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This has come up a few times in a few other questions, but I've heard varying opinions from different people in a number of answers and comments on here - some saying that just boiling for a moment is enough, others saying that to be sure you should boil for around 10 minutes, and others saying it doesn't matter how long you boil for because some bacteria can survive it with no problem.

So in short, how long should you boil water you've collected for to ensure it's safe to drink? (I'm discounting filtering like metals which boiling won't remove.) It'd be good if answers could be referenced / backed up since people do seem to have different ideas on the subject!

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  • related: outdoors.stackexchange.com/questions/7/…
    – user2169
    May 3, 2013 at 16:43
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    The question doesn't say anything about where the water is being collected, but the unspoken assumption may be that backcountry water contains bacteria, viruses, or protozoan cysts capable of causing disease when you drink it. That's not true, as discussed in this answer to a related question: outdoors.stackexchange.com/a/4058/2169 . Of course boiling may still be wise at home after a natural disaster, in third-world travel, when hiking near livestock farms, or when the water is from an obviously untrustworthy backcountry source, such as a puddle full of mule poop.
    – user2169
    Aug 20, 2013 at 17:45
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    Keep in mind that boiling only kills pathogens and destroys some toxins. Toxins that are not destroyed by high heat will actually be concentrated by the boiling process, making the water less drinkable than it was before. So exercise some care in choosing your water source.
    – Perkins
    Sep 17, 2015 at 0:54
  • Also related: (191)
    – Mr.Wizard
    Nov 20, 2015 at 13:52
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    It's unfortunate that the incorrect answer by Justin C has been accepted and is currently the most upvoted answer.
    – user2169
    Jan 19, 2017 at 23:07

4 Answers 4

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The EPA recommends boiling for one minute for most people, and three minutes for anyone above 5000ft or 1000m (sic) in elevation.

reference - https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water

That is what I have always done when backpacking. I don't boil for 10-20 minutes because it will kill my fuel supply and waste too much of the water. If I were camping and had plenty of fuel I might boil longer to be safe.

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    +1 for mentioning the altitude issues. Water boils at decreasing temperatures as altitude increases, but that doesn't mean pathogens die at those lower temperatures.
    – Matthew Flaschen
    Feb 19, 2012 at 3:24
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    @Matthew - I'm not sure that's true. My understanding is that they are killed by the water inside them reaching a temperature that forces a state change (liquid to gas). Not 100% sure of that, but that's how I understand it.
    – Greg.Ley
    Feb 19, 2012 at 6:39
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    @Greg.Ley, no. As shown by this graph linked by Graham, different pathogens die at different temperatures (and various times at that temperature). No mention is made of altitude or phase change, and most temperatures are far lower than sea-level boiling point. Of course, boiling at sea level for sufficient time is one way to achieve those times and temperatures.
    – Matthew Flaschen
    Feb 19, 2012 at 6:46
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    That looks right. Makes it easy, too, because unless you're boiling above 14000 ft you'll kill anything by the time it hits boiling (depending on how you interpret Hep A).
    – Greg.Ley
    Feb 19, 2012 at 7:54
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    A recommendation by an agency like the EPA isn't the same as scientific evidence. As explained in the correct answers by Graham and Michael Pryor, the scientific evidence is that all disease-causing organisms are dead long before the water reaches a boil.
    – user2169
    May 3, 2013 at 16:42
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This article (*) gives a good summary of the efficiency of boiling as a method for making water safe for consumption. In particular, Table 2 provides a summary of the temperature and time required to kill various micro-organisms.

A distinction should be drawn between killing all pathogens and making water safe to drink. Sterilisation of water (killing all living containments) is not necessary to make water safe to drink. For example, boiling may not be effective against bacterial spores such as Clostridium which can survive at 100°C (212°F), however, as Clostridium is not an enteric (intestinal) pathogen, ingestion will not cause infection.

All enteric pathogens are quickly killed above 60°C (140°F), therefore, although boiling is not necessary to make the water safe to drink, the time taken to heat the water to boiling is usually sufficient to reduce pathogens to safe levels. Allowing the boiled water to cool slowly will also extend the exposure of enteric pathogens to lethal temperatures.

Boiling also gives a simple visual indicator that a high enough temperature has been reached when a thermometer is not available.

