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If you've ever read about water purification on internet forums or social media, it's likely that you encountered at least one person who asked, “how long should water be boiled before it's considered safe?” This is a common question, and one that's prudent to ask — after all, you wouldn't want to take any chances of consuming bacteria or protozoa that will make you severely ill. Unfortunately, this question has also led to the spread of some misinformation on the topic.
First of all, you may hear self-proclaimed “experts” on the internet say that water must be boiled for a minimum time of 5 minutes, 10 minutes, or even 20 minutes to ensure that all waterborne pathogens have been killed. This minimum boiling time constraint is a myth, and we'll explain why.
Scientific research tells us that waterborne pathogens (bacteria, protozoa, and viruses) are killed or inactivated at high temperatures. According to the World Health Organization, a water temperature of 158°F (70°C) will kill 99.999% of bacteria, protozoa, and viruses in less than 1 minute.
Since we know that water boils* at 212°F (100°C), this means that by the time water has reached a rolling boil, it will be safe to drink. For an added margin of safety, the Centers for Disease Control (CDC) recommend boiling water for 1 minute just to be sure.
*Now, there is one important clarification: the 212°F boiling point we mentioned is at sea level, and boiling temperature changes with altitude. The higher you are above sea level, the lower the boiling point of water will be. For example, at 10,000 feet, the boiling point drops to 193.6°F (89.8°C). At the summit of Mt. Everest, an immense 29,029 feet, the boiling point is 158°F (70°C). So, even at the highest point on earth, bringing water to a rolling boil will kill pathogens in less than 1 minute. For an added margin of safety, the CDC recommends boiling for 3 minutes at altitudes above 6,562 feet.
Anyway, don't just take our word for it — listen to the scientists. Here's a direct quote from an article titled “Water Disinfection for International and Wilderness Travelers” from the Oxford Journal of Clinical Infectious Diseases:
“Because enteric pathogens are killed within seconds by boiling water and are killed rapidly at temperatures >60°C [or >140°F], the traditional advice to boil water for 10 min to ensure potability is excessive. Because the time required to heat water from a temperature of 55°C [or 131°F] to a boil works toward disinfection, any water that is brought to a boil should be adequately disinfected. Boiling water for 1 min or keeping water covered and then allowing it to cool slowly after boiling can add an extra margin of safety. The boiling point decreases with increasing altitude, but this is not significant when compared with the time required to achieve thermal death at these temperatures.”
So, here's the bottom line: Water does not need to be boiled for a minimum of 5, 10, or 20 minutes in order to be considered safe to drink. By the time it reaches a rolling boil, it can be considered safe, regardless of your altitude. (Note: This assumes that no harmful chemicals or heavy metals, such as pesticides or lead, are present. To remove those contaminants, you'll need a water filter/purifier.) However, you may continue to boil for 1 to 3 additional minutes for an extra margin of safety — if you've got a few minutes to spare, it won't hurt, but shouldn't be considered mandatory.