An article is circulating on the internet about the dangers of reboiling water and concentrating dissolved chemicals. It amounts to baseless fear-mongering. Lets consider fluoride as an example. Suppose you put a liter of water containing 1 ppm fluoride in a kettle and boil it. You then take 200 mL to make a cup of coffee or tea. That means you will ingest 0.2 mg of fluoride. If you now let the water keep boiling until 100 mL evaporates...which would take a long time...and you take 200 mL from the remaining water to make your next cup of coffee, you will be ingesting 0.22 mg of fluoride. This is an insignificant difference, even if there were an issue with that amount of fluoride, which there isn't. The same argument applies to any other solutes in water. To imply that making your next cup of coffee from boiled water is going to have any sort of impact on health is absolute nonsense. With tea there may actually be a slight difference but that has nothing to do with taste not toxicity. Boiling drives oxygen out of the water and deoxygenated water tends to taste more flat.
An article is circulating on the internet about the dangers of reboiling water and concentrating dissolved chemicals.