The World Health Organization, looking at global public health, identifies fluoride as one of a few chemicals (along with the arsenic and nitrate, and to a lesser extent lead, selenium and uranium) that are found in excessive levels in many parts of the world and cause negative health effects; for fluoride this is especially true in large regions of India, China, Central Africa and South America, and locally in many parts of the world. Only for fluoride does it recommend adjusting the level in places where the chemical is low to reach a threshold; this is because there is clear evidence that low concentrations provide protection against cavities, both in children and in adults.[43][44]