Salt is iodized by adding a tiny, controlled amount of an iodine compound, either sprayed on as a liquid solution or blended in as a dry powder, during the final stages of salt production. The amount is small enough that you can’t taste or see it: iodized salt in the U.S. contains about 45 micrograms of iodine per gram of salt. Globally, about 89 percent of people use salt with at least some added iodine, a public health measure that prevents thyroid problems and developmental issues caused by iodine deficiency.
The Two Iodine Compounds Used
Salt producers use one of two chemicals to deliver iodine: potassium iodide or potassium iodate. Before the 1990s, potassium iodide was the standard in many countries. The problem is that iodide is chemically unstable. When exposed to humid air, the iodine can convert to a gas and escape from the salt entirely. In 1991, after recommendations from the WHO and the International Council for the Control of Iodine Deficiency Disorders, many countries switched to potassium iodate, which holds up much better in heat and humidity.
The U.S. still uses potassium iodide for table salt because the climate, packaging, and distribution chain are controlled enough to keep it stable. Countries with tropical climates, where heat and humidity are constant threats, almost universally use potassium iodate instead.
Wet Spray Method
The most common large-scale approach is wet spray iodization. Workers dissolve the iodine compound in water to create a concentrated solution, which is stored in a stainless steel reservoir (typically around 80 liters). As salt moves along an inclined rubber belt conveyor, the solution is misted onto it through specialized stainless steel nozzles powered by an air compressor. The salt then passes through a screw conveyor that tumbles and mixes the grains so the iodine distributes evenly.
This method works best with fine, free-flowing salt. One drawback: in tropical countries, adding water raises the salt’s moisture content by 2 to 3 percent, which ironically speeds up iodine loss during later storage. Some facilities add a drying step afterward, using a fluidized bed dryer to pull that extra moisture back out, but this adds cost and complexity to the production line.
Dry Mixing Method
Dry mixing skips the water entirely. Instead, the iodine compound is first blended with a carrier powder, usually food-grade calcium carbonate, to make it easier to distribute evenly through a large batch of salt. A typical ratio is one part potassium iodate to ten parts calcium carbonate. This premix is then fed into a ribbon blender or worm-screw mixer along with crushed salt, where mechanical tumbling distributes the iodine throughout the batch.
In one well-documented Indian production method, workers load 25 kilograms of crystal salt into a roller crusher, add a measured dose of the iodate-carbonate premix, then repeat the cycle continuously. After crushing, the salt and premix travel along a conveyor belt and drop into a worm-screw mixer for a final blending pass. The target is typically around 30 parts per million of iodine in the finished product.
Dry mixing has a significant advantage in hot, humid regions: because no water is added, the salt stays drier and retains its iodine longer on the shelf. The equipment is also simpler, with no pumps, spray nozzles, or air compressors required.
Drip-Feed Method
A third, simpler technique is the drip-feed method, used in smaller-scale operations. A concentrated iodine solution drips slowly through a hollow needle onto salt as it moves through a screw conveyor or roller grinder. A regulating device controls the drip rate to match the flow of salt. This approach requires less equipment than spray iodization but offers less precision in how evenly the iodine is distributed.
Stabilizers and Anticaking Agents
Iodine is volatile. It wants to escape, especially when exposed to heat, light, moisture, or acidic conditions. To slow this down, producers add small amounts of stabilizing compounds. Light basic magnesium carbonate, typically at about 0.3 percent, serves double duty as both a stabilizer and an anticaking agent, keeping the salt free-flowing while protecting the iodine from breaking down. Calcium carbonate, used as the carrier in dry mixing, also provides some stabilizing effect. In the U.S., dextrose (a simple sugar) is commonly added to iodized table salt for the same reason: it helps prevent the potassium iodide from oxidizing.
How Much Iodine Survives Storage
Even with stabilizers, iodine loss during storage is a real challenge, and humidity is the biggest factor. Salt stored at moderate humidity (around 60 percent) loses between 0 and 20 percent of its iodine over six months. But in saturated humidity, the losses are dramatic: samples stored in open containers lost half their iodine within one month, and after a year, only about 1 percent of the original iodine remained.
Packaging makes a major difference. Salt in solid polyethylene bags retains most of its iodine for the first six months, with losses in the 10 to 15 percent range. After that, losses climb more steeply. This is why iodized salt is almost always sold in sealed plastic or lined packaging rather than paper or cloth bags, especially in developing countries where storage conditions are less controlled.
Iodine Loss During Cooking
Heat also drives iodine out of salt. When heated to 120°C (about 250°F), salt can lose up to 57 percent of its iodine in just three hours. This is one reason public health guidelines set iodine levels at production higher than what people strictly need: the assumption is that a meaningful portion will be lost between the factory and the dinner table. Adding iodized salt at the end of cooking, rather than during prolonged boiling or frying, preserves more of the iodine.
Why Some Salts Aren’t Iodized
Sea salt, kosher salt, and specialty salts like Himalayan pink salt are generally not iodized unless the label specifically says so. Unfortified sea salt contains only trace amounts of naturally occurring iodine, far less than the 45 micrograms per gram found in standard iodized table salt. The coarse grain size of kosher salt also makes it harder to iodize evenly, since the iodine compound clings better to fine, uniform crystals. If you cook primarily with these salts, you’re likely getting very little iodine from salt and would need to get it from other sources like dairy, seafood, or eggs.