The notion that boiled water left overnight automatically becomes unsafe or carcinogenic is a common misconception. If the initial water source is safe, boiled correctly, and then stored in a clean, covered container, there is no scientific basis to suggest it transforms into a "toxic" substance after just one night.

The primary concern regarding water safety lies in its post-boiling storage. While boiling effectively eliminates most pathogenic microorganisms, this protection is not indefinite. Once the water cools, if it is poured into an unclean bottle or cup, left uncovered, or frequently exposed to the external environment, microorganisms can re-enter. Therefore, instead of worrying excessively about "overnight water causing cancer," attention should be directed towards the cleanliness of the container and the storage environment.

Another important consideration is partially consumed water. When drinking directly from a bottle or cup, microorganisms from the mouth can enter the remaining water. If this bottle is then kept for an extended period, particularly in a warm environment, the water's hygienic quality will significantly degrade from its initial state. It is advisable to avoid the habit of drinking directly from a container and then storing it at room temperature for too long.

To minimize the risk of bacterial contamination, the simplest approach is to use a safe water source, boil it thoroughly, and then store it in clean, covered containers. Water containers should be washed regularly. Additionally, water should be kept in a clean area, away from direct sunlight and high temperatures. If possible, it is best to boil only the amount of water needed for daily consumption rather than preparing a large quantity for prolonged storage.

Doctor Nguyen Trong Thuy

Former head doctor of the Vietnam National and U23 Football Teams