The hardness of drinking water (i.e., total hardness) has a certain impact on human health. According to research data from various sources at home and abroad, the higher the hardness of drinking water, the higher the mortality and morbidity rates for diseases such as hypertensive heart disease, coronary heart disease, cerebrovascular disease, and kidney stones.
Boiling drinking water can reduce its hardness. The hardness of water will change differently after boiling for different durations, being placed in a cup with or without a lid, and being stored in a thermos for different periods.
As the boiling time of drinking water is extended, its hardness decreases accordingly; it takes about 10 minutes of boiling to reduce the hardness by 50%. When boiled water is poured into a cup, the hardness decreases more with a lid on than without; there is a significant decrease within 1 minute of covering, and little change from 1 minute to 1 hour. After boiled water is poured into a thermos, its hardness continuously decreases, dropping by 50% after 8 hours, with little change from 8 hours to 1 day.
Furthermore, according to relevant experimental reports, within 5 minutes of boiling, the nitrite content in water rises slowly; the increase accelerates significantly between 5-10 minutes of boiling. After 15 minutes of boiling, the nitrite content in the water can be 9 times higher than it was at the 5-minute mark.
5-10 minutes is better
In order to maximize the killing of bacteria and viruses in the water, reduce the hardness of drinking water as much as possible, and decrease the nitrite content, it is best to control the boiling time of water to 5-10 minutes. Moreover, one should only boil as much water as will be consumed on the same day. Any remaining water in the kettle or water heater must be emptied and refilled with fresh, unboiled water the next day. Any leftover water from the previous day in household thermoses or water pitchers should also be replaced; it is not advisable to pour freshly boiled water on top of the leftover water and drink them together.