin-nails claim that sells it has almost nothing behind it by comparison.

Too little

No human deficiency syndrome. Silicon-deprived chicks and rats develop skull and long-bone abnormalities, which is the basis of the essentiality argument and has never been reproduced as a human disease.

Too much

Oral silicon is of low toxicity and is cleared renally. The serious toxicology of this element is inhalational and industrial: crystalline silica dust causes silicosis and is a lung carcinogen. That distinction matters for anyone handling diatomaceous earth, where food grade is amorphous and pool grade is calcined to the crystalline form.

A rare case where the supplement is benign and the same element in a different physical form is a well-established occupational carcinogen.

What it is

A non-metal in group 16, the same column as oxygen above it and selenium below. Its usefulness in biology is one property: the thiol group, –SH, which gives up a hydrogen easily and forms a reversible bond with another thiol. That reversibility is the point. A disulfide bridge can be made and unmade, which is how a protein holds a shape it can later be talked out of.

Why it matters

Because the body's two most important redox systems — glutathione and the thioredoxins — are both built on sulfur, and because the sulfur amino acids methionine and cysteine sit at the head of a pathway that produces homocysteine, taurine, glutathione and the methyl groups for DNA methylation. It is also the element the supplement aisle sells hardest under other names: MSM, NAC, alpha-lipoic acid, glucosamine sulfate and garlic's allicin are all sulfur chemistry.

Too little

There is no recognised dietary sulfur deficiency in people eating adequate protein, because methionine and cysteine carry it. Where sulfur amino acids genuinely run short — in severe protein restriction, and in some inborn errors — glutathione synthesis is what fails first, and that is a general loss of antioxidant capacity rather than a specific deficiency syndrome.

Too much

Elemental sulfur is poorly absorbed and essentially non-toxic by mouth. The harms in this column belong to specific sulfur COMPOUNDS, not to the element: hydrogen sulfide is acutely lethal by inhalation at concentrations that paralyse the sense of smell first, and sulfites trigger bronchospasm in a subset of asthmatics.

A 'sulfur allergy' is almost always a sulfonamide antibiotic allergy, and it has nothing to do with sulfites, sulfates, MSM or the element. This confusion causes real harm — see the tests below.

What it is

The principal cation inside the cell, where sodium is the principal cation outside it. About 98% of the body's potassium is intracellular, which is why the serum value — the one every blood test reports — is a poor guide to how much is actually there. It is also naturally radioactive: potassium-40 is the largest source of internal radiation dose in an ordinary human body.

Why it matters

Higher potassium intake lowers blood pressure and, in the salt-substitute trials, lowers stroke and death. Adequate intake is set at 3,400 mg/day for men and 2,600 for women, and the actual median intake in most Western countries is well under that. At the same time an intravenous potassium bolus stops the heart in diastole, which is precisely why it is used in lethal injection and in cardioplegia. The difference between the nutrient and the poison is entirely in the route and the rate.

Too little

Weakness, cramps, ileus, and ventricular arrhythmia — especially in anyone on digoxin or with existing heart disease. The commonest causes are diuretics and gastrointestinal losses, and a low potassium that will not correct is usually a low magnesium that has not been noticed.

Too much

Muscle weakness and then cardiac conduction failure: peaked T waves, a widening QRS, a sine wave, asystole. Almost always a kidney that cannot excrete, a drug that blocks excretion (ACE inhibitors, ARBs, spironolactone, trimethoprim), or cells spilling their contents.

Dietary potassium in people with normal kidneys is safe in a way intravenous potassium never is. Conflating the two is how 'potassium is dangerous' gets said about a banana.

What it is

Ninety-nine per cent of the body's calcium is in bone and teeth as hydroxyapatite. The remaining one per cent, in blood and inside cells, is the fraction that is tightly regulated and that actually does the signalling — muscle contraction, neurotransmitter release, coagulation. Serum calcium is defended so hard by parathyroid hormone that it tells you almost nothing about how much calcium is in the skeleton.

