Why are certain geographical areas more prone to fluoride toxicity?

While Western countries like the United States intentionally add a small amount of fluoride to drinking water for dental health, some geographical locations have groundwater that is naturally excessively high in fluoride, and people who live in these areas have a greater risk of developing fluoride toxicity. In some regions of India and China, geothermal and volcanic activity has led to the formation of rocks that have high levels of fluoride. When these rocks are exposed to alkalinity and heat, fluoride is released into the soil and freshwater sources, resulting in high-fluoride food crops and drinking water. Fluoride does not have a taste, odor, or color, and since groundwater is not tested often, high fluoride exposure can go undetected for a long time. It is predicted that some countries in Central Africa are also at high risk for drinking water with excessively high fluoride.^[1]