Have you ever boiled water in a kettle, then got busy with a phone call or started watching a movie?
The water is soon to be forgotten about, and will sit there to get cold. The temptation is to just reboil the water. But this should not be done and for good reasoning. The composition in water will change as it is boiled. Gases will be dissolved while volatile compounds evaporate. When the water is reboiled the chemical composition of the water will change. Instead of substances being eliminated, they will begin to accumulate. Substances that are dangerous include arsenic, fluoride and nitrates. Calcium salts can also lead to kidney stones or gallstones when taken in large amounts.
Arsenic can cause physical effects when exposed for several years. Cancer, skin lesions, renal system dysfunctions, cardiovascular disease and diabetes have a high risk when arsenic is in the body.
Fluoride is found in drinking water and can affect the health of a person. The government does not feel that is all that harmful to the health, so the fluoride is left in the water. A study in 2013 found that male mice had their fertility rate lowered as a result of fluoride.
Nitrates are found all over the earth. However, it is very dangerous when it is added to foods. When high temperatures are added in, the nitrates will morph into nitrosamines, which are carcinogenic.
This will lead to various cancers, including colon, bladder, pancreatic, stomach, ovarian, and esophageal, as well as leukemia.