Iodised salt (also spelled iodized salt) is table salt mixed with a minute amount of one of four Iodine-containing salts of Hydriodic acid or Iodic acid: potassium iodate or potassium iodide, and sodium iodate or sodium iodide. It is used to prevent and remedy iodine deficiency. Worldwide, iodine deficiency affects about two billion people and is the leading preventable cause of mental retardation. It also causes thyroid gland problems, specifically endemic goiter. In many countries, iodine deficiency is a major public health problem that can be cheaply addressed by iodisation of salt. Iodine is a micronutrient that is naturally present in the food supply in many regions. However, 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 needed by humans. Edible salt can be iodised by spraying it with a potassium iodate solution. Two ounces of potassium iodate, costing about USD$1.15, are required to iodise a ton of salt. Salt is an effective vehicle for distributing iodine to the public because it does not spoil and is consumed by everyone in the population, in fairly predictable amounts.