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Iodised 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 goitre. 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.
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 table salt with iodide may rapidly lose its iodine content in high temperature and high relative humidity conditions through the process of oxidation and iodine sublimation.