Bottom line: It is the temperature at which the water boils at a given altitude (atmosphere pressure) that is the key, NOT the time it takes to actually cook the food or boil water to make it safe.
When it comes to ‘safe water’ boiling, remember that it is NOT just the altitude we have to consider, it’s the type of contaminant we are trying to get out of the water.
Basically, in this scenario, boiling water is only good for bacteria, virus and microscopic life that we will be removing. Metals and many human-created chemicals will NOT be removed. In fact some metals, like lead and mercury will be concentrated by boiling, making the water even more deadly.
Many of us have no clue at what altitude we live at and in an emergency this is even more so. So how do we tell?
For finding out what altitude you are at you can go to the USGS web site or just do an internet search on “altitude for [your city]” or the altitude of any address @ http://veloroutes.org/elevation/. There are also several iPhone apps that utilize your GPS coordinates to calculate your elevation. Once you have established your altitude, adjust your ‘boiling’ times accordingly or you can address the actual temperature of the water itself.
Then when the water reaches 212 F (boiling at sea level) I give the normal times. For safe water this is instantaneous to 1 minute at 212 F.
- Tepid Water - 85 to 105°F. The water is comparable to the temperature of the human body.
- Warm Water - 115 to 120°F. The water is touchable but not hot.
- Hot Water - 130 to 135°F. The water is too hot to touch without injury.
- Poach - 160 to 180°F. The water is beginning to move, to shiver.
- Simmer - 185 to 200°F. There is movement, and little bubbles appear in the water.
- Slow boil - 205°F. There is more movement and noticeably larger bubbles.
- Real boil - 212°F. The water is rolling, vigorously bubbling, and steaming.
Note: Adding a little salt to water will cause the water to boil at a slightly higher temperature, which can be helpful while cooking especially at high altitudes.
To download this quick single page ‘cheat sheet’ see Altitude & Water Boiling Points chart. This chart includes some major US cities as altitude references.