Health experts and consumer protection agencies, as of February 24, 2026, warn against re-boiling leftover water in electric kettles, a common practice in millions of German kitchens. This habit can significantly worsen water quality by concentrating dissolved minerals like nitrates, arsenic, and calcium compounds as water evaporates with repeated heating. While German tap water is highly controlled, regular re-boiling increases these concentrations. Furthermore, standing, cooled water can become a breeding ground for bacteria, posing risks, particularly for immunocompromised individuals, pregnant women, seniors, and infants. Limescale buildup from hard water also affects appliance lifespan and water taste. Experts recommend using only fresh water, boiling the exact amount needed, and regularly descaling kettles to ensure health, water quality, and appliance longevity.