Legionella in swimming pools and spa areas - what should we know and what risks does it pose?

Legionella in swimming pools and spa areas - what should we know and what risks does it pose?

Legionella, widely known for its resistance to adverse environmental conditions, is a major problem in indoor hot water systems, as well as in relaxation areas in many complexes. What is it, what can cause it and how can it be removed? You will find out by reading the article.

Legionella - what is it and where does it occur?

Legionella bacteria are commonly found in the human environment. Under natural conditions, they live in moist or watery environments, such as soil, mosses and lichens. When conditions are favourable for these bacteria, they multiply rapidly, particularly in man-made reservoirs, including hot water systems.

Legionella in water has optimal conditions for growth, particularly at temperatures between 25 and 45oC. These bacteria often colonise both hot and cold water mains, so swimming pools, spa areas and other hot water installations maintain favourable conditions for their proliferation. Transferred into the water supply system, they can multiply rapidly and produce bacterial biofilms. A bacterial biofilm is an agglomeration of both bacteria and fungi together with other organisms and inorganic matter. These clusters of micro-organisms produce substances that bind them to each other and to the substrate to which they adhere.

Bacterial biofilms create ideal conditions for the growth of Legionella bacteria. They find nourishment and protection from disinfectants as well as thermal exposure. Often, after thermal or chemical disinfection processes of indoor hot water systems, the system is recolonised by bacteria that have survived in the biofilm.

Legionella in water systems, how to get rid of it?

Legionella bacteria show the ability to adapt quickly in unfavourable conditions, which, together with their ability to form biofilms, causes problems with their complete removal from the installation. For this reason, it is important to choose the right method of cleaning the installation (physical or chemical), particularly if a Legionella test reveals the presence of Legionella in the water. Each method has its own advantages and disadvantages, which are summarised in the table below:



Chlorination (ClO2)

Chlorination (NaOCl)

Ionisation (Cu2+ and Ag+)

UV light (disinfection)

Thermal disinfection (70°C)

Biofilm removal


No (at disinfection doses of 2/3 mg/l)

Yes, depending on the dose



Effectiveness against Legionella

Short-term (need for constant dosing)


LONG-TERM, dose-dependent (effective 0.2-0.8 mg Cu/l and 0.02-0.08 mg Ag/l - for about 1.5 to 3 months)

Short-term (up to approx. 2 min)

Short-term (up to approx. 1 week)

Impact on the surroundings

None (at low doses)

Proven carcinogenic effect

None (at low doses)


Risk of burns


A good method to help keep the system cleaner for longer is to ensure flow in all parts of the system. It is important that the expansion vessel of the water system is flowable, then it is not a reservoir of standing water and makes it more difficult for bacteria to colonise.

Using a method that is not strong enough only leads to a partial and temporary removal of the problem. This is because the aforementioned recolonisation by bacteria that have survived in the biofilm occurs.

Legionella in water and risk of disease

Microbiological testing of water for Legionella from swimming pools, paddling pools or similar places is one of the most important microbiological tests, as it allows us to detect the bacilli of these dangerous bacteria and thus prevent infection.

Infection can occur when bacteria enter the respiratory tract. It is worth bearing in mind that Legionella bacteria are not transmitted from one person to another, and that infection occurs through the inhalation of water aerosols (water particles containing bacteria suspended in the air). These aerosols are formed in places such as:

  • showers,
  • spa-type swimming pools,
  • jaccuzi,
  • fountains
  • air humidifiers.

In addition, aerosols are also formed in the natural environment (rivers, lakes, damp ground), although their numbers in such places are usually low, and high numbers are found in inadequately maintained artificial water systems.

Legionella infections can take the form of Pontiac fever or Legionnaires' disease.

Legionellosis and its forms

There are three main forms of Legionellosis, so-called extrapulmonary mild forms, acute forms and Legionnaires' disease (pneumonia). It develops 5-6 days after infection and often begins with typical flu-like symptoms (fever, headache and muscle pain), followed by coughing and breathing difficulties, which progress to severe pneumonia.

In Poland, legionellosis was included in the list of infectious diseases in the Act of 5 December 2008 (Journal of Laws 2008, item 1570, as amended). on combating infections and infectious diseases in humans.

In the Regulation of the Minister of Infrastructure of 12 April 2002 on the conditions to be met by buildings and their location, §120 contains regulations concerning hot water installations. Among other things, it stipulates that the installation should make it possible to achieve a hot water temperature of not less than 55 °C and not more than 60 °C. In addition, it must be made of such materials and designed in such a way that it can be periodically or continuously disinfected by chemical or physical methods. The regulation also specifies the conditions for periodic thermal disinfection of the network - ensuring a temperature of not less than 55 °C and not more than 80 °C at the tapping points.


Water testing for Legionella bacteria

SGS Poland has laboratories offering accredited testing for bacteria in water, including the presence of Legionella bacteria in accordance with PN-EN ISO 11731: 2017-08. We also offer identification of the main serogroups of Legionella spp.

Buy special testing package in our store: https://sklep.sgs.com/products/legionella-water-testing-package


[1] Chudzicki J.: Wybrane metody usuwania i unieszkodliwiania bakterii Legionella w instalacjach basenowych, Instal 6/2003.

[2] Gałdysz I. i inni,: Ocena zagrożenia bakteriami Legionella spp. W domach pomocy społecznej w Polsce w latach 2009-2013, Hygeia Public Health 2018, 53(1): 74-7

[3] Rygała A, Kręgiel D., Legionella spp. – występowanie i wykrywanie. Laboratorium – Przegląd Ogólnopolski, 4:35-40 . 2019

[4] Szczerbiński R. i inni., Occurrence of Legionella sp. bacteria in hot utility water systems in health care centers and residential and nursing homes in the Podlaskie voivodeship. Probl Hig Epidemiol 2011, 92(4): 920-923

[5] Wanot B, Krzypkowska A., Legionella – wciąż aktualny problem. Technologia wody 5/2018.

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