Certain areas of the world, due to natural deficiency and unavailability of iodine, are severely affected by iodine deficiency, which affects approximately two billion people worldwide. It is particularly common in the Western Pacific, South-East Asia and Africa. Among other nations affected by iodine deficiency, China and Kazakhstan have begun taking action, while Russia has not. Successful campaigns for the adoption of the use of iodized salt require education of salt producers and sellers and a communication campaign directed at the public. The cost of adding iodine to salt is negligible—"Only a few cents a ton. "Iodine deficiency has largely been confined to the developing world for several generations, but reductions in salt consumption and changes in dairy processing practices eliminating the use of iodine-based disinfectants have led to increasing prevalence of the condition in Australia and New Zealand in recent years. A proposal to mandate the use of iodized salt in most commercial breadmaking was adopted in Australia in October 2009. In a study of the United Kingdom published in 2011, almost 70% of test subjects were found to be iodine deficient. The studys authors suggested an investigation regarding "evidence-based recommendations for iodine supplementation".Micronutrient deficiencies, including iodine deficiency, impair the development of intelligence. Lacking iodine during human development causes a fall, in average, of 12 intelligence quotient (IQ) points in China.
A study of U.S. military data collected during the First and Second World Wars found that the introduction of salt iodization in the U.S. in the 1920s resulted in an increase in IQ, by approximately one standard deviation, for the quarter of the U.S. population most deficient in iodine, explaining about "one decades worth of the upward trend in IQ" in the U.S. (i.e., the Flynn effect). The same study documented "a large increase in thyroid-related deaths following the countrywide adoption of iodized salt, which affected mostly older individuals in localities with high prevalence of iodine deficiency".In iodine-deficient or mildly iodine-deficient areas of Europe, iodine deficiency is frequent during pregnancy despite the widespread use of iodised salt, posing risks to the neurodevelopment of foetuses. In one study performed in a mildly iodine-deficient area, iodine deficiency was found to be present in more than half of breastfeeding women; in contrast, the majority of their newborns had iodine excess, mostly due to neonatal exposure to iodine-containing disinfectants. A 2014 meta-analysis found that iodine supplementation "improves some maternal thyroid indices and may benefit aspects of cognitive function in school-age children, even in marginally iodine-deficient areas".