What is Legionella?

Legionella is a gram-negative, rod shaped bacteria found in freshwater environments world-wide.

Gram-negative bacteria feature an outer membrane or envelope that offers protection to the bacteria.

There is a potentially life-threatening pneumonia called legionellosis caused by Legionella, including the most severe type of infection, Legionnaires’ disease.

There are many different species of Legionella bacteria, and it is likely all could possibly cause human disease given the right conditions, however the Legionella pneumophila species has been shown to contribute the highest number of human infections.


Where is legionella bacteria found?

Legionella is found in all freshwater environments however in this natural state it is unlikely the bacteria would pose a threat to humans. As freshwater is drawn into human-made water systems the risk factors increase.

Some examples of human-made or engineered water systems that are particularly susceptible to legionella bacteria are drinking water systems (both human and animal), cooling towers, agricultural drip lines, and shower heads.

These water systems provide ideal grounds for legionella bacteria to multiply as often the water temperature is raised. The bacterium multiplies at temperatures in the range 25 – 42ºC, with an optimal temperature for growth of 35ºC. These raised temperatures result in rapid multiplication of the bacteria.

Legionellae also find refuge in biofilms that form on surfaces within a buildings water system. The biofilm offers protection, shelter, and the required nutrients for diverse bacterial colonies to form and flourish.

It has been suggested that the prevention of biofilm formation in a water system is the most effective measure of controlling legionella.


The effect of biofilms

Planktonic or freely suspended bacteria can travel throughout a water system, they can also become sessile and settle on a surface forming a colony called a biofilm.

Protozoa like Amoeba are single celled micro-organisms that also exist ubiquitously. Legionella cells can invade a wide range of protozoa hosts parasitically and multiply within them.

Legionella bacteria and other waterborne pathogens can reside and flourish in biofilms which offer both protection and access to nutrients, including waste from other micro-organisms.

Within building water systems, biofilm is found in pipe internal surfaces, plumbing fixtures, hvac equipment and areas where changes in temperature and water flow rate may be increased or reduced due to changes to plumbing or system design.

These conditions can often be ideal places for bacteria to colonize and form biofilms. [5]

Biofilm in pipework diagram

How does legionella bacteria spread?

Free floating, planktonic bacteria can spread throughout a water system and can settle to form a biofilm on surfaces within a water system.

Mature biofilms can also enter a lifecycle stage where micro-organisms exit the biofilm structure for dispersal back into the water system.

Large portions of a biofilm can also become detached and (depending on water flow rate) be relocated. This could be due to a physical disturbance or in some cases the use of a chlorine-based disinfectant. At regulatory compliant dose rates chlorine-based disinfectants cannot fully remove deeper layers of a biofilm but can cause surface damage providing a route for bacteria to escape.



What are the risks of legionella bacteria in a water system?

Once a water system has legionella multiplying within it there is a risk to human health.

Water contaminated with legionella can form into droplets that are small enough for people to breathe in. If these droplets are suspended into the air (aerosolised) via cooling towers, showers or decorative fountains for example they can be inhaled.

Drinking contaminated water is unlikely to cause a risk (unless the water accidentally gets into the lungs) however inhalation of contaminated aerosolised droplets poses a serious risk of potentially life-threatening pneumonia referred to as Legionellosis.

The most common Legionellosis is Legionnaires Disease which every human is potentially susceptible to however those at a higher risk are:

  • Over 45 years of age.
  • Smokers and/or heavy drinkers.
  • People with underlying health issues.
  • People with impaired immune systems.

Legionnaires disease symptoms usually begin 2-14 days after exposure and include:

  • Cough
  • Shortness of breath
  • Fever
  • Muscle aches
  • Head aches

Legionnaires Disease Symptoms

Further associated symptoms such as diarrhea, nausea and confusion are possible.

Although the disease is very serious treatments are possible with antibiotics. Hospital stays are mostly required with 9 out of 10 infected making full a recovery.



How to control legionella in water systems and prevent outbreaks?

The health and safety executive in the UK provide a wealth of information and guidance on the control of legionella. Their brief guide for duty holders identifies the risks and health & safety duties building controllers or operators are legally required to follow.

To prevent or control legionella risk:

  • Water becoming aerosolized or the release of water spray needs to be properly controlled.
  • Avoid water stagnation in the system. Use short pipe-lengths and remove redundant pipework.
  • Manage water temperature to avoid conditions that micro-organisms flourish in.
  • Keep the system and water within it clean.
  • Treat water to kill or inhibit growth of micro-organisms.



The HSE provide an approved code of practice and guidance relating to the control of legionella bacteria in water systems.

Dutyholders, employers and landlords who control premises are all required to comply with the legal duties set out by various acts and regulations such as the Health and Safety at Work etc Act 1974 (the HSW Act), the Control of Substances Hazardous to Health Regulations 2002 (COSHH) and Management of Health and Safety at Work Regulations 1999 (the Management Regulations).

To legally comply responsible parties must:

  • Identify and assess sources of risk.
  • Prepare a written scheme for controlling or prevention of risk.
  • Implement, manage and monitor control measures.
  • Keep records of the measures.
  • Appoint a competent person with authority and knowledge to ensure compliance.



Water treatment and legionella control options for hot and cold water systems

In HSG274 Part 2: The control of legionella bacteria in hot and cold water systems, The HSE publish guidance to prevent or control legionella exposure risk.

Initial basic considerations like regular movement of water in distribution pipework and regular flushing of outlets to prevent water stagnation are covered. However, the regimes they recommend to control legionella in a water system are either via temperature control or biocidal control.


Temperature control

Temperature control is the most common and traditional approach however this is often difficult due to water system complexity or the age of the building. Hot water must be stored at a minimum of 60C and distributed at a minimum temperature of 50C within one minute at outlets.

