The reason why water boils under 100 degrees Centigrade or 100 degrees Celsius is because commercial airplanes are not pressurized to air pressure at sea level, but to about 8,000 feet or about 2,400 meters. At this effective altitude, water boils around 91.6 degrees Centigrade.

Contrary to popular belief, the water used for coffee and tea on airplanes is typically heated but not fully boiled. Aircraft coffee makers and tea urns generally heat water to temperatures between 180°F and 190°F (82°C to 88°C). This is below the 212°F (100°C) required to kill all bacteria and pathogens. Furthermore, numerous studies by health organizations have found that the "potable water" stored in the large tanks on airplanes can sometimes contain high levels of coliform bacteria or even E. coli, as these tanks are difficult to clean and sanitize perfectly. This is why flight attendants and seasoned travelers often advise against drinking the "tap" water from the galley or the lavatory. If you are concerned about water safety, it is always better to drink bottled water or order a canned beverage. Most airlines use bottled water for cold drinks but continue to use the tank water for hot beverages because the heating process kills many (but not necessarily all) microorganisms. If you have a sensitive stomach or are traveling with an infant, always ask for bottled water to be used in formula or for drinking.