Why it matters

The recommendation to supplement calcium for bone is among the most widely followed in medicine, and the evidence that it reduces fractures in community-dwelling adults is weak. Meanwhile randomised data raised a signal for myocardial infarction with calcium supplements that does not appear with dietary calcium. The interesting question is not whether calcium matters — it plainly does — but whether a bolus of it in a tablet behaves like calcium in food.

Too little

Chronically low intake contributes to low bone mass, and in children with vitamin D deficiency, to rickets. Acute hypocalcaemia is a different thing entirely — perioral tingling, carpopedal spasm, a positive Chvostek sign — and is usually a parathyroid or magnesium problem, not a dietary one.

Too much

Hypercalcaemia: stones, bones, abdominal groans and psychiatric overtones. The milk-alkali syndrome, once historical, came back with calcium carbonate supplements. Supplemental calcium also raises kidney stone risk in the trials, while dietary calcium lowers it — one of the clearest dietary-versus-supplement dissociations in nutrition.

Where the calcium goes matters more than how much there is. The concern with supplements is an acute post-dose rise in serum calcium that food does not produce.

What it is

A transition metal named after Vanadís, a name for the Norse goddess Freyja, because its compounds are so strikingly coloured. Biologically it is best known as a phosphate mimic: vanadate has a shape and charge close enough to phosphate that enzymes handling phosphate will accept it, and then fail to let go.

Why it matters

That mimicry is the whole pharmacology. Vanadate inhibits protein tyrosine phosphatases — the enzymes that switch insulin signalling off by stripping phosphate groups from the receptor. Block the off-switch and insulin signalling persists longer, which in cells and in rodents looks convincingly like improved insulin sensitivity.

Too little

No human deficiency has ever been demonstrated. Vanadium is not an established essential nutrient for people, and the entries that list it as one are extrapolating from other species.

Too much

Gastrointestinal intolerance is dose-limiting and begins well below the doses that showed effects in animals. The upper limit is set at roughly 1.8 mg/day; supplement trials have used fifty times that.

The gap between the dose that works in a rodent and the dose a person can tolerate is the reason this never became a drug.

What it is

A transition metal whose biology depends entirely on which form you mean. Trivalent chromium, Cr(III), is the dietary form and is poorly absorbed. Hexavalent chromium, Cr(VI), is an industrial pollutant and a confirmed carcinogen. They share a name and almost nothing else, and conflating them is the single most common error in writing about this element.

Why it matters

Chromium has been proposed since the 1950s as a cofactor for insulin signalling, acting through an oligopeptide called chromodulin that is said to amplify insulin receptor activity. The mechanism is plausible and much cited. Whether it translates into a clinical effect at the doses people actually take is a separate question, and the answer from randomised trials is largely no.

Too little

Genuine dietary deficiency is essentially confined to long-term intravenous feeding without added chromium, where glucose intolerance and neuropathy have been reported and reversed on repletion.

Too much

No upper limit has been set for the trivalent dietary form, which is itself a statement about how little is absorbed. Hexavalent chromium is a different matter entirely and is an occupational and environmental exposure, not a supplement.

An element where the interesting question is not the dose but whether the effect exists.

What it is

A transition metal with a half-filled 3d shell, which is why it holds so many oxidation states and why biology uses it in redox enzymes. It is required by manganese superoxide dismutase — the mitochondrial one — by arginase, and by pyruvate carboxylase. Dietary intake comes mainly from whole grains, nuts and tea, and deficiency in free-living people is close to unheard of.

Why it matters

Manganese is the clearest example of an element whose toxicity runs through a route food does not use. Inhaled manganese bypasses the liver's first-pass control and travels up the olfactory nerve directly into the brain, where it accumulates in the basal ganglia and causes a parkinsonism that does not respond to levodopa. Welders get it; people eating oats do not.

Too little

Experimental deficiency produces dermatitis, hypocholesterolaemia and impaired clotting. It has essentially never been described in a person eating ordinary food.

