Water fluoridation is the controlled addition of fluoride to a public water supply to reduce tooth decay.
Its use began in 1945, following studies of children in a region where higher levels of fluoride occur naturally in the water.
Researchers discovered that moderate fluoridation prevents tooth decay, and as of 2004 about 400 million people worldwide received fluoridated water.
Although excessive water fluoridation can cause dental fluorosis (which can alter the appearance of developing teeth) it is not considered to be a public-health concern.
There is no clear evidence of any other adverse effects. Although this is true, water fluoridation is still a controversial practice and therefore its application and use is strictly monitored and controlled.