The goal of water fluoridation is to prevent tooth decay by adjusting the concentration of fluoride in public water supplies. Tooth decay (dental caries) is one of the most prevalent chronic diseases worldwide. Although it is rarely life-threatening, tooth decay can cause pain and impair eating, speaking, facial appearance, and acceptance into society, and it greatly affects the quality of life of children, particularly those of low socioeconomic status. In most industrialized countries, tooth decay affects 60–90% of schoolchildren and the vast majority of adults; although the problem appears to be less in Africa's developing countries, it is expected to increase in several countries there because of changing diet and inadequate fluoride exposure. In the U.S., minorities and the poor both have higher rates of decayed and missing teeth, and their children have less dental care. Once a cavity occurs, the tooth's fate is that of repeated restorations, with estimates for the median life of an amalgam tooth filling ranging from 9 to 14 years. Oral disease is the fourth most expensive disease to treat. The motivation for fluoridation of salt or water is similar to that of iodized salt for the prevention of congenital hypothyroidism and goiter.