Too much

Manganism: a parkinsonian syndrome with early psychiatric change, a characteristic 'cock-walk' gait, dystonia, and T1 hyperintensity in the globus pallidus on MRI. Crucially it is poorly responsive to levodopa, which is how it is told apart from Parkinson's disease. The other route to it is liver failure or long-term parenteral nutrition, where biliary excretion — the body's only route out — fails.

This element's dose-response has two different curves depending on whether the exposure is swallowed or breathed, and only one of them is in the band above.

What it is

A transition metal cycling between Fe²⁺ and Fe³⁺, and the most abundant element on Earth by mass. In biology that redox flexibility is both the point and the danger: iron carries oxygen precisely because it can change state, and free iron generates hydroxyl radicals for exactly the same reason.

Why it matters

Most of the body's iron is in haemoglobin, carrying oxygen. The rest runs cytochromes, the enzymes of the electron transport chain, and the ribonucleotide reductase that lets a cell copy its DNA. Because there is no regulated route for getting rid of iron, the entire control system sits at absorption — and its master switch is a hormone.

Too little

The commonest nutritional deficiency in the world. Fatigue and breathlessness are late signs; the stores empty long before the haemoglobin falls, which is why ferritin is the test that matters.

Too much

Haemochromatosis — inherited over-absorption — deposits iron in liver, pancreas, heart and pituitary, and is treated by the deliberately old-fashioned expedient of removing blood. Acute overdose in children is a leading cause of poisoning death.

An element with no excretion pathway punishes both ends of the curve, and it punishes the high end slowly enough to be missed for decades.

What it is

A transition metal whose entire established human requirement is as the atom at the centre of vitamin B12. There is no free cobalt requirement: a person cannot use inorganic cobalt to make B12, because only bacteria and archaea can build the corrin ring around it. That single fact explains why B12 is a strictly animal-source or bacterially fermented nutrient.

Why it matters

Cobalt is the element that makes the B12 story make sense — cyano-, hydroxo-, methyl- and adenosylcobalamin differ only in what is attached to the cobalt. It is also a case study in metal toxicity that medicine created twice: once by using cobalt salts to stabilise beer foam, and once by implanting metal-on-metal hip prostheses that shed cobalt into blood.

Too little

There is no cobalt deficiency, only B12 deficiency. In ruminants, whose gut bacteria make their B12 for them, cobalt-deficient pasture does cause a wasting disease — which is a nice demonstration of where the requirement really sits.

Too much

Cobalt cardiomyopathy, first described in beer drinkers in Quebec in 1965 when cobalt sulphate was added as a foam stabiliser; and the modern version, from metal-on-metal hip wear, with cardiomyopathy, hypothyroidism, optic and auditory neuropathy and polycythaemia. Cobalt also stabilises HIF, which is why it raises erythropoietin and why it has been used as a doping agent.

The clinical syndrome from an implant is called arthroprosthetic cobaltism, and the reason it is missed is that nobody connects a failing heart to a hip replaced years earlier.

What it is

A transition metal that cycles readily between Cu⁺ and Cu²⁺, which makes it a superb biological catalyst and a dangerous free ion. The body never lets it circulate unescorted: almost all plasma copper is bound to ceruloplasmin, and the chaperone proteins that hand it between enzymes exist specifically so that no copper atom is ever loose in a cell.

Why it matters

Copper runs cytochrome c oxidase — the final complex of the mitochondrial electron transport chain, where oxygen is actually reduced to water. It also runs lysyl oxidase, which cross-links collagen and elastin, and dopamine β-hydroxylase, which makes noradrenaline from dopamine. Deficiency therefore hits energy, connective tissue and catecholamines at once.

Too little

Anaemia unresponsive to iron, neutropenia, and a myeloneuropathy — numbness, gait disturbance, loss of position sense — that is clinically almost indistinguishable from B12 deficiency and is frequently misdiagnosed as it.

Too much

Rare from diet. Wilson's disease is the inherited failure to excrete copper into bile, and it presents as liver disease, or as a movement disorder, or as psychiatric illness, depending on where the copper accumulates first.

Menkes disease is the mirror image — a failure to absorb copper at all — and shows what the enzymes above are worth by removing them.

