Is it true that mixing 'raw' and boiled water can be dangerous?

There is an opinion about the inadmissibility of mixing 'raw' and boiled water, which dates back to ancient times. Back then, people used water from rivers and lakes, which they always boiled before consumption. Water was considered 'raw' before boiling. In those times, mixing boiled and 'raw' water was indeed risky, as pathogenic microorganisms and parasites could be present in untreated water.

Boiling kills most bacteria and worms, but if treated water is mixed with 'raw' water, the entire effect will be lost. Nowadays, tap water is already purified water that has undergone disinfection in the purification system. Many people further filter it, which helps remove dissolved elements and chemical compounds that can negatively affect health.

Filtered water is most often boiled before consumption, which allows for the removal of even more microorganisms. The ancient rule against mixing 'raw' and boiled water has changed: it is now believed that boiling removes accumulated heavy metals and salts from the water, making it potentially healthier than filtered water.

However, the dissolved salts in the water do not disappear. Some of them, such as magnesium and calcium bicarbonates, break down into insoluble carbonates when the temperature rises and settle out. Therefore, in boiled water, one may sometimes notice white flakes of sediment. Both of these substances are safe and are even used as food additives. So there is no need to be concerned about this.

Nevertheless, we do not recommend mixing tap water with boiled water—the quality of tap water in our country leaves much to be desired. The optimal option would be to use a filter. In this case, mixing filtered and boiled water can be done in any proportions without special concerns. However, if you experience pain or other unpleasant symptoms after consuming the water, you should immediately consult a doctor.