Above 50 degreesC (122 degreesF They can survive but do not multiply 35 to 46 degreesC (95 to 115 degreesF Ideal growth range 20 to 50 degreesC (68 to 122 degreesF Legionellae growth range.  Hydro Quebec, (admittedly an electric utility) To reduce the risk of burns from hot tap water, the temperature setting on the water heater can be turned down.  Even at 60 degreesC - the setting on most electric water heaters - an estimated 25% of all water heaters are contaminated by legionella bacteria.  Legionella bacteria tend to grow in the lower temperatures at the bottom of water heater ; such bacteria can cause a form of pneumonia.  The Canada Safety Council: In 2000, the Walkerton disaster had sent a wake-up call about the safety of Canada's drinking water.  For example, temperatures under 50 C may increase the risk of Legionnaires' disease, a form of pneumonia, due to bacterial growth in the tank.  Legionella, named for an outbreak that killed 34 at an American Legion gathering in 1976, thrives in warm environments ranging from 77 - 113 degreesF, especially in electric water heaters. 
- Is it Safe To Turn Down Your Water Heater Temperature? (treehugger)
- Danger in Your Water Heater, Maybe (Green Daily)
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