Answer: Legionellosis

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. That tip, if written by a knowledgeable and responsible party, will always include a caveat, however: don’t turn it down too far.

If you have no problem with lukewarm showers, why can’t you turn your water heater down super low? 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.

The bacteria’s name, the medical name of the symptoms (legionellosis), and the common name of the infection (“Legionnaires’ disease”), are all derived from the first publicly identified outbreak in July of 1976 among attendees of an American Legion convention at a hotel in Philadelphia. Of the 221 reported cases, 34 patients perished.

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.