(*) Backer, H. Water Disinfection for International and Wilderness Traveler. Clinical Infectious Diseases. (2002) 34 (3): 355-364. Available from: http://cid.oxfordjournals.org/content/34/3/355.full

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    +1 for giving a reliable scientific source and for a correct explanation of why it's not necessary to kill all heat-resistant spores
    – user2169
    Aug 20, 2013 at 17:16
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    Note that in Table 2 of the article, some temperatures as high as 98 C are listed, but that doesn't mean that lower temperatures were found to fail. The table just lists some temperatures that were found to succeed. This answer gives the correct interpretation, which is that all disease-causing organisms are dead long before the boiling point is reached.
    – user2169
    Aug 20, 2013 at 17:35
  • +1 but note that the article implies (to a skim reading at least) the filtration or sedimentation is necessary as a first step - I'm not sure how much of a protective/insulating effect particles of dirt have given the time at temperature in complete immersion, but it's also going to be a lot nice to drink.
    – Chris H
    Sep 5, 2013 at 16:07
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    I have a large bit of dirt in the water, just getting the water to boiler point, my not get the inside of the dirt hot enough unless you boil for some time.
    – Ian Ringrose
    Oct 3, 2014 at 17:25
  • @BenCrowell, I agree that this answer gives the right advice for most drinking water purposes, but it's not true that all disease-causing organisms are dead long before the boiling point is reached. That's why hospitals use autoclaves and autoclave for 3min at 134°C or 15 min at 121°C. Admittedly, this is not relevant - or rather, not practical - for outdoor purposes. (The Wiki articles on Prions or Autoclaves give a good overview on the topic).
    – fgysin
    Nov 25, 2019 at 7:56
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0 minutes.

“According to the Wilderness Medical Society, water temperatures above 160° F (70° C) kill all pathogens within 30 minutes and above 185° F (85° C) within a few minutes. So in the time it takes for the water to reach the boiling point (212° F or 100° C) from 160° F (70° C), all pathogens will be killed, even at high altitude. To be extra safe, let the water boil rapidly for one minute, especially at higher altitudes since water boils at a lower temperature (see page 68.)”

Source: http://www.princeton.edu/~oa/manual/water.shtml

More info on this survival blog.

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    -1 Boiling certainly reduces the viability of bacteria and viruses, but to say "All pathogens will be killed" is incorrect. Most prions and many bacterial and fungal spores are resistant to the temperatures reached by boiling.
    – Graham
    Feb 17, 2012 at 14:54
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    I believe the CDC recommendation is based on the fact that your average person doesn't know what "boil" means, not because it is actually necessary. I have some references for that if it's appropriate.
    – Greg.Ley
    Feb 17, 2012 at 22:18
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    @Greg.Ley -- From a scientific standpoint you are absolutely correct that the minute is not necessary. From an applied science in the wild standpoint, where I am in the woods with no thermometer, I'm going to err on the side of using that minute to make sure I don't get Hepatitis (which can require temperatures up to 98C to kill).
    – Russell Steen
    Feb 18, 2012 at 16:46
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    @MichaelPryor: this question in itself is obviously hard to answer and the OP admitted that. Different experts feel differently about it. Nope, there is no scientific controversy. "To be extra safe..." doesn't indicate there is any controversy about whether it's necessary to keep the water at 100 C for a certain amount of time. It indicates the concern that people might see a few bubbles coming up and incorrectly interpret that as boiling.
    – user2169
    Aug 20, 2013 at 17:21
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    @BenCrowell -- No, the statement was based on research I did at the time of clinical studies of the viability of various pathogens. Off the top of my head that was based on a paper I got from the CDC. I don't have it saved anymore.
    – Russell Steen
    Aug 27, 2013 at 20:46
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The adequate minimum is 1-2 minutes (100C water at the sea level). Mind the altitude. To kill bacteria you need a combination of temperature and time. At some altitudes (miles), without a pressurizer, you just can't kill some resilient strains in a reasonable time (low boiling temperature). Use other disinfection methods in addition.

Why no less?

For this method, you just need to know the local boiling temperature. Any pre-boiling temperature demands a thermometer to be sure that you're heating the water as intended. And the 1-2 minutes are enough to assure that it was boiling, not -5 -7C from the boiling point near the surface or so while you're thinking there was a full volume already boiled.

Why 1-2 min. is safe enough at most elevations?

See the tables about temperatures for pathogen killing and boiling temperature at different altitudes. https://www.who.int/water_sanitation_health/dwq/Boiling_water_01_15.pdf https://en.wikipedia.org/wiki/High-altitude_cooking

The maximum is 30 min. It's for Anthrax as an edge case, again 100C at the sea level. The most important thing - it's a misconception that 70C for 30 minutes; 85° C for 2 minutes, or alike low energy procedures are ultimate protection. It's a somewhat recommendation for your protection on average, not a guarantee. It isn't killing everything, just lowering the most common harmful pathogens (including parasites, viruses, etc., excluding spores and toxins) to safe for a healthy individual levels. Alas, 30 min 100C boiling either won't kill everything, but it's already a paranoid level of heating.

https://en.wikipedia.org/wiki/Anthrax

You can meet something like this bacterium in the wilderness; however, most of the time you will not.

Decide yourself.

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