Final answer:
The optimal amount of fluoride in the city water supply is determined by national and international guidelines. The World Health Organization recommends a maximum fluoride ion concentration of 1.0 ppm, while the US Environmental Protection Agency sets a maximum level of 4 ppm. These guidelines aim to balance the benefits and risks of fluoride exposure.
Step-by-step explanation:
Fluoride in the city water supply is beneficial for preventing tooth decay. The optimal amount of fluoride in the water supply is determined by national and international guidelines. The World Health Organization recommends a maximum fluoride ion concentration of 1.0 ppm in drinking water, while the United States Environmental Protection Agency sets a maximum level of 4 ppm for fluoride in drinking water. These guidelines aim to balance the benefits of fluoride in preventing tooth decay with the potential risks of excessive fluoride exposure.