Home > Legionella and Hot Water > Water Heater History
Legionella and Hot Water
Hot water helped clean the American house in the 1900s. Later, tanks were set to a “safe” warm temperature so no one would get burned. That same warmth let bacteria live inside the tank. This page is about that second history.
What Legionella is
Legionella is a kind of bacteria that lives in water. It is not a virus. It does not grow in your body and then spread from person to person the way a cold does.
It grows in warm, still water — inside tanks, pipes, showerheads, hot tubs, and the cooling systems on big buildings.
People usually get sick by breathing in tiny drops of water, not by drinking it. Shower mist is the common way at home. A hot-tub spray can do it too.
Two illnesses can follow:
-
Legionnaires’ disease — a serious pneumonia (lung infection). It can put people in the hospital. Older adults, smokers, and people with weak immune systems are at higher risk.
-
Pontiac fever — a milder, flu-like illness from the same bacteria. It usually passes on its own.
Most healthy people who meet Legionella never get sick. The bacteria still matter because the tank can grow them quietly for a long time.
1976: when the bacteria got a name
In July 1976, American Legion members met at the Bellevue-Stratford Hotel in Philadelphia. Afterward, 182 people got a strange pneumonia. 29 died.
Doctors did not know the cause at first. The news called it “Legionnaires’ disease.” In early 1977, scientists found a new bacterium and named it Legionella pneumophila. In that hotel, it was growing in the air-conditioning cooling tower, then spreading as mist through the building.
That was the famous start. It was not a home water heater.
Later work showed the same bacteria also grow in hot-water tanks, pipes, and showers. Hospitals, hotels, apartments, and regular houses have all had cases. The water heater became part of the medical story after 1976, not during it.
Warm is the problem, not “hot”
Legionella does not like ice-cold water. It also does not like truly hot water. It likes the middle.
So a tank that feels “safe” on your skin can feel perfect to the bacteria.
That is the fight in American plumbing:
-
In 1988, water-heater makers followed a Consumer Product Safety Commission idea: preset tanks around 120°F so kids and older people would not get burned. At 140°F, skin can burn in seconds. At 120°F, it takes much longer.
-
Health agencies later said: store water at 140°F (60°C) if you want to keep Legionella down. Keep water moving through the pipes at 120°F or hotter.
Both sides are right about their own danger. The engineering fix is a mixing valve at the faucet or shower. The tank stays hot. Cold water is blended in at the tap so the water you touch is cooler. Your glossary already names this: an anti-scald / thermostatic mixing valve.
Why the water heater is a good home for bacteria
A storage tank is a dark, wet tank that sits for hours. That is not automatically dirty. It becomes a problem when four things stack up.
1. The bottom of the tank is cooler and dirtier than the thermostat thinks.
Minerals and rust settle as sediment. Hard water — common warmer states like Arizona — makes more of it.
The thermostat reads water higher in the tank. The sludge layer can stay in the growth range even when the dial looks fine.
2. Electric tanks are often worse than gas tanks.
A gas burner heats from below and warms the bottom. An electric element hangs in the water. It may never heat the muck underneath. Studies of homes (including a large Quebec study) found Legionella much more often in electric heaters than in gas or oil ones.
3. Water that sits still gets friendlier to germs.
A guest bath, a vacant house, a capped pipe that goes nowhere (“dead leg”) — the water cools into the danger band. Chlorine, the leftover disinfectant from the city, also fades faster in hot water.
4. The last few feet of pipe can be the dirtiest.
Showerheads, hoses, and faucet screens grow a thin slime called biofilm. Bacteria live in that slime. You meet it as mist when you shower.
Other bacteria (and why viruses don’t belong here)
Legionella is the headline. It is not the only germ that likes plumbing.
Nontuberculous mycobacteria (NTM) — relatives of tuberculosis bacteria, but not TB. They can cause lung infections, especially in people who already have lung disease. Homes with tanks below about 125°F turn them up more often than homes above about 130°F. Some types even tolerate water around 113°F. Shower mist is again the usual route.