What it is

A transition metal that, unusually for its class, does not change oxidation state under physiological conditions — it is always Zn²⁺. That stability is exactly why biology uses it so heavily: an ion that will not accidentally run a redox reaction can be used as scaffolding.

Why it matters

Zinc is a structural component of roughly three thousand human proteins, most famously the zinc finger — a fold that grips DNA and that exists only because a zinc ion holds it in shape. It is also catalytic in carbonic anhydrase and in the matrix metalloproteinases that remodel tissue, and it is required for the maturation of T cells, which is why deficiency shows up first as infections that will not clear.

Too little

Impaired wound healing, loss of taste and smell, recurrent infection, and in children, stunting. Acrodermatitis enteropathica is the inherited form and is fatal untreated.

Too much

Copper deficiency — and this is the important one, because it is caused by the supplement rather than by the diet. Sustained high-dose zinc induces intestinal metallothionein, which traps copper and prevents its absorption. The result can be anaemia, neutropenia and an irreversible myelopathy.

Zinc's toxicity is almost entirely a copper story. Judging zinc in isolation is how the harm gets missed.

What it is

A metalloid whose toxicity depends entirely on its chemical form. Inorganic arsenic — arsenite and arsenate — is the carcinogen. Organic arsenic, mostly arsenobetaine, is what seafood contains; it passes through essentially unchanged and is not toxic. A total arsenic measurement that does not speciate cannot tell those two apart, which is the single most consequential laboratory point about this element.

Why it matters

Arsenic is the show's argument about dose and form in one element. It is a confirmed human carcinogen for skin, lung and bladder at drinking-water concentrations found across Bangladesh, West Bengal and parts of the American southwest; it is in rice, because rice takes it up from flooded paddy far better than other grains; and arsenic trioxide, with all-trans retinoic acid, turns acute promyelocytic leukaemia from the most lethal acute leukaemia into one curable without chemotherapy in most patients.

Too little

Arsenic has no established human requirement. Some animal work suggests an ultratrace role; it has never been shown to matter in people, and no deficiency state exists.

Too much

Chronic: hyperkeratosis and hyperpigmentation of the palms and soles, peripheral neuropathy, peripheral vascular disease including blackfoot disease, and cancers of skin, bladder and lung with latencies measured in decades. Acute: vomiting, profound hypotension, a garlic odour, and a late Mees' line across the nail.

The largest mass poisoning in history was a public-health intervention: tube wells were drilled across Bangladesh in the 1970s to escape cholera in surface water, and the groundwater turned out to be arsenical.

What it is

A metalloid sitting directly under sulphur in the periodic table, which is the whole reason it matters: it is chemically similar enough to sulphur that the body's machinery keeps mistaking one for the other. Named after Selene, the moon, by Berzelius in 1817 — and named badly, because he thought he had found a relative of tellurium, which he had named after the earth.

Why it matters

Selenium is built into twenty-five human proteins as a twenty-first amino acid, selenocysteine, inserted by a piece of cellular machinery that exists for this purpose alone. Those selenoproteins run two systems you cannot live without: the glutathione peroxidases, which dismantle peroxides before they damage cells, and the deiodinases, which convert the thyroid's inactive T4 into the active T3 that actually reaches your tissues.

Too little

Keshan disease — a fatal cardiomyopathy in selenium-poor regions of China, which is also where the coxsackievirus work showed a nutritional deficiency can reach past the host and mutate the pathogen itself.

Too much

Hair loss, brittle nails, garlic breath, gastrointestinal upset, and in acute overdose, neurological damage. A mislabelled supplement in 2008 poisoned 201 people across ten US states.

The dose-response is a U, not a slope, and it is biochemistry rather than a statistical artefact.

What it is

A transition metal that, like chromium, breaks the expected electron filling: one 5s electron rather than two, because a half-filled 4d shell is more stable. In biology it is never used bare — it is inserted into a pterin scaffold to make the molybdenum cofactor, and that cofactor is what four human enzymes require.

