In high-altitude cities, water boils at a lower temperature than the standard 212 degrees Farenheit. For example, in Denver the boiling point is about 203 degrees. Can one kasher utensils with hagalah under such conditions? What if the previous use of that utensil was in a lower-altitude city, and actually reached 212 degrees? And if such hagalah is indeed acceptable, where do we draw the line (e.g., what about on Mount Everest, where the boiling point is 156 degrees)?
Per the OU Daf HaKashrus Volume 15 No 6 dated March 2008 this question was answered by Rabbi Gersten as follows "In a near sea level environment water boils at 212 degrees Fahrenheit. at higher elevations water boils at lower temperatures. In Denver for example the average boiling temperature is 202 degrees Fahrenheit. when Hagalah is performed in high elevations, it is sufficient to heat the water until it boils, provided the temperature is Rochsim which is above 190 degrees Fahrenheit".
Rabbi Moshe Tendler says that the principle of "as it absorbs, so it exudes" (k'bol'o kach polto) means METHOD of absorption, not temperature. Fire kashering for things that became unkosher through dry heat (roasting, grilling) and boiling for liquids. His example was that a spoon that became unkosher on a camping trip to the Dead Sea through immersion in liquid (meat spoon in hot milk) can certainly be kashered by boiling water in Jerusalem, even though the water is boiling at a lower temperature.