Boiling Water: How Long To Disinfect?

a pan and water boiing on the top of a stove
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You and I know that we need water to live, but the quality of that water is important. So important, as a matter of fact, that it could mean the difference between good health and life-threatening disease. Sickness caused by contaminated water has stopped armies in their tracks and changed the course of history.

How long do you have to boil water for it be safely drinkable?

The old saying goes “ask a bunch of doctors the same question, get a bunch of answers”. You ask a bunch of survivalists about boiling water, well, you might get a bunch of answers. If you consider that the fuel and time required to boil water might be limited in certain survival scenarios, however, it’s a serious question.

There are all sorts of disease-causing microbes, also called pathogens, that are harmful to humans and can be found in water. These include protozoa, bacteria and viruses. The protozoal microbes that could get you sick include cryptosporidium and giardia. Harmful bacteria include salmonella, shigella, and e. coli. Viruses that contaminate water include things like hepatitis, enterovirus, and norovirus.

There are various ways to disinfect water. Bleach is popular, iodine will work, and UV sterilization using direct sunlight on clear bottles of water for a good 8 hours is another way. Of all ways you can disinfect water, however, the Centers for Disease Prevention and Control (the CDC) recommends boiling as the best method. They have an excellent PDF you can download, by the way, called Drinking Water Treatment Methods for Backcountry and Travel Use”.

The CDC believe that none of the methods other than boiling are 100% effective in killing all disease-causing bugs. Even bleach takes several days to kill some organisms like cryptosporidium, something we talked about on the Survival Medicine podcast a few months ago. Of course, The CDC (and I) suggest that cloudy water should be filtered as well as disinfected. You can improvise a filter but some popular commercial lightweight filters include the Mini-Sawyer and the Lifestraw.

Here’s a fun fact: How much wood does it take to boil one liter of water? it takes one kilogram (2.2 pounds) of wood fuel to boil one liter (1.06 quarts) of water. And how much wood could a woodchuck chuck if a woodchuck could chuck wood? I’m conducting an experiment on this at present, but it’s hard to convince these critters to chuck wood at all.

Disinfection means the removal or destruction of harmful pathogens, if you’re alone in the woods with nothing but a firestarter, boiling is an effective option. Freezing, on the other hand, doesn’t kill bacteria, it just prevents them from multiplying. As soon as the water warms, the bacteria will begin multiplying again.

Having said that, if you get two pots of water, and you freeze one, and leave the other half at room temperature, the frozen water will certainly have less bacteria in it than the room temperature water. It’s just not the same as killing the bacteria.

How long should water be boiled? I had a good discussion of this with Eric of the Woodsman’s Journal Online blog recently. Some say that water should brought to a rolling boil for three minutes, five minutes, ten minutes or maybe even longer. However, the CDC and the World Health Organization have both shown that this is not necessary, and for survival purposes, I think it’s wasteful from the standpoint of fuel consumption and time. Thinking of survival scenarios, the smoke from a fire to boil water might give away your position to others in hostile areas, even if you use a Dakota fire hole.

Eric noted that The Backpacker’s Field Manual states that water temperatures above 160° F will kill pathogens within 30 minutes and at 185 degrees, just a few minutes. Neither of these is boiling temperature, which is 212 degrees at sea level. If you manage to get water to boiling, you’ve already passed the temperatures that kill just about every pathogens. It should be noted that, according to the National Institute of Health, some viruses are resistant to heat and may require longer times at sub-boiling temperatures, such as poliovirus.

If most microbes die before water gets to boiling, why boil water? In survival settings, a good boil is a simple recognizable sign that doesn’t require special temperature monitoring materials.

According to the CDC, heating water to a roiling boil for one minute at sea level has “high effectiveness in killing” protozoa such as Giardia and cryptosporidium (which is resistant even to bleach), bacteria like E. coli, Salmonella, Shigella, and others I mention in our new book Alton’s Antibiotics and Infectious Disease, as well as viruses such as enterovirus, hepatitis A, and norovirus.

The temperature to boil water is dependent on the altitude you’re at. It’s thought that for every 500-foot increase in elevation, the temperature required goes down by just under a degree Fahrenheit.
Why does this happen?

As water heats, there is a point at which it begins to change to a gas. This is known as the boiling point. At sea level, when water reaches 212°F or 100°C, it begins to turn to steam or vapor. Once this point is reached, the water can’t get any hotter. This process is different than evaporation, which takes place at the surface of the water; boiling takes place within the entire volume of water.

The vapor being produced in the water causes something called vapor pressure. This pressure fights the pressure of the atmosphere that’s pressing upon it, so water can’t release the vapor (that is, boil) until the vapor pressure can overcome the atmospheric pressure. The atmospheric pressure decreases with altitude, so the temperature required to make steam decreases as well.

at elevations above 2,000 meters (6,562 feet), 3 minutes of additional boiling are recommended by the CDC to ensure that the water has remained hot enough, for long enough, to destroy all risky bugs. Some of you may have learned to bring water to a rolling boil for a minute at sea level and boiling it for an additional minute for every 1,000 feet (305 meters) above sea level. This is how I was first taught, and even if it’s not the standard, I don’t think you can go wrong using that method. It’s just that the CDC thinks it’s longer than necessary.

Joe Alton MD

Joe Alton MD

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