Pseudomonas aeruginosa — lives in slime inside pipes, showerheads, and hot tubs. It can cause “hot tub rash,” ear infections, and much more serious illnesses in people with weak immune systems.
Other plumbing bacteria you may see in research papers: Stenotrophomonas, Acinetobacter, Aeromonas, and a few more. They rarely bother healthy people. They matter in hospitals, in home care, and for anyone whose body already has a harder time fighting infection.
Amoebae — tiny animals, not bacteria. Acanthamoeba can live in warm pipes and carry Legionella inside it. That is one reason the bacteria are hard to kill. Flushing or heating the water does not always reach what is hiding in the slime.
Viruses do not run this story. A virus cannot multiply in a steel tank. It needs a living host. Heat tends to knock viruses down, not grow them. Germs like norovirus or hepatitis A can contaminate drinking water after sewage problems. That is a different page.
Naegleria fowleri, the so-called “brain-eating amoeba,” likes very warm freshwater. Home heaters are not a typical U.S. source. The practical warning is smaller: do not use untreated tap water in a neti pot or sinus rinse. Use distilled, sterile water that was boiled and then cooled.
What actually helps
This is shop advice, not a medical exam.
-
Keep the tank hotter than the tap. Store water around 130–140°F. If children or older adults live in the house, mix it cooler at the fixture so no one is scalded.
-
Drain the sediment. Follow the maker’s schedule. In hard-water country, that often means once or twice a year, not “someday.”
-
Run unused hot taps. After a vacation or in a guest bath, let the hot water run until it is actually hot.
-
Clean shower heads and faucet screens. Vinegar helps if they are crusted with scale.
-
Treat a long shutdown as stagnant water. A house that sits empty is not the same as a house in daily use. Flush it.
If someone in the home is older, has lung disease, or has a weak immune system, ask a doctor about heater temperature versus burn risk. That is a personal call, not a slogan.
How does this fit the rest of the site?
The home page already says reliable hot water improves public health. That was true. Cleaner bodies, cleaner dishes, cleaner laundry.
This page is the sequel. Once the tank became normal in American houses, the country also had to learn how warm water behaves when it sits. Codes, mixing valves, and tank setpoints are part of that lesson. So is the 1976 outbreak, even though that first famous case was a hotel cooling tower, not a basement heater.
Related on this site
-
Technical Explainers & Glossary (mixing valve, sediment, thermostat range)
Other known or related
1. Mycobacterium xenopi
Call this out by name, not just “NTM.” It likes hotter water than most mycobacteria (around 113°F / 45°C) and shows up in hot-water taps, especially in hospitals. That is a true hot-water organism, not a generic tap-water note.
2. “Hot tub lung” (MAC hypersensitivity)
Same family as M. avium, different disease. It is an allergic-type lung reaction to mist from a dirty hot tub — sometimes showers too — not the usual slow NTM lung infection. One short subsection under related devices.
3. Burkholderia cepacia complex
On the CDC premise-plumbing list. Serious for people with cystic fibrosis or weak immune systems. Hospital water is more than home tanks, but it belongs in the research.
4. Elizabethkingia (E. meningoseptica, E. anophelis)
Rare. Documented in healthcare water: sinks, baths, and contaminated equipment. Worth a line in a “hospital water” note, not a home-tank headline.
5. Hot tubs, humidifiers, CPAP / nebulizers
Same bacteria, different machines. Pseudomonas “hot tub folliculitis” (itchy rash) is the common one.
Sources and limits
Main public sources: CDC guidance on Legionella in building water, EPA notes on Legionella indoors, OSHA on domestic hot-water systems, and the CPSC scald recommendation that pushed many tanks toward 120°F.
This page is for learning. It is not a diagnosis, and it is not a substitute for local code or a licensed plumber.
Historical systems on this site should not be copied as modern safety advice.