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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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4 Answers 4

Boiling kills everything -- giardia, cryptosporidium, other bacteria, and viruses. 185F (140.6C) 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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I know there is some question whether you need 'minutes' or just bringing to a rolling boil is enough. I lean toward the latter, and have never had issues. –  LBell Jan 25 '12 at 2:06
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 '12 at 2:37
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 '12 at 6:05
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. –  Dangeranger Jan 26 '12 at 0:34
And what about eggs of roundworms (Ascaris lumbricoides) and other parasites? I've heard some of them are very hard to kill even by high temperature... –  ВГДЕЖЅZЗИІКЛМНОПҀРСТȢѸФХ Jun 19 '12 at 7:42

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 at 1:51
@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 at 20:09

Botulinum toxin is particularly tough, as is Bacillus cereus. B. cereus is more likely found while camping. For a good reference see: http://aggie-horticulture.tamu.edu/extension/poison.html 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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+1 for the pragmatic view about "where it does no harm". But can you give more detail about pathogens in bottled water?! –  robserverdavista Feb 25 '12 at 23:45
From linked article: "The botulinum toxin is destroyed by boiling the food for 10 minutes." I've written it here, as the very important information –  ВГДЕЖЅZЗИІКЛМНОПҀРСТȢѸФХ Jun 19 '12 at 7:44

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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