data from a survey performed in 1981, data from a survey from 2008 and 2009, an annual assessment on endemic fluorosis from 2010, and monitoring data of endemic fluorosis from the years 2015 and 2017.

From 2009 to 2014, skeletal fluorosis of endemic fluorosis data were collected from 27 sample villages in 9 regions, i.e., Lingwu City, Litong District, Tongxin County, Yanchi County, Qingtongxia City, Pengyang County, Xiji County, Haiyuan County, and Shapotou District. All these data were used in combination to investigate the variation in endemic fluorosis over a period of 40 years.

Local river water sampling {#Sec5}
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Water samples were collected from the Qingshui River (main stream and tributary waters) and groundwater in 2018--2019. For this, 20 sampling points were evenly set up, and sampling was conducted three times atless than 5%, and the qualified rate of sample test is 100%.

The color scale shows the number of affected villages with fewer than 200 affected villages are considered as low endemic; areas with between 200 and 400 affected villages, medium endemic; and areas with over 400 affected villages, high endemic.

Results {#Sec7}
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Analysis of epidemiological data of endemic fluorosis (1981--2017) {#Sec8}
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Endemic fluorosis in the investigated area was first described in the survey of 1981. This covered a population of 4.87 million in 20 cities/counties of Ningxia, in which all but one fluorosis was endemic to various degrees (the exception was Jingyuan County). In that year, endemic fluorosis was reported for 539 villages in the region. The situation worsened over time. In 2010, there were 3449 villages with endemic fluorosis; in