If you look up articles about saving money around your home, you’ll more often than not find a tip instructing you to turn down the thermostat on your home’s water heater. 

The problem is the near ubiquitous presence of Legionella bacteria in water supplies. At temperatures of 68–122° F, the bacteria thrives and multiplies rapidly. Then, the next time you take a shower or use the water in your home in any application where contaminants in it becomes aerosolized, you’re at risk of breathing in the bacteria which can lead to a particularly nasty form of atypical pneumonia.

Because lower water temperatures encourage rapid growth of the bacteria, home owners have since been encouraged to always keep their hot water heaters turned up hot enough to effectively kill off the bacteria. At just above 122° F, it can take hours for all of the Legionella bacteria in a tank to die off, but above 140° F it dies off within minutes, and above 158° F the bacteria dies almost instantly.

Read more: http://www.howtogeek.com/trivia/homeowners-are-encouraged-to-keep-their-hot-water-tanks-set-higher-to-prevent/?answer=2