Why is iodine added to table salt?

Iodized salt is table salt mixed with a minute amount of various salts of the element iodine. The ingestion of iodine prevents iodine deficiency. Worldwide, iodine deficiency affects about two billion people and is the leading preventable cause of intellectual and developmental disabilities. Deficiency also causes thyroid gland problems, including "endemic goiter". In many countries, iodine deficiency is a major public health problem that can be cheaply addressed by purposely adding small amounts of iodine to the sodium chloride salt. According to public health experts, iodization of salt may be the world's simplest and most cost-effective measure available to improve health, only costing US $0.05 per person per year.

Iodine is a micronutrient and dietary mineral that is naturally present in the food supply in some regions, especially near sea coasts, but is generally quite rare in the Earth's crust, since iodine is a so-called "heavy" element and abundance of chemical elements generally declines with greater atomic mass. Where natural levels of iodine in the soil are low and the iodine is not taken up by vegetables, iodine added to salt provides the small but essential amount of iodine needed by humans.

An opened package of iodized table salt may rapidly lose its iodine content through the process of oxidation and iodine sublimation. Canning or pickling salt has no iodine added because it can cause oxidation, a visually undesirable and harmless darkening of food.