Temperature control requires routine monitoring and inspection and also presents a scald risk making it far from ideal.

Biocidal control

Biocidal treatments also require thorough monitoring and control however reduction of temperatures when replaced with water treatment measures reduces the scald risk and can provide energy savings.

It goes without saying that there can be no lapse in biocide treatment when temperatures are reduced however cost effective, automated constant dosing solutions are available to negate risk.

The biocidal water treatment options listed in HSE HSG274 pt. 2 are as follows:

  • Chlorine dioxide
  • Chlorine
  • Copper and silver ionization
  • Stabilised Hydrogen Peroxide



Legionella control options compared


Chlorine based disinfectants


  • Effective against planktonic bacteria in water
  • Cost effective (low unit cost)
  • Most common established chemistry


  • Hazardous disinfection by-products such as trihalomethanes (THMs).
  • Studies suggest THMs are carcinogenic to humans and animals.
  • Effective in a narrow pH range.
  • Not effective against Cryptosporidium.
  • Difficult manual handling and has a risk of forming a toxic gas.
  • Noticeable taint to taste and smell.
  • Dries and irritates skin and hair.
  • Can cause respiratory and eye irritation.
  • Cannot remove biofilm at regulatory compliant dose rates.
  • Corrosive to metals rubbers and fabric increasing repair and maintenance costs.
  • Decreases in efficacy at higher temperatures and becomes unstable above 45C.
  • Requires neutralisation before disposal.
  • Ineffective against amoebae.


Copper & Silver Ionisation


  • Not corrosive.
  • No harmful disinfection by-products.


  • Decreasing efficacy above a pH of 8.
  • Can leave behind a grayish residue.
  • It is possible for micro-organisms to build up a resistance.
  • Efficacy can be affected by the temperature and water composition.
  • Difficult to control legionella within regulatory silver content levels.


Stabilised Hydrogen Peroxide


  • Highly effective at removing biofilms.
  • Bactericidal, virucidal, sporicidal, fungicidal, algacidal and amoebicidal.
  • No irritation, taste, smell or taint at dose levels.
  • No toxic disinfection by products, just water and oxygen.
  • No possible toxic gas formation
  • No neutralization requirements at dose rates.
  • Non corrosive at dose rates.
  • Effective at a wide range of pH and temperatures (retains stability).
  • Effective against Cryptosporidium.


  • Possible higher purchase cost than traditional chemistries like chlorine (offset by reduced repair and maintenance costs).
  • Undiluted product requires safety precautions.


What is the best treatment for legionella in a water system?

As is clear in the comparison tables above Stabilised Hydrogen Peroxide compares very favourably to the other biocidal treatments the HSE published in the approved code of practice.

EndoSan is the market leading stabilised hydrogen peroxide. Due to its strong oxidizing properties stabilised hydrogen peroxide can be used to disinfect against bacteria, viruses, mould, fungi, and more. Also, at correct dose rates it can penetrate and remove biofilms making it the ideal water treatment chemical for legionella control.

EndoSan is proven to kill legionella bacteria according to EN 13623:2010-12 – Quantitative suspension test for the evaluation of bactericidal activity against Legionella of chemical disinfectants for aqueous systems.

If you have any questions relating to controlling legionella in a water system using EndoSan Stabilised Hydrogen Peroxide please contact the team who are on hand and ready to assist.



[1] Fields, B. S., Benson, R. F., & Besser, R. E. (2002). Legionella and Legionnaires’ disease: 25 years of investigation. Clinical microbiology reviews, 15(3), 506–526. https://doi.org/10.1128/CMR.15.3.506-526.2002

[2] Marston, B. J., Lipman, H. B., & Breiman, R. F. (1994). Surveillance for Legionnaires’ disease. Risk factors for morbidity and mortality. Archives of internal medicine, 154(21), 2417–2422.

[3] Katz, S. M., & Hammel, J. M. (1987). The effect of drying, heat, and pH on the survival of Legionella pneumophila. Annals of clinical and laboratory science, 17(3), 150–156.

[4] Rogers, J., Dowsett, A. B., Dennis, P. J., Lee, J. V., & Keevil, C. W. (1994). Influence of temperature and plumbing material selection on biofilm formation and growth of Legionella pneumophila in a model potable water system containing complex microbial flora. Applied and environmental microbiology, 60(5), 1585–1592. https://doi.org/10.1128/aem.60.5.1585-1592.1994

[5] Michael Taylor; Kirstin Ross; Richard Bentham (2009). Legionella, Protozoa, and Biofilms: Interactions Within Complex Microbial Systems. , 58(3), 538–547. doi:10.1007/s00248-009-9514-z

[6] Rumbaugh, K.P., Sauer, K. Biofilm dispersion. Nat Rev Microbiol 18, 571–586 (2020). https://doi.org/10.1038/s41579-020-0385-0

[7] Centers for Disease Control and Prevention – Legionairres Disease Causes and Spread https://www.cdc.gov/legionella/about/causes-transmission.html

[8] The Health & Safety Executive – What is Legionnaires’ disease? https://www.hse.gov.uk/legionnaires/what-is.htm

[9] The Health & Safety Executive – Legionnaires’ disease – A brief guide for dutyholders https://www.hse.gov.uk/pubns/indg458.htm

[10] The Health & Safety Executive – Legionnaires’ disease. The control of legionella bacteria in water systems Approved Code of Practice and guidance https://www.hse.gov.uk/pubns/books/l8.htm

[11] The Health & Safety Executive – Legionnaires’ disease Part 2: The control of legionella bacteria in hot and cold water systems https://www.hse.gov.uk/pUbns/priced/hsg274part2.pdf


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