Why it matters

Molybdenum is a rare element in this atlas in that its deficiency is genetic rather than dietary. Molybdenum cofactor deficiency presents in the first days of life with intractable seizures and a progressive encephalopathy, and it is fatal in infancy for most of those affected — and one form of it, type A, now has a drug. Dietary deficiency, by contrast, has been described in a handful of patients on long-term parenteral nutrition and essentially nowhere else.

Too little

Acquired deficiency is essentially confined to long-term parenteral nutrition without added molybdenum, where it produces a syndrome of tachycardia, headache, night blindness and coma with raised methionine and low uric acid. The genetic cofactor deficiency is a different and far more severe disease.

Too much

Industrial exposure has been associated with a gout-like syndrome and raised uric acid, and high intakes interfere with copper. Oral molybdenum in supplement doses is of low toxicity, which is partly why the upper limit is set so far above ordinary intake.

Tetrathiomolybdate turns molybdenum's copper antagonism into a drug: it is used deliberately to strip copper in Wilson disease.

What it is

Element 43 was the gap in Mendeleev's table that would not fill, because every isotope of technetium is radioactive and none has a half-life long enough to have survived since the earth formed. It was produced in a cyclotron in 1937 and named from the Greek for 'artificial'. There is no stable technetium anywhere, and there are trace amounts on earth only as a fission product.

Why it matters

Technetium-99m is the workhorse of nuclear medicine: bone scans, cardiac perfusion, sestamibi parathyroid scans, renal scans, sentinel node mapping. It is close to an ideal imaging isotope — a six-hour half-life, a 140 keV gamma that escapes the body and images cleanly, and no particulate emission to dose the patient. It also has a supply problem that a handful of ageing reactors are responsible for.

Too little

There is no deficiency and no biological requirement: this is an element with a purely medical existence. The band above is a radiation dose scale, not a nutrient one.

Too much

The risk from a single diagnostic study is small and stochastic — a modest theoretical increase in lifetime cancer risk, with no deterministic threshold effect at these doses. The harm worth naming is cumulative and comes from repeated imaging, particularly in young patients, and from studies ordered without a question they can answer.

This is the only entry in the atlas whose dose–response is drawn in millisieverts. It is here because the same discipline applies: name the dose, name the benefit, and do not pretend either is zero.

What it is

A soft metal in zinc's group and its chemical shadow: cadmium follows zinc into zinc transporters, into metallothionein and into enzymes where it does not belong. It has no biological role. What makes it different from most toxic metals is the kinetics — once absorbed it is retained in the kidney cortex and liver with a half-life of ten to thirty years, so exposure integrates over a lifetime.

Why it matters

Cadmium is the argument for thinking about total lifetime dose rather than current level. It caused itai-itai disease in Japan — a painful osteomalacia with renal failure in people eating rice grown on cadmium-contaminated water — and at the much lower exposures found in ordinary populations it still associates with reduced bone density, renal tubular damage and mortality. Smokers carry roughly twice the body burden of non-smokers, because the tobacco plant concentrates it from soil.

Too little

No requirement, no deficiency. Every part of the curve above zero is exposure.

Too much

Proximal tubular damage with low-molecular-weight proteinuria, then hypercalciuria and osteomalacia. Itai-itai — 'it hurts, it hurts' — described the fractures. Cadmium is also an IARC Group 1 lung carcinogen by inhalation.

The bone disease is largely secondary to the kidney disease: damaged tubules waste calcium and cannot make active vitamin D. A cadmium story told as a bone story alone gets the causal order wrong.

What it is

A soft, ancient, unreactive metal that gave the Bronze Age its name and has essentially no toxicity in its metallic or simple inorganic forms. It also has ten stable isotopes, more than any other element — a consequence of fifty protons being a nuclear magic number, which makes the nucleus unusually stable in an unusual number of configurations. The element's entire biological interest lies in what happens when carbon is attached to it: organotins are a different proposition from tin in every way that matters.

