For decades, tiny amounts of fluoride have been added to community water supplies as a way to protect people’s teeth against cavities. It’s an invisible program—when’s the last time you thought about fluoride in your water?—yet its impact has been enormous. More than 200 million people in the US have access to fluoridated water, and the program has been so successful that the CDC named it 1 of the 10 greatest public health achievements in the twentieth century.
