In some people's opinion, water can't be burned repeatedly, and a part of the water can't be burned again, because it will cause the accumulation of nitrite, calcium, magnesium and other substances. People drinking such water will be poisoned and even cause cancer. In fact, can the water be drunk repeatedly? Is it unhealthy to boil some of the remaining water in the kettle? Can the overnight water be drunk? Can I drink the water repeatedly?

Some people believe that repeated heating of water will increase the amount of nitrite in the water, and that drinking such water will cause poisoning and even cancer.

The nitrite contained in drinking water is generally converted from the original nitrate in the water. After the drinking water is boiled repeatedly, due to high temperature and lack of oxygen, part of the nitrate is converted into nitrite. In addition, due to the evaporation of water, the concentration of nitrite will increase

Ingestion of foods containing low levels of nitrite can replenish nitrite in the human body. However, if too much nitrite is ingested at one time, it is easy to cause acute poisoning, which will oxidize low-iron hemoglobin, which has normal oxygen-carrying capacity in the blood, into methemoglobin, lose the oxygen-carrying capacity, and cause tissue hypoxia, called methemoglobin Blood disease.

When the intake of nitrite reaches 0.2-0.5 grams, it can cause poisoning, and when it exceeds 3 grams, it can cause death [2].

The Joint Food Additives Expert Committee (JECFA) of the Food and Agriculture Organization of the United Nations and the World Health Organization (JECFA) stipulates that the daily allowable intake of nitrite is 0-0.2 mg per kilogram of body weight. For an adult weighing 60 kg, the permissible daily intake of nitrite is equivalent to 0-12 mg.

So how much nitrite is in the water that is repeatedly boiled?

In 2007, a study by the Shanghai Chongming County Center for Disease Control and Prevention showed that for fresh bottled mineral water, as the number of repeated heating increases, the nitrite content in the hot water gradually increases. After heating to 52 times , The content of nitrite is 2.3 micrograms/liter.

In 2010, a study conducted by Hengyang Normal University reheated barrels of purified water 181 times. As a result, the content of nitrite in the water was between 3-4 micrograms/liter.

Whether it is heated 52 times or 181 times, the result is less than the maximum limit value of 5 micrograms/liter and 100 micrograms/liter of nitrite in my country's packaged drinking water (except mineral water) and mineral water.

More importantly, if you want to reach the acutely toxic dose of 0.2 grams of nitrite, you can't just drink a few glasses of water. You need to drink tens of thousands of liters of water at a time. Obviously, this is impossible.

What if it is heated by tap water?

In 2011, Harbin Institute of Technology conducted a study on tap water that met the national standard to boil it five times, and the results did not detect the presence of nitrite. Even if a certain amount of nitrite is added artificially, the quality of nitrite does not improve as the number of boiling increases.

In our daily life, it is rare that drinking water is repeatedly heated for dozens or hundreds of times. As long as it is qualified drinking water, there is no need to worry about poisoning caused by excessive nitrite content.

Remember that simple and rude sentence-talking about toxicity without a dose is a hooliganism.

As for carcinogenesis, there is no need to worry. Because nitrite is not a carcinogen, the "nitrosamines" produced under certain conditions are carcinogens.

Therefore, we generally do not need to worry about drinking a few cups of repeatedly boiled water, which will cause nitrite poisoning or carcinogenic problems.

Is it unhealthy to boil some of the remaining water in the kettle?

Some people wonder, is it unhealthy to boil some water left in the kettle? If it refers to the problem of excessive nitrite, there is no need to worry, because the reason is the same as repeated heating of the water in the drinking fountain.

If it is the hardening of the water and the increase of harmful substances, there is no need to worry too much.

Let me talk about the hardening of water. The hardness of water is mainly due to the effect of minerals such as calcium and magnesium in the water. If there are many minerals in the water, the water quality will be hard, otherwise it will be soft.

If the water itself is relatively hard and contains more calcium and magnesium, after being heated and boiled, insoluble substances such as calcium carbonate, commonly known as scale, will be formed, which will precipitate, and the result will decrease the hardness of the water.

If it is pure water that is basically free of impurities, a part of the water remaining in the kettle will be boiled, although the calcium and magnesium minerals in it will accumulate to a certain extent, but eventually scale can also be formed.

The point is that minerals such as calcium and magnesium are not harmful substances to the human body. Drinking harder water can also help us replenish minerals such as magnesium and calcium to a certain extent. Some studies have shown that water with higher hardness may have a beneficial effect on a variety of cardiovascular diseases.

Let's talk about the increase of harmful substances. If it is a qualified drinking water, as long as it is not polluted by harmful substances, then harmful substances will not arise out of nothing. Even if the last remaining part of the water is reheated, it will not accumulate harmful substances. Too much, on the one hand, because the water is generally boiled with a lid, and not much water evaporates, on the other hand, it is because new water is added to it and burned together.

In addition, the premise of this harmfulness is that harmful substances are always precipitated during heating after heating. What if the harmful substances can be volatilized? Doesn't repeated heating reduce the content of harmful substances?

Therefore, there is no need to worry too much about whether it is unhealthy to boil some of the remaining water in the kettle.

Can the overnight water be drunk?

Overnight water is also a concern for many people, worrying about bacterial contamination and worrying about the increase in nitrite.

In fact, for boiled plain water, most of the bacteria in the water have been killed because of high temperature boiling. If it can be stored reasonably, there is no need to worry about the growth of bacteria. Of course, if there is no sealed storage, high temperature, etc., there are risks in this regard.

As for the issue of nitrite, some studies have found that the nitrite content in the overnight water after boiling is 1090 micrograms/liter, which has exceeded the limit value of nitrite specified in the current "GB 5749-2006 Sanitary Standard for Drinking Water" in my country. 1000 micrograms/liter.

However, as mentioned earlier, acute poisoning can only be caused when the intake of nitrite reaches at least 0.2 grams. Therefore, drinking one or two glasses of overnight water will not cause nitrite poisoning so easily.

What about packaged drinking water? Studies have found that 5 brands of drinking water began to detect nitrite after being opened overnight. The levels were 1.5 micrograms/liter, 1.3 micrograms/liter, 5.0 micrograms/liter, 2.1 micrograms/liter, and 1.9 micrograms/liter. The results also did not exceed the maximum limit value of 5 micrograms/liter and 100 micrograms/liter of nitrite in packaged drinking water (except mineral water) and mineral water in my country.

Therefore, for overnight water, as long as it can be stored reasonably, such as sealed preservation [10], keeping the temperature not too high, etc., generally speaking, you can drink it with confidence. Of course, don’t leave the boiled water for too long, try not to leave it overnight, especially when the weather is relatively hot.