Repeated boiling does not suddenly create toxins, but evaporation can gradually concentrate minerals and contaminants already present in the water. Using fresh water for each boil can also improve taste and reduce mineral buildup.
Another problem is using an old, rusty, damaged, or heavily scaled kettle. Regular cleaning and descaling can help, while kettles with corrosion, peeling coatings, or persistent metallic tastes should be replaced.
Leaving boiled water sitting for many hours can also reduce freshness. For better quality, boil only what you need, keep stored water covered, clean the kettle regularly, and refill it with fresh drinking water whenever practical.
