A friend recently bought an electric kettle. However, he always unplugs it after boiling water. When asked why, he explained he worries that leaving it plugged in will cause continuous heating, repeatedly boiling the water.Based on past experience, the longer water boils, the higher its nitrite content becomes—nitrites are potentially carcinogenic and harmful to health—such water is also called "thousand-boil water."
Can electric kettles and water dispensers used in countless households withstand repeated heating? After boiling water, should you unplug the device?
Nitrites don't appear out of thin air
In reality, nitrites in water don't materialize spontaneously. They typically form through the conversion of existing nitrates in the water.Qualified drinking water contains minimal nitrate; however, nitrate levels may rise if the water source is contaminated with nitrogen-containing organic compounds. Typically, repeated boiling increases nitrite levels primarily through evaporation and concentration, raising nitrite concentration without increasing total quantity. Another possibility is that certain nitrates decompose into nitrites when heated.
How much nitrite is produced by repeated boiling?
China's hygiene standards for bottled water stipulate that nitrite levels must not exceed 2 micrograms per liter.How much nitrite is actually present in repeatedly boiled water?
In 2007, the Shanghai Center for Disease Control and Prevention investigated this issue. After repeatedly heating water from a water dispenser's barrel, they found that nitrite levels did indeed rise with increasing number of boils. After 52 boils, the water contained 2.3 micrograms per liter of nitrite [1].
In 2011, Harbin Institute of Technology also verified this claim. Researchers used six beakers containing untreated water and water boiled 1 to 5 times, measuring nitrite, nitrate, and other substances.The results showed that tap water meeting national standards contained no detectable nitrite after repeated boiling. Even when nitrite was intentionally added, its concentration remained unchanged during the boiling process [2]. In contrast, repeated heating of bottled water 20 times resulted in nitrite levels exceeding the standard at 2.1 micrograms per liter [3], just 0.1 micrograms above the limit.
What about more frequent heating? Another study repeatedly heated bottled water in a water dispenser over an extended period. After 181 heating cycles, the nitrite ion concentration reached 3.53 micrograms per liter—five times the initial level [4].
Don't panic at this "fivefold increase." While exceeding China's bottled water hygiene standards, this level remains safe. Consider that many popular ham sausages contain far higher nitrite levels: qualified ham sausages allow up to 30,000 micrograms (30 milligrams) of nitrite per kilogram.
If ham is safe, this level of nitrite in water is even less cause for concern. Besides, outside of scientific research, who boils water over 180 times in daily life? Moreover, household water dispensers and electric kettles typically hold small volumes—water is usually consumed long before it undergoes multiple reheats.
Do not store boiled water for extended periods
However, it's worth noting that when stored in a thermos for 5-6 days, boiled water may indeed see increased nitrite levels due to bacterial activity. This risk is heightened in rural areas where nitrate content in water is naturally higher.Therefore, it is advisable to consume boiled water promptly after boiling and avoid prolonged storage. If storage is necessary, it is best to keep it in a sealed container to minimize bacterial growth and reproduction.
From an energy conservation perspective, turning off the power after boiling is indeed a good habit. Additionally, failing to unplug an electric kettle promptly after boiling may pose an electric shock hazard. The risks of fire or personal injury from electrical leakage warrant greater caution than the trace amounts of nitrite in the water.
Therefore, for both electrical safety and energy conservation, it is recommended to turn off the power promptly after water reaches a boil.