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If there is suddenly a shower ban in the school gym, it could be because dangerous bacteria are lurking in the water. Again and again, sanitary facilities in Germany have to be temporarily closed because legionella have settled in the water. And they can be extremely dangerous for your health. Here you can find out everything you need to know about Legionella in drinking water.

1. What makes legionella so dangerous?

The dangerous rod-shaped pathogens viewed under the microscope: Legionella pneumophila at 1000x magnification in the laboratory of the ifp Institute for Product Quality The rod-shaped bacteria are almost always present in small quantities in water, in seawater as well as in the bathtub. They are also harmless in low concentrations. Legionella only become a problem when they get into the lungs in high concentrations, for example when water vapor that is contaminated with the bacteria is inhaled while showering or in the whirlpool. Because Legionella, especially the species Legionella pneumophila, can cause Legionnaires' disease, a severe form of pneumonia. The bacteria also get into the ambient air via the air conditioning system or water slides in the swimming pool. If the pathogens have accumulated in local recooling plants and sewage treatment plants, they can also spread via the water vapor generated there and become a health hazard for humans. However, according to the current state of knowledge, there is no risk of infection if water containing germs is drunk or used to wash hands.

2. How do Legionella multiply in drinking water?

Legionella feel particularly comfortable at temperatures between 20 and 50 ° C. At this temperature they multiply most. Their number hardly increases below 20 ° C and they are killed at temperatures above 60 ° C. Especially in artificial water systems such as water pipes in buildings, the bacteria find good conditions due to the temperatures there. "The growth of Legionella is encouraged by a corresponding supply of nutrients, for example a biofilm in water pipes or sediments such as rust and scale," explains Alexander Wolff, head of the water management department at the ifp Institute for Product Quality in Berlin.

The pathogens can multiply particularly well in standing water. Therefore, after a long absence - for example after a vacation - you should first let a few liters of water run off when you open the tap. In addition, experts warn against lowering the hot water temperature of the in-house drinking water system in the summer months. This saves energy and costs, but also creates ideal conditions for legionella to multiply in the water.

It can also be dangerous if the hot water cools down too much in the pipe on the way to the tap. Because heated drinking water at a low temperature is only hygienic if it is heated at the time of consumption, for example with a flow heater or a heat exchanger.

Samples are examined for Legionella in such Petri dishes. The pathogens form white "colonies" on the black culture medium. Consumers can order such a legionella test from

3. Up to 30,000 cases of Legionnaires' disease annually in Germany

Anyone who is infected with legionella can get legionellosis. The most important forms of legionnaires 'disease are Legionnaires' disease and Pontiac fever. Legionnaires' disease - a form of pneumonia - is usually associated with chest pain and headache, cough, chills and high fever. It can also lead to severe confusion. Pneumonia is often severe and lasts about four weeks. It is fatal in around one in ten people affected.

With Pontiac fever, the course of the disease is not that bad. It starts with symptoms like the flu. Typical are fever, malaise, headache and body aches, chest pain and cough are also possible. The disease usually goes away on its own within two to five days.

Even a small number of germs that have been inhaled can cause serious illnesses. Elderly people, people with a weakened immune system or chronic illnesses and babies are particularly at risk. If the number of germs is high, healthy people can also become infected with legionnaires.

Experts assume that Legionnaires' disease occurs 15,000 to 30,000 times a year in Germany alone. However, doctors often do not initiate a laboratory test for pneumonia. And without it, the cause of the pneumonia remains hidden. This is why it is difficult to obtain reliable figures on the frequency of Legionnaires' disease, despite the obligation to report.

4. How do you recognize Legionnaires' disease?

For patients suffering from Legionnaires' disease, it is vital to start the appropriate therapy as quickly as possible. Getting the correct diagnosis is not that easy. Because the symptoms do not have to be clear and can also indicate another form of pneumonia. Therefore, it is best for patients to consider for themselves whether they might have been exposed to a risk of infection with Legionella in their drinking water. For example, you should tell the doctor if you have recently been on vacation and stayed at a hotel. Because this can be an important clue that will help the doctor make a diagnosis.

