Iodine (I)

Iodine is the heaviest element with a confirmed, non-negotiable role in human biology, and its role is unusually literal. Thyroid hormone is a small molecule built from two tyrosine units with either three or four iodine atoms bolted onto the aromatic rings — that is what T3 and T4 mean. The iodine is not a catalyst or a cofactor; it is structural, and no lighter halogen substitutes. The thyroid gland exists in large part to capture iodide from the bloodstream and concentrate it by a factor of tens against its gradient, because the element is scarce in most terrestrial environments and the hormone cannot be made without it.

Seaweed ash, gunpowder and too much acid

Napoleon's armies needed saltpetre, France was running short of the traditional sources, and producers turned to burning kelp — the ash contains the sodium and potassium salts the process needed. Bernard Courtois ran such an operation in Paris, and by 1811 he had noticed that the copper vessels in his works were being corroded by something in the liquors that had no business being there.

Adding sulfuric acid to the residues one day, he used too much, and a dense violet vapour rose from the vessel and condensed on the cold surfaces above as lustrous dark crystals. Courtois recognised he had something new and had neither the money nor the laboratory to chase it. He gave samples to Charles Desormes and Nicolas Clément, who made the discovery public in late 1813.

Who named it, and who got nothing

What followed was a scramble between two of the most eminent chemists in Europe. Joseph Louis Gay-Lussac addressed the Institut de France on 6 December 1813, argued that the substance was a new element analogous to chlorine, and named it iode from Greek ioeides, violet-coloured, after the vapour.

Humphry Davy was in France at the time, travelling under a passport granted by Napoleon despite the war, and had examined a sample. He reached the same conclusion and sent a letter to the Royal Society dated 11 December, claiming he had communicated his findings to the Institut's secretary more than a week earlier. A sharp priority dispute followed between Davy and the French establishment over who first recognised iodine as an element.

Both sides agreed, and said explicitly, that the discoverer was Courtois. Neither the credit nor the naming brought him anything. He lost his saltpetre business as the wars ended, tried and failed to make a living selling iodine, and died in poverty in 1838.

Salt, and the intervention nobody notices

Iodine deficiency is not a rare disease. Iodine leaches out of soil with rainfall and glacial erosion, so inland and mountainous regions — the Alps, the Himalayas, the Andes, the Great Lakes basin — have historically had almost none in their food and water. The visible result is goitre, an enlarged thyroid straining to capture what iodide there is. The invisible and far more important result is that severe deficiency in pregnancy impairs fetal brain development irreversibly.

Iodine deficiency remains the leading preventable cause of intellectual disability worldwide, and the remedy is one of the cheapest interventions in public health: add a few parts per million of potassium iodate or iodide to table salt. Switzerland began in 1922, and Michigan — the centre of the American "goitre belt" — followed in 1924, with iodised salt on grocery shelves nationally within a year. Goitre rates in the affected regions fell by most of their value within a decade.

The global programme is more recent and larger. In 1990 something like one household in five worldwide used iodised salt; today the figure is around nine in ten, and the number of countries classed as iodine deficient has fallen from over a hundred to a couple of dozen. It is arguably the most cost-effective public health measure ever undertaken, and almost nobody who benefits from it is aware of it.

Thirty-three kilo-electronvolts

The largest industrial use of iodine is medical imaging, and the reason is a sharp feature in its X-ray absorption.

An element absorbs X-rays much more strongly just above the energy needed to eject an electron from its innermost shell. For iodine, that K-edge sits at about 33 keV — right in the middle of the energy range used in diagnostic radiography and computed tomography. Blood vessels and soft tissue are nearly transparent at those energies and nearly identical to one another; fill them with an iodine-containing compound and they stand out sharply.

Iodinated contrast agents are therefore designed as molecules that carry three or six iodine atoms, dissolve at very high concentration, and are excreted unchanged by the kidney without ever releasing free iodide. Millions of doses are administered every year, and the manufacture of these compounds consumes more iodine than any other single application.

Iodine's other markets are more varied: povidone-iodine antiseptics, animal feed supplements, the polarising film in liquid crystal displays — which uses stretched polyvinyl alcohol stained with iodine chains — and silver iodide, used both in photographic emulsions and, on the strength of its crystal structure resembling ice, as a cloud-seeding agent.

From kelp to Chilean caliche

Nobody extracts iodine from seaweed at commercial scale any more. Two very different sources supply the world.

Chile's Atacama Desert holds caliche, the nitrate-bearing crust mined since the nineteenth century, in which iodine occurs as iodate. It comes out as a byproduct of nitrate production, and Chile is the largest producer by a wide margin. Japan's supply comes from deep brines beneath the Kantō plain in Chiba Prefecture — ancient seawater trapped in sediments, pumped up alongside natural gas and stripped of its iodide. Between them the two countries account for the great majority of world output, from geological accidents about as different as two accidents can be.

Iodine-131, and eight days that mattered

Only iodine-127 occurs naturally. The radioactive isotopes matter enormously anyway, because the thyroid cannot tell them apart from the stable one and concentrates whatever arrives.

Iodine-131, a fission product with a half-life of eight days, is the reason for both the greatest health consequence of the Chernobyl accident and one of the most successful cancer treatments in medicine. Released in quantity in 1986 and taken up through contaminated milk, it produced a documented epidemic of thyroid cancer in children across Belarus, Ukraine and western Russia — the one health effect of that accident that epidemiology established beyond dispute. The countermeasure, distributing stable iodide so the thyroid is already saturated, works precisely because the gland has no way to discriminate.

The same avidity is therapeutic. A dose of iodine-131 given to a patient with thyroid cancer or an overactive thyroid is delivered by the gland to itself, and the short-range beta radiation destroys the tissue that absorbed it while sparing everything else. It is among the oldest targeted cancer therapies in use and remains one of the most effective.

Iodine-129 is the opposite case. Fission makes it too, but it endures for nearly 16 million years per half-life, which on any human timescale means permanently; it travels freely in groundwater, and it consequently appears in every long-term repository assessment. Its presence in the environment is also used as a tracer of where reprocessing discharges have travelled in ocean currents.