How long does water need to be boiled for to kill all bacteria / viruses?
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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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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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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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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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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