In order to finally identify the disease beyond any doubt, the pathogens must be detected in the body. This is often quick and easy with a urine sample. However, only certain types of Legionella can be recognized in this way. The pathogens can also be detected in the lung secretions. Bacterial components or - much later - antibodies against the pathogen can possibly also be detected in the blood. If a Legionella infection is detected, this must be reported.

5. Again and again illnesses and deaths from regional Legionella outbreaks

Legionella first became aware of Legionella in July 1976 at the Bellevue-Stratford Hotel in Philadelphia (USA). There, 180 of 4,400 delegates fell ill at a congress. The disease claimed 29 lives.

Cooling towers can pose a legionella risk for the population. A dramatic legionella epidemic in Germany occurred in early January 2010 in the Ulm area in southern Germany. At that time, 64 people were infected with legionnaires' disease and five patients died. The authorities identified the cooling towers belonging to a block-type thermal power station as the cause. The power plant was still in trial operation in December 2009. That is why it was switched off again and again so that the legionella could multiply explosively in the lukewarm water of the cooling system. The pathogens got into the air. Whoever breathed it in became infected.

The most recent outbreak of legionnaires' disease in Germany with 165 cases of illness took place in Warstein in 2013. Three patients died at the time. A large-scale investigation showed that the germs had accumulated in local recooling plants and sewage treatment plants. The pathogens had got into the ambient air and had been inhaled by many people.

6. Protection of the population through new compulsory tests for evaporative cooling systems and cooling towers?

In order to avoid such cases in the future, the Federal Council passed the 42nd Federal Immission Control Ordinance (BImSchV) on June 2, 2017 to protect the population from outbreaks of legionnaires. The new regulation entails extensive notification, operating and monitoring obligations for the operators of evaporative cooling systems, cooling towers and wet separators. Because cooling systems contaminated with bacteria - just like legionella in drinking water - pose a risk to the population.

Accordingly, the operators from now on have to meet various requirements. For example, they have to provide proof of specialist knowledge and undertake to regularly train responsible persons. You must also check the cooling system regularly and have an inspection carried out by an expert or a type A inspection body every five years. The test reports are to be passed on to the appropriate authority. In addition, the cooling water must be checked regularly for Legionella. Regular microbiological monitoring of the cooling water is to be carried out by an appropriately accredited water laboratory.

These notification and investigation obligations enable the local authorities to act faster and more effectively in the event of a Legionella outbreak and to identify possible sources of infection more quickly.

7. What hardly anyone knows: Danger of infection from Legionella in dental practices

Legionella are transmitted through atomized or misted water. Therefore, you can not only become infected with legionnaires' disease while showering, through air humidifiers, taps or air conditioning. Caution is also advised at the dentist. This is because the water from the treatment chair reaches the patient's mouth and airways in atomized form.

"Even if standards are adhered to, the water-carrying systems, for example for transmission instruments, multi-function syringes, ultrasound for cleaning teeth or mouth rinses, are often colonized or contaminated by various microorganisms," explains Alexander Wolff. "When using the devices, the water can be misted and there is a risk of infection."

The water-carrying systems in dentists' chairs can be colonized by Legionella. In many practices, however, this is not checked, although the Robert Koch Institute recommends annual water tests for dental practices. That is why the Robert Koch Institute (RKI) recommends dentists to use only microbiologically harmless water during treatment in accordance with the "Infection Prevention in Dentistry - Hygiene Requirements". In addition, the water in dental practices should be regularly checked for Legionella by an accredited water laboratory The Robert Koch Institute emphasizes that contamination and microbial contamination of the water in dental practices can also occur if the fed-in water meets the requirements of the Drinking Water Ordinance.

Incidentally, dentists do not have a statutory examination obligation at this point. In some regions, the health authorities refer to the RKI recommendation as part of their practice monitoring. However, that depends heavily on the responsible office or federal state - there is no uniform handling across Germany. The drinking water tests recommended by the Robert Koch Institute are therefore mostly dependent on the dentists' own initiative and sense of responsibility towards their patients.

