Iodine originates primarily in the ocean, where seawater contains roughly 60 parts per billion of dissolved iodide. From there, it cycles into the atmosphere, deposits onto land, concentrates in certain geological formations, and enters the food supply through seaweed, dairy, fish, and iodized salt. Understanding this chain explains why some regions have plenty of iodine and others are chronically deficient.
The Ocean Is the Primary Source
About 75% of atmospheric iodine comes from a single chemical reaction: when ozone in the lower atmosphere contacts iodide ions dissolved in surface seawater, it triggers the release of inorganic iodine gases. The remaining 25% comes from organic iodine compounds produced by marine algae and phytoplankton. Once airborne, iodine travels with weather systems and eventually deposits onto soil and vegetation on land. This ocean-to-atmosphere-to-land pipeline is the fundamental reason coastal regions have more iodine in their soil, water, and local food than inland areas do.
The Earth’s continental crust contains only about 119 parts per billion of iodine, while the oceanic crust (including seawater) holds roughly 777 parts per billion. That sixfold difference matters. Deep-sea organisms concentrate iodine into sediments over millions of years: deep-sea carbonates contain around 30 parts per million, while continental limestones hold just 2.5 parts per million. These ancient marine sediments are the geological reservoirs that eventually become the iodine deposits mined today.
Where Iodine Is Mined and Extracted
Two countries dominate global iodine production: Chile and Japan. They tap into completely different geological sources.
In Chile’s Atacama Desert, iodine occurs as iodate minerals locked inside caliche, a crusty mineral deposit found near the surface. Mining operations use heap leaching or vat leaching to dissolve the iodine-bearing minerals, then chemically reduce the iodate in solution to recover pure iodine. Chile’s caliche deposits are unusual because they formed in one of the driest places on Earth, where ancient marine sediments were never washed away by rain.
Japan takes a different approach entirely. Beneath parts of the Chiba Prefecture east of Tokyo, ancient seawater became trapped in porous rock layers millions of years ago. These underground brines are pumped to the surface, and the dissolved iodide ions are oxidized into molecular iodine for collection. Japan’s brine deposits are so rich that the country produces roughly a third of the world’s supply from this single method. Newer extraction techniques using specialized ionic liquids can concentrate trace iodide from brine samples by 40 to 100 times, improving efficiency.
Why Some Regions Have Almost None
The farther you go from the coast and the higher the elevation, the less iodine reaches the soil. Mountain ranges and landlocked plains are the classic iodine-deficient zones. Research in Morocco’s Atlas Mountains illustrates this clearly: coastal soils near Agadir average 2.76 micrograms of iodine per gram, while soils in the Ounein Valley just 150 km inland hold only 1.47 micrograms per gram. The “available iodine,” the portion plants can actually absorb, is three times higher at the coast.
This pattern repeats worldwide. The Himalayas, the Andes, the Alps, and large swaths of Central Africa and Central Asia are historically iodine-poor. People living in these areas who depend entirely on locally grown food often develop iodine deficiency disorders, including goiter and developmental problems in children. This geographic vulnerability is exactly why salt iodization programs were introduced in the 1920s and remain one of the most cost-effective public health interventions ever deployed.
How Iodine Gets Into Food
Seaweed is by far the most concentrated natural food source of iodine, but the range across species is enormous. Kombu (a type of kelp) averages 2,524 milligrams per kilogram of dry weight, with some samples reaching nearly 5,000 mg/kg. Wakame averages about 140 mg/kg, and nori sits around 37 mg/kg. A single small piece of kombu used to flavor soup stock can contain several times the daily recommended intake of 150 micrograms for adults.
Dairy products are the leading iodine source in many Western diets, but this isn’t because cows naturally produce iodine-rich milk. It’s largely an artifact of farming practices. Cattle feed is often supplemented with iodine compounds, sometimes at levels above recommendations, particularly to prevent foot rot. Iodine-based teat dips and udder washes used during milking add even more. Some dairy herds produce milk exceeding 1,000 micrograms of iodine per liter as a result. This means your milk’s iodine content varies significantly depending on the farm’s practices and the season.
Fish and shellfish pick up iodine directly from seawater, with cod, shrimp, and tuna being reliable sources. Eggs contain moderate amounts because poultry feed is also supplemented. Beyond these, iodized table salt remains the most consistent source globally, typically providing 45 micrograms per quarter teaspoon.
How Your Body Uses It
When you eat iodine-containing food, your digestive system converts it to iodide ions that enter the bloodstream. Your thyroid gland, a butterfly-shaped organ at the base of your neck, actively pulls iodide out of the blood using a specialized transport protein embedded in its cell membranes. This transporter is so efficient that it concentrates iodide inside the thyroid to levels 40 times higher than what’s circulating in your blood.
Once inside thyroid cells, iodide is incorporated into thyroid hormones, which regulate metabolism, brain development, body temperature, and heart rate. The thyroid is essentially the sole reason your body needs iodine at all. Adults require about 150 micrograms daily, pregnant women about 220 micrograms, because fetal brain development depends heavily on adequate thyroid hormone levels.
The Discovery That Started It All
Iodine was discovered by accident in 1811. Bernard Courtois, a French chemist manufacturing saltpeter for Napoleon’s armies, noticed unusual violet fumes rising when he added too much sulfuric acid to seaweed ash (called “varec” in French). The purple vapor condensed into dark, shiny crystals of a substance no one had seen before. Other chemists, including Joseph Gay-Lussac, confirmed it was a new element and gave it the name “iodine” from the Greek word for violet. The seaweed ash Courtois was working with had concentrated iodine from seawater over the course of the algae’s lifetime, the same biological concentration process that makes seaweed such a potent dietary source today.