context1: The first toothbrush to resemble the modern toothbrush is believed to have been invented in China. Japanese Zen master Dōgen Kigen recorded on Shōbōgenzō that he saw some monks clean their <strong>teeth</strong> with a brush when he had been in China in 1223. This brush used horse tail hairs attached to a stick made of ox <strong>bone</strong>.
context2: According to the WHO over 26 million people in China suffer from dental fluorosis (weakening of <strong>teeth</strong>) due to elevated fluoride in their drinking water. In addition, over 1 million cases of skeletal fluorosis (weakening of <strong>bones</strong>) are thought to be attributable to drinking water. High levels of fluoride occur in groundwater and defluoridation is in many cases unaffordable.