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I know that boiling does little to break down chemical contaminants or heavy metals. But I'm unsure if it kills all microbial life. Are there any waterborne illness-causing viruses/bacteria/protozoa/cysts that can survive boiling? And if so, how serious is the illness they cause?

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    I would like to point out that "boiling" is not actually a specific heat. You can boil water at extremely low heat if you are at high altitude.
    – That One Actor
    Mar 8, 2016 at 19:09
  • Right, heavy metals can only break down by radioactive decay, and most isotopes not even that. (To be exact, shooting stuf into them, like neutrons, may actually help). It fascinates me that they can harm you and are basically indestructible...
    – Volker Siegel
    Nov 6, 2019 at 14:16

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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.

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 a waterborne enteric (intestinal) pathogen, ingestion will not cause infection.

All waterborne 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 waterborne 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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  • What about Clostridium Botulinum? Ingestion of this bacteria can and frequently does give people botulism which can cause death if left untreated. That would appear to be counter to what that article is suggesting.
    – BenAlabaster
    Feb 26, 2015 at 1:51
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    @BenAlabaster As I understand it, Clostridium spores are heat resistant but are not toxic by ingestion. Clostridium bacteria and botulism toxin which can be toxic by ingestion are easily destroyed by boiling water.
    – Graham
    Feb 26, 2015 at 20:09
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Boiling kills everything -- giardia, cryptosporidium, other bacteria, and viruses. 185°F (85°C) for a few minutes will do it, and boiling for one minute will do it. (Boiling is lots of big bubbles, not just a few small bubbles on the side of the pot.)

Some people recommend longer boiling times at higher altitudes because water boils at cooler temperatures there.

(Source: Wilderness Medical Society, The Backpacker's Field Manual by Curtis)

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    Not quite true... There are some bacteria that can survive boiling, but I seriously doubt you would ever find them in your traditional hiking water sources! en.wikipedia.org/wiki/Hyperthermophile
    – Timothy Strimple
    Jan 25, 2012 at 2:37
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    There's one more caveat: While boiling kills most bacterias responsible for food-borne illnesses, boiling doesn't necessarily destroy their spores/toxins. One infamous example is botulism.
    – Lagerbaer
    Jan 25, 2012 at 6:05
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    Note that the water must be clean to kill all bacteria, meaning that there cannot be particulates like silt, or any detritus within it. Bacteria and cryptosporidium can hide in those places, and survive the boiling process. So filter or settle your water if it is dirty, or boil for 15 minutes.
    – JoshuaBurke
    Jan 26, 2012 at 0:34
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    Clostridium Botulinum will survive to around 120 deg C
    – BenAlabaster
    Feb 25, 2015 at 22:24
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    I know this answer is over 3 years old, but I can't help but point out that 185F (85C) isn't anywhere close to 140.6C. Incidentally, 140C liquid water would kill ANYTHING exposed to it, even if it survived the pressure necessary to raise the water's boiling point that high.
    – Dallium
    Jun 23, 2015 at 20:39
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Botulinum toxin is particularly tough, as is Bacillus cereus. B. cereus is more likely found while camping.

But your goal is not so much to kill everything as reduce the level to the point where it does no harm. The dust you breathe, the things you touch, and (yick!) the people you are with all bring various pathogens, as does your tap water and (often especially) bottled water. Do your best, and your body takes care of the rest.

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Some types of spores can survive boiling. But they're either not disease-causing or they're not in any condition to make you sick as far as I know.

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Wikipedia has some interesting points on this:

The elimination of micro-organisms by boiling follows first-order kinetics—at high temperatures it is achieved in less time and at lower temperatures, in more time. The heat sensitivity of micro-organisms varies, at 70 °C (158 °F), Giardia species (causes Giardiasis) can take ten minutes for complete inactivation, most intestine affecting microbes and E. coli (gastroenteritis) take less than a minute; at boiling point, Vibrio cholerae (cholera) takes ten seconds and hepatitis A virus (causes the symptom of jaundice), one minute. Boiling does not ensure the elimination of all micro-organisms; the bacterial spores Clostridium can survive at 100 °C (212 °F) but are not water-borne or intestine affecting. Thus for human health, complete sterilization of water is not required.[3]

The traditional advice of boiling water for ten minutes is mainly for additional safety, since microbes start getting eliminated at temperatures greater than 60 °C (140 °F) and bringing it to its boiling point is also a useful indication that can be seen without the help of a thermometer, and by this time, the water is disinfected. Though the boiling point decreases with increasing altitude, it is not enough to affect the disinfecting process.

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  • The bolded material about Giardia isn't relevant, since it talks about 70 C, but the question is about boiling. The bolded material about Clostridium isn't relevant, because, as noted in the text, Clostridium doesn't cause disease in drinking water.
    – user2169
    Jun 23, 2015 at 22:43

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