It stays right with the water vapor, and gets condensed back into liquid at the end of the process. So, don't let anyone tell you otherwise and don't trust those that do. Even bathing or showering in fluoridated water will cause you to absorb an unhealthy amount of it through your skin. Fruit juices often have very high concentrations of Fluorine, due to the use of Fluorine in pesticides . Pop and other packaged beverages are also common and substantial sources of Fluorine, not only because of the fluoridated water but because of fluoride-laced ingredients, and the concentration of same during the manufacturing process.
Bottled water can be an option, but choose carefully.
Mineral Waters of the World provides a free and very detailed analysis of the composition of over 600 types of bottled water available throughout the world. Ditto toothpaste manufacturers and thus dentists. Any tooth decay reducing effect attributable to fluoride occurs by topical mechanisms involving action on tooth surfaces and on oral bacteria that promote dental caries. Then visit their web sites. I did this for the largest bottlers in the Phoenix area and learned exactly nothing about the content of the water they are selling.