We are all guilty of it. Putting the kettle on to boil water and then forgetting about it. The hot water cools down, and consequently, we boil the same water again. This is where we run into a dangerous scenario, something we might not have been warned about.
The composition of water changes when boiled, which is typically a good thing as the volatile compounds evaporate and gases dissolve. This is why it's safe to drink boiled water.
If you have perfectly pure distilled deionized water, nothing will happen if you reboil it. However, ordinary water contains dissolved gases and minerals. When it is boiled for too long or is reboiled, the chemical compounds change for the worst.
Dangerous substances accumulate instead of being disposed. When water is reboiled, compounds amass, thus an increased risk of consuming certain chemicals.
These harmful substances generally include arsenic, nitrates, and fluoride. Even healthy minerals can become detrimental when amassed; calcium salts can lead to kidney stones and gallstones when consumed in large quantities.
The Damaging Effects Of Reboiled Water
When nitrates are exposed to high heat, like boiling water, it becomes highly toxic. High temperatures transform nitrates to nitrosamines, which are carcinogenic. Nitrates have been linked to diseases like leukemia, non-Hodgkin lymphoma, ovarian, colon, rectal, bladder, stomach, esophageal, pancreatic, and thyroid cancer.
An accumulation of arsenic over the years may result in arsenic intoxication which has been shown to cause numerous health issues: cancer, cardiovascular disease, neurological issues, infertility, and developmental disorders. The World Health Organization states that drinking water poses the greatest threat to human health because of arsenic.
Despite the scientifically proven dangers of fluoride,
government insists on keeping it
in the water supply. Fluoride exposure showed damaging effects on neurological
and cognitive development in children according to data from US .
Source - Chemistry Forum