The results from scientific studies vary greatly concerning the amount of tooth decay reduction that can be credited to public water-fluoridation. Some studies indicate that the reduction in cavities is as much as 40%. Other researchers believe the benefit is only around 18%. Most dentists believe that the benefits are minimal. Access to fluoridated toothpaste, as well as regular in-office treatments are believed to provide the greatest benefit. The reasons for that are easy to understand. When you consume a beverage, the liquid is only on the teeth for a short period of time. It passes over the teeth and into the digestive system. Consumption of fluoride is of no benefit. In fact, consuming too much can be deadly. There have been several instances of over-fluoridation at public treatment facilities. In the US, there were four such instances between 1991 and 2010. Human error is the problem. There doesn't seem to be anything we can do about that. There is a negative role of fluoride in water when concentrations are too high. The World Health Organization recommends a maximum concentration of 1.5mg/L. In one case, the concentration was over 200mg per liter. The accident resulted in hundreds of illnesses and the death of one person. The usual symptoms of excessive intake include nausea and vomiting, which should reduce the likelihood of poisoning. But if a person had a weakened immune system or other health problems, the poisoning could still be fatal. Excessive consumption does not always cause illness. That's another problem, because the element has no smell or taste. You cannot tell whether a glass contains it or not, without scientific testing. So, long-term consumption of excessive amounts is possible and can lead to another negative role of fluoride in water; dental and skeletal fluoriosis. The symptoms include discoloration of the teeth, especially when children are exposed before their permanent teeth come in, and weakened bones. Fluoriosis usually occurs only in areas where the excessive concentration is naturally occurring. Fluoride is present in the rock formations. Rainwater passes over the rocks and carries the element into the groundwater. The concentrations in groundwater are highest after heavy rainfall or heavy snow melt. In some cases, it is necessary to use activated alumina to reduce the concentration to safe levels. Most public health organizations believe that the positive role of fluoride in water outweighs any negative side effect. Other contaminants cause more problems for public health. Now might be a good time to learn about those hazardous contaminants.