When camping, travelling overseas, or even following some malfunction at your local water treatment plant, you are urged to boil water before drinking it. How effective is boiling - does it really make the water safe to drink? And how long do you boil the water for?
Why Boil Water?
The main reason boiling is recommended as a water treatment is because it can be done simply and because the disease causing organisms are destroyed by heat. Holding the water at a high enough temperature for long enough will destroy pathogens. The organisms of interest are cysts such as cryptosporidia and giardia, and bacteria such as E.coli, typhoid etc. Although the boiling does not remove them from the water, it does kill them and thus they will cause no illness.
How Long Should Water Be Boiled?
There are many times suggested: 1 minute, 3 minutes, 5 minutes, 10 minutes etc. Some advocates even suggest adding a minute to the boiling time for each 1000 feet of altitude. So which of these times is correct? After all, the amount of time the water needs to boil determines the amount of fuel needed (to boil 1 litre of water requires about a kilogram of wood). As it turns out, none of them. You do not even need to boil it for one minute - you just need to bring it to the point of boiling. Let's consider why.
According to the Wilderness Medical Society, water temperatures above 70° C (160° F) kill all pathogens within 30 minutes and above 85° C (185° F) within a few minutes. So the higher the temperature, the less time is needed to kill the micro-organisms. Bringing the water to boiling point raises it well over the 85°C mark. Not only that, but the water must then be allowed to cool (unless you are making a cuppa). So the total time the water spends above 85°C is significant and sufficient to kill pathogens.
If it is sufficient just to bring water to 85°C for a few minutes, why are you told to boil it? Basically because that is a foolproof way of guaranteeing the water will be the right temperature. Most people, do not carry thermometers with them when camping so it would be difficult to determine when the right temperature was met - once it reaches boiling you know what temperature it's at within a couple of degrees.
How Effective is Boiling Water?
Boiling water is an effective method for destroying bacteria and other pathogens. If the water is turbid, filter it through a clean cloth, or coffee filter to remove particulate matter before boiling as that will improve its appearance.
Boiling will not provide any safeguard against other things such as heavy metals, pesticides, herbicides, pharmaceuticals etc that may contaminate a water supply. It may remove chemicals which have a lower boiling point than water but what about the others? Neither does it remove turbidity, foul tastes and odours.
In short, boiling water does not purify your water. It is certainly effective at eliminating the target pathogens but will not be effective against other contaminants - you really need a decent water purifier for that. However, drinking boiled water is certainly better than dehydration.
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