Why it matters

Tin is in this atlas for the gap between a substance and its compounds, which is the same lesson mercury teaches and nobody applies. Nothing about tin metal predicts that tributyltin would turn out to be a nanomolar agonist at two nuclear receptors controlling fat cell formation. It is also the quiet example of a heavy metal doing genuine good in an ordinary product: stannous fluoride is one of the better-evidenced ingredients in a supermarket, and it is the same element.

Too little

No established requirement. Tin has been proposed as an ultratrace element on the basis of old rat studies showing impaired growth on depleted diets, and the finding has never been replicated convincingly or shown in humans. Treat any supplement selling it as a nutrient as selling a hypothesis.

Too much

Inorganic tin from unlacquered cans — acidic foods such as tomatoes and pineapple leach it — causes nausea, cramps and diarrhoea at high concentrations, and stops there. It is poorly absorbed and cleared. Organotins are the opposite: lipophilic, persistent, immunotoxic and neurotoxic, and trimethyltin in particular is a selective hippocampal poison used deliberately in animal models.

The dose–response for tin the metal is unremarkable. The dose–response for tributyltin is the interesting one, because the receptor effects appear at nanomolar concentrations — which is to say at levels a conventional toxicology screen designed around organ damage would call clean.

What it is

A halogen, heavy and violet-black as a solid, which sublimes straight to a purple vapour. Biologically it is unusual in that the body has essentially one use for it, and builds a dedicated organ around that use: the thyroid gland concentrates iodine against a steep gradient using a sodium-iodide symporter, holding most of the body's supply in one place.

Why it matters

Thyroid hormone is iodine. Not iodine-dependent — iodine, structurally: T4 is a molecule carrying four iodine atoms, T3 carries three, and the number in the name is the count. Remove the iodine and there is no hormone to make. Because thyroid hormone of maternal origin drives fetal brain development in the first trimester, before the fetal thyroid works at all, maternal deficiency at that window is not recoverable later.

Too little

Goitre, hypothyroidism, and — at severity, in pregnancy — cretinism. Randomised trials in New Guinea and Congo in the 1970s showed that correcting severe deficiency early in pregnancy prevents it.

Too much

Paradoxically, hypothyroidism. Large iodine loads shut down thyroid hormone synthesis (the Wolff-Chaikoff effect); most thyroids escape this within days, but some do not, and the ones that do not are often already damaged.

Both ends present as an underactive thyroid, which is why the deficiency and the excess are so easily confused.

What it is

A lanthanide with a half-filled 4f shell: seven electrons in seven orbitals, all spins parallel, giving it the largest number of unpaired electrons of any stable element. That is what makes it magnetic enough to be useful. The free Gd³⁺ ion is also almost exactly the ionic radius of Ca²⁺, which is why it blocks calcium channels and why it must never be administered unchelated.

Why it matters

Gadolinium is the clearest example in imaging of a benefit-and-harm ledger that changed twice. It was considered essentially inert for twenty years; then nephrogenic systemic fibrosis appeared in patients with renal failure and killed some of them; then, after the dosing rules for renal impairment fixed that, MRI studies showed gadolinium depositing in the brains of people with entirely normal kidneys. The second finding has no demonstrated clinical consequence, and it is still being argued.

Too little

No requirement and no deficiency. This is a diagnostic exposure scale.

Too much

Nephrogenic systemic fibrosis — progressive fibrosis of skin, joints and internal organs, with no reliable treatment — in patients with severe renal impairment given the less stable linear agents. Since those agents were restricted and eGFR screening became routine, new cases have become very rare. Separately, dose-dependent signal change in the dentate nucleus and globus pallidus is seen in patients with normal kidneys after repeated studies.

The retention finding is real and the harm from it is unproven. Both halves of that sentence have to be said, because dropping either one produces a different and dishonest story.

What it is

A dense, unreactive transition metal, so inert as bulk metal that it is used for electrodes and jewellery and implanted hardware without consequence. Its medicinal chemistry lives entirely in the square planar Pt(II) complexes, and its electron configuration is one of the periodic table's exceptions: 5d⁹ 6s¹ rather than the expected 5d⁸ 6s².

