How does water kill bacteria?

The heat energy (in the water) is transferred to the bacterial cells, inactivating proteins needed for the bacteria to function and reproduce. The proteins are denatured, meaning the structure is changed, much like a fried egg.

Water begins killing some bacteria at about 120ºF. Pasteurization takes place at 140ºF. Water temperatures between 140ºF - 165ºF will kill most bacteria considered harmful to humans. Boiling water (212ºF) will kill most bacteria.

The hotter the water, the less time it takes to kill bacteria. Legionella (the cause of Legionnaires' disease) will grow in water at temperatures from 68°F - 122°F. At 131°F, Legionella die within 5 to 6 hours. At 140°F, Legionella die within 32 minutes. At 151°F, Legionella die within 2 minutes. 158°F - 176°F is considered the disinfection range for Legionella.

Water temperatures over 122ºF can burn the skin. Most hot water heaters are set somewhere between 120ºF - 130ºF. Therefore, when you wash your hands with soap and hot water, you aren't killing many germs... you're just washing them down the drain.