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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/… –  Ben Crowell May 3 '13 at 16:43
    
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. –  Ben Crowell Aug 20 '13 at 17:45
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3 Answers

up vote 24 down vote accepted

The EPA recommends boiling for one minute for most people, and three minutes for anyone above one mile in elevation.

reference - http://water.epa.gov/drink/emerprep/emergencydisinfection.cfm#method

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 '12 at 3:24
    
@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 '12 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 '12 at 6:46
    
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 '12 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. –  Ben Crowell May 3 '13 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 –  Ben Crowell Aug 20 '13 at 17:16
    
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. –  Ben Crowell Aug 20 '13 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 '13 at 16:07
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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 '12 at 14:54
    
I think we need some scholarly references here, because the Wilderness Medicine Institute also says to bring water just to a boil to be safe. It is conflicting, so I think we need better sources to answer this question accurately. –  Greg.Ley Feb 17 '12 at 18:44
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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 '12 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 '12 at 16:46
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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 '13 at 20:46
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