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According to Dr Nguyen Duy Thinh, former lecturer at the Institute of Biotechnology and Food Technology, many families boil water, let it cool, and drink it over the course of several days or even a week. He said some even refill the container before finishing the previous portion of boiled water, a common mistake that should be corrected.

Boiling helps destroy microorganisms and parasites in the water using heat. The process does not produce harmful or carcinogenic substances. But boiled water should not be stored for too long because it can become more contaminated over time.

When water is boiled, microorganisms are destroyed and broken down into organic matter, which becomes a food source for new bacteria entering from the environment. This makes recontamination likely and causes bacteria to multiply quickly.

Many people find that boiled water left overnight tastes different and worry that its quality has declined. The change in flavor comes from a small amount of carbon dioxide in the air dissolving into the water. While this is not harmful, it is best to avoid drinking boiled water that has been left out for too long. Ideally, cooled boiled water should be consumed within the day and not left overnight. Avoid adding new water to an old container, especially when the water source or storage conditions are unhygienic.

Boiled water should be kept in clean, sterilized glass containers that meet hygiene standards, not in plastic or recycled bottles. It should be stored in a clean area, tightly sealed, and equipped with a covered spout for pouring. The best practice, Dr Thinh said, is to finish boiled water within the same day and avoid leaving it overnight.