Legionnaires’ disease – why should I worry, I’m not in the Foreign Legion?

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There was a gathering of U.S. war veterans arranged by the American Legion in a hotel in Philadelphia in 1976, and 221 of those attending fell ill with severe pneumonia, with 34 succumbing. What caused it was a previously unknown pathogenic microorganism Legionella and these bacteria were found in the lungs of the patients. The bacteria were assumed to have spread via the ventilation system of the hotel.

Where Legionella are found?

Legionella bacteria exist everywhere in nature, but usually in small numbers. The health risks are caused by the fact that they may live their modest lives in water supply systems, where there are few competing bacteria. There they may then multiply and reach unnaturally high concentrations. In natural waters, one litre of water may contain at most a few hundred bacteria, whereas water from the worst water systems is teeming with millions of bacteria. The most troublesome water systems are warm water systems, whirlpool-types of baths and humidifiers. The newest detected sources of legionella bacteria and also infections have been the active sludge basins where waste water is being treated. Aerosols, small droplets of water containing bacteria, dispersed in air are a special risk.

Illnesses caused by Legionella

The most severe disease caused by Legionella is pneumonia which may be fatal even when treated. The number of patients with the disease is not known, because the cause may go undiagnosed. It has been calculated that 1 to 2 percent of pneumonia cases throughout the world may be caused by Legionella. Most often frail or elderly people contract the disease just like the old Second World War veterans in Philadelphia, but occasionally even previously healthy people become sick.

Temperature is important

Legionella bacteria grow in 20–45ºC, the optimal temperature for them to flourish is 30–40ºC. Therefore in order to prevent their growth, the hot water temperature should be at least 50ºC, but to be certain, the initial temperature of the outgoing hot water should be at least 60–65ºC to guarantee a high enough temperature also in the piping system. On the other hand, cold water should be as cold as possible to discourage Legionella growth.


The material used for piping can have an influence on the growth of Legionella just as it can for other bacteria. A new copper surface inhibits bacterial growth; therefore replacing copper piping with plastic materials is not invariably a good idea, although even the copper surface will lose its effect as it ages. It is also important that piping is not too long and there should not be dead spaces with slow flow rates. To prevent bacterial growth, the shorter and simpler the water piping systems, the better. This is especially true for systems used only every now and then.

Cooling towers and air humidifying systems are a special risk, because respirable aerosols are easily formed. In cooling systems, some kind of biocide treatment is needed to keep them clean of Legionella. Water in swimming pools and spa pools should be properly disinfected. Continuous care and cleaning are also essential in any water system to avoid the risks posed by Legionella.

Legionnaires’ disease caused by Legionella bacteria is a “new” infectious disease, often causing symptoms of pneumonia. It may be fatal but it is not transmitted from one person to another. The most common sources are aerosols from showers and taps, and if hot water is not hot enough, the bacteria are able to grow in the pipelines.

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