Boiling water to sterilize germs and harmful bacteria is a recommended health. However, if you have a habit of reheating water that has been ripe to just brew the drink, this is not recommended because it can endanger health.
Reheating again water that has previously reached the boiling point, will produce some toxic substances. Reported by Goody Feed page, Monday (3/7/2017), this applies only if you use ground water directly.
Groundwater contains minerals, natural gas, and chemicals that will change its composition and properties in the boiling process to boiling. If boiled back, it will happen again the form of substances called toxic changes.
Different if you boil more distilled water or water by deionization process. This water is safer if you want to boil back.
It does not produce toxic substances, such as nitrate, arsenine and fluoride produced by repeatedly boiled ground water.
Nitrates that are heated many times will turn into nitrosamine, and are carcinogenic or cancer-causing.
Some studies link nitrosamine with blood cancer or leukemia, as well as lymphomas and some other cancers.
For arsenic, it is known to include toxins that in small amounts will trigger cancer, heart disease, infertility and neurological problems. Kidney problems are also feared will arise, coupled with the risk of gallstone disease.
While fluoride substances are believed to damage the growth of neural and cognitive development. To prevent these dangerous risks, you should boil water only once.