8. How can an infestation by the pathogen be prevented?

A possible multiplication of Legionella in drinking water should be avoided at all costs. Therefore, special technical rules apply to the establishment and maintenance of drinking water installations. But everyone can also help prevent legionella infestation: If the shower has not been used for a long time, it should be rinsed with hot water for a few minutes so that the water in the pipe can drain off. The best way to do this is to open the window and leave the room. "In addition, after a long absence, the drinking water should run off before it is used for food and drinks until fresh water flows in," advises Wolff. "The fresh water can be recognized by the fact that it comes out of the pipe a little cooler than the stale water." In addition, the jet regulators of the faucets (also known as aerators or mixing nozzles) should be cleaned and decalcified regularly - this removes the breeding ground for legionella.

In general, all water pipes in buildings should be used regularly so that there is no water standstill in the pipes. In systems for heating drinking water, the temperature controller should be set to at least 60 ° C. The water temperatures in the pipes should not fall below 55 ° C. Otherwise legionella can easily multiply there. However, it is essential to be aware of the risk of scalding from water that is too hot.

9. The "technical measure value" for legionella in drinking water and the consequences of exceeding it

In the German Drinking Water Ordinance, a so-called “technical measure value” of 100 colony-forming units (CFU) per 100 milliliters of water is specified for legionella. This “technical measure value” should be complied with without any problems, provided that all technical rules are observed. If, on the other hand, it is exceeded, this is an indication that something is wrong in the drinking water system and that appropriate measures should be taken. However, this value does not provide any information about the Legionella concentration above which an infection occurs in humans. Because so far there are no reliable findings. There is no such thing as a “limit value” that indicates the risk of infection from Legionella and that could be used as a guide.

If an examination of the drinking water reveals that 100 colony-forming units (CFU) per 100 milliliters are exceeded, this must be reported immediately to the responsible health department. In addition, depending on the pathogen concentration in the water, measures must be taken to avert the risk of legionella. To do this, the drinking water system must first be carefully examined. If the examination reveals an extremely high level of contamination, i.e. more than 10,000 CFU per 100 ml, shower bans are usually imposed as an immediate measure. In this case, the drinking water installation must also be comprehensively renovated.

10. Who does the statutory Legionella test obligation apply to?

For operators of large systems for heating drinking water, i.e. for hospitals, schools, kindergartens, hotels or nursing homes, the German Drinking Water Ordinance requires regular testing for Legionella. These facilities have to have their drinking water systems tested for the pathogens once a year. Even for landlords of houses with at least three apartments, housing associations and property management companies, there is an obligation to inspect if there is a corresponding large-scale system for heating drinking water, but only every three years. There is only no legal obligation for one and two-family houses. For residential complexes with condominiums, the following applies: As soon as even a single condominium in a residential complex is rented and is not used by the owner himself, a Legionella test must be carried out.

In the legionella laboratory of, the water samples from consumers and commercial customers are filtered. The germs remain on the filter and grow into visible bacterial colonies in the incubator, but there are exceptions: In buildings with drinking water systems without central heating or without a central hot water tank, no legionella test has to be carried out. This means that buildings with drinking water installations without hot water supply and systems in which only instantaneous water heaters are used do not have to be checked for Legionella in the water. Apartments or commercial spaces without showers or other aerosol-forming units are also not subject to the statutory inspection obligation.

11. How can Legionella be detected in drinking water?

Legionella can be detected by taking a water sample from the drinking water system. Since the bacteria feel particularly comfortable between 20 and 50 ° C, the Legionella test is usually carried out on the basis of a hot water sample. However, if the cold water does not cool down to below 25 ° C even after it has been running for a long time, a test for Legionella is also recommended in cold water.

If the landlord wants or has to have a legionella test carried out, the tenant is obliged to grant him or a specialist for legionella tests for sampling access to his apartment. Of course, the landlord must observe the statutory provisions on viewing rights. This means that he must not appear suddenly at his tenant's, but must announce his visit or the visit of the expert in good time and also give reasons.

To investigate Legionella in the water, samples are taken from several points in the drinking water system. Then the bacteria contained in the water are cultivated on a nutrient medium in a laboratory, i.e. multiplied so that the colonies can be counted after a few days. For an officially and judicially recognized Legionella test in accordance with the Drinking Water Ordinance, the sampling must be carried out by certified personnel.

If you are unsure whether Legionella may have accumulated in the water in the pipes of your own apartment, you should ask your landlord whether he has already examined the water. For their own control, however, each individual can have a legionella test carried out by a professional laboratory.