Many people believe that boiled water is safe to drink no matter how long it has been stored, but experts remind everyone that boiled water should not be consumed after three days.
When water is stored for too long, it can become contaminated with bacteria and produce nitrites. Once a large amount of nitrites enters the human body, it can cause tissue hypoxia, leading to symptoms such as nausea, vomiting, headaches, and palpitations. In severe cases, it can even lead to death from lack of oxygen. Nitrites can also form nitrosamines in the human body, which can promote liver cancer, stomach cancer, and other diseases. Boiled water stored in a thermos flask breeds bacteria faster and produces more nitrites once it cools down.