Why it matters

Cisplatin was found by accident. A 1965 experiment on whether electric fields affect bacterial growth produced platinum complexes from the supposedly inert electrode, and those complexes stopped E. coli dividing. Following that observation to its end turned metastatic testicular cancer from a disease with roughly a 10% survival into one cured in the large majority of cases. It is the strongest argument in medicine for funding people who are looking at something else.

Too little

No requirement, no deficiency. The band is a therapeutic dose scale, and the 'too little' end of it is a treatment that does not work.

Too much

Cisplatin's toxicities are the reason the other platinums exist: nephrotoxicity, profound emesis, peripheral neuropathy, and irreversible high-frequency hearing loss. Ototoxicity in children treated for solid tumours is permanent and under-recognised. Carboplatin trades nephrotoxicity for myelosuppression; oxaliplatin trades both for a cold-triggered neuropathy.

This is an atlas entry where the dose–response curve is a genuine therapeutic window rather than a nutritional one, and where the harms are accepted because the alternative is death from the cancer.

What it is

The only metal liquid at room temperature, and the element where speciation decides everything. Elemental mercury vapour is absorbed through the lung and crosses into the brain. Inorganic mercury salts are corrosive and nephrotoxic and barely reach the brain. Methylmercury, the organic form that accumulates in fish, is almost completely absorbed from the gut, crosses the placenta, and targets the developing nervous system. Ethylmercury, from thiomersal, is cleared several times faster than methylmercury and is a fourth thing again.

Why it matters

Mercury is the atlas's case for insisting on the chemical form before the conversation starts. The dental amalgam argument is about elemental vapour; the fish advisory is about methylmercury; the vaccine argument was about ethylmercury; and skin-lightening cream poisoning is about inorganic salts. They have different routes, different targets, different half-lives and different tests, and treating them as one substance is how both alarmism and dismissal get made.

Too little

No requirement. Selenium is the relevant counterpart here: selenium and mercury bind each other with extraordinary affinity, which is the basis of the argument that a fish's selenium content partly offsets its mercury content.

Too much

Methylmercury: paraesthesia, constricted visual fields, ataxia, and — the reason the fish advisories exist — impaired neurodevelopment after prenatal exposure. Elemental vapour: tremor, erethism (the Mad Hatter's irritability and social withdrawal), gingivitis. Inorganic salts: corrosive gastritis and acute tubular necrosis.

The two great prospective cohorts of prenatal methylmercury — the Faroe Islands and the Seychelles — reached different conclusions, and that disagreement is itself the story. The Faroese ate whale, with PCBs alongside the mercury; the Seychellois ate ocean fish, with selenium and long-chain omega-3s alongside it.

What it is

A heavy, soft, corrosion-resistant metal that human beings have been smelting for seven thousand years, and whose usefulness is exactly why it is everywhere. It has no biological role. Roughly 90–95% of the adult body burden sits in bone with a half-life of decades, which means lead is not an exposure that ends — it is an exposure that is stored and slowly released, notably in pregnancy and after the menopause.

Why it matters

Lead is the strongest illustration in toxicology that a threshold can fail to exist. The 'acceptable' blood level for children has been revised downward six times since 1960 — 60, then 40, then 30, 25, 10, 5 and now 3.5 µg/dL — and each revision happened because better studies found effects below the previous line. The dose–response for cognitive harm is also supralinear: the steepest part of the curve is at the lowest exposures.

Too little

There is no low end and no deficiency. Every named level is a line drawn by a committee, and every one of those lines has since been moved down.

Too much

In children: reduced IQ, attention and behaviour problems, at levels once considered normal. In adults: hypertension, chronic kidney disease, and cardiovascular death. At very high levels, encephalopathy, wrist drop, colic and a microcytic anaemia with basophilic stippling.

The most consequential recent finding is not about children. In a cohort of over 14,000 US adults, blood lead concentrations well inside the 'normal' range predicted cardiovascular mortality — implying a very large number of attributable deaths a year from an exposure treated as historical.