Whilst we might assume that the temperature of boiling water is 100°C, that’s’ only the case at sea level. The temperature at which water boils reduces the higher the altitude. This is caused by a reduction in the atmospheric pressure.
For every 300m of altitude the boiling temperature drops by 1°C. So, for example, if you’re boiling water on top of Scafell Pike, which is 978m above sea level, it’ll boil at 96.8°C; if you’re on the top of Yr Wyddfa (Snowdon) at 1,085m it’ll boil at 96.5°C; on top of Ben Nevis at 1,345m it’ll boil at 95.7°C.
In the context of water safety, high altitude is most commonly defined as being 2,400m above sea level (1). At this elevation the boiling point of water is 92°C. Whilst we don’t have any peaks in the British Isles that are considered to be at high altitude, there are hundreds across Europe, the highest of which is Mount Elbrus in Russia at 5,642m; at this altitude water boils at 86.4°C.
If you’re travelling further afield this altitude calculator might be a useful resource.
When it comes to purifying water by boiling, the standing advice is to bring the water to a rolling boil for 1 minute or 3 minutes at high altitude. This should always be your goal.
You could also use a chemical purifier.
But out of interest I thought I’d have a look at some water borne organisms that can harm us and the temperatures at which they are neutralised.
Parasites
Liver flukes, specifically the larvae, are killed at 63°C although the Centers for Disease Control and Prevention (CDC) recommends heating water to between 70°C and 80°C for at least 10 minutes. (2)
Protozoa
Cryptosporidium is killed at 72°C within 1 minute and above 65°C within 5 minutes. (3)
Giardia is killed at 72°C within 1 minute and above 60°C within 5 minutes. (4)
It’s worth noting that both sources cited here recommend a rolling boil for 1 minute wherever possible or 3 minutes at high altitudes.
Bacteria
Water borne bacteria include cholera, typhoid, salmonella, E. colli, Legionnaires disease and shigella. The recommendation is, again, to bring the water to a 1 minute rolling boil or 3 minutes at high altitudes.
The CDC suggests, however, that where this is not possible you should bring the water to as much above 60°C as possible. (5)
This ‘too hot to touch’ approach is better than nothing, but I’ve found the following information which gives us some further guidance:
Cholera – 60°C for 10 minutes (6)
Typhoid – 65°C for 20 minutes (7)
Salmonella – 65°C for 1 minute (6)
E. colli – 60°C for 10 minutes (6)
Legionnaires – 70°C within minutes (8)
Shigella – 65°C for 20 minutes (9)
Viruses
Water borne viruses include hepatitis A, hepatitis E, polio virus, rotavirus, echovirus (meningitis).
For viruses, not unexpectedly, the CDC recommends bringing the water to a rolling boil for 1 minute or 3 minutes at high altitudes.
The New York State Department of Health provides the following further information (10):
60°C to 65°C – this range begins to kill many enteric viruses (those that affect the gut), but it often takes 30 minutes or more to achieve a significant reduction.
70°C to 75°C: – at these temperatures most viruses are inactivated in less than one minute.
Conclusion
Boiling water is a great way to purify it. In the British Isles bring your water to a rolling boil for 1 minute to make it safe to drink. If you’re in Europe and at altitude (over 2,400m), even on top of Mount Elbrus, a 3 minute rolling boil will purify your water.
If for whatever reason you’re unable to bring the water to a rolling boil, getting it too hot to touch, and keeping it that way for 30 minutes, is better than nothing.
References
- https://education.nationalgeographic.org/resource/altitude/
- https://www.cdc.gov/liver-flukes/clonorchis/index.html#:~:text=To%20avoid%20Clonorchis%20infection%2C%20do,can%20also%20contain%20infectious%20parasites.&text=Cook%20fish%20to%20an%20appropriate,%5B~63%C2%B0%20C%5D)
- https://www.epa.gov/sites/default/files/2015-10/documents/cryptosporidium-report.pdf
- https://bestfriendsvet.com/library/giardia/
- https://wwwnc.cdc.gov/travel/page/water-disinfection
- https://pmc.ncbi.nlm.nih.gov/articles/PMC7310936/
- https://www.health.ny.gov/environmental/water/drinking/boilwater/response_information_public_health_professional.htm
- https://legionellacontrol.com/guidance/temperature-influences-legionella-growth/
- https://pubmed.ncbi.nlm.nih.gov/16524613/#:~:text=The%20results%20of%20this%20study,minimum%20of%2060%20degrees%20C.
- https://www.health.ny.gov/environmental/water/drinking/boilwater/response_information_public_health_professional.htm#:~:text=Although%2C%20some%20bacterial%20spores%20not,factor%20for%20boil%20water%20responses
We talk about the importance of boiling water on our 2 Day Bushcraft Course, 5 Day Bushcraft Course and the Institute for Outdoor Learning Bushcraft Competency Certificate course.
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