Glossary

The terms that recur in Legionella guidance, in plain English. Each links to where we cover it in depth. For the long-form version with worked examples, see the A-Z jargon-buster.

ACoP L8: The HSE’s Approved Code of Practice and guidance “Legionnaires’ disease: the control of Legionella bacteria in water systems”. It sets out the legal duties and the standard of control expected for managing Legionella risk in workplaces and other premises.; ACoP L8: understanding the UK Legionella Code of Practice Is ACoP L8 law? Approved Codes of Practice vs guidance explained L8 vs HSG274: what's the difference and how they fit together
Aerosol: A fine spray of water droplets small enough to be breathed deep into the lungs, produced by showers, taps, cooling towers, spa pools and similar equipment. People catch Legionnaires’ disease by inhaling contaminated aerosol, not by drinking the water.; How Legionella spreads through water systems Can you catch Legionnaires' disease from your home shower?
Approved Code of Practice (ACoP): Guidance approved by the HSE on how to comply with a piece of health and safety law. It has a special legal status: you do not have to follow it, but if you do not and something goes wrong, you must be able to show you achieved compliance some other way.; Is ACoP L8 law? Approved Codes of Practice vs guidance explained ACoP L8: understanding the UK Legionella Code of Practice
Augmented care: Healthcare settings where patients are especially vulnerable to waterborne infection — for example those who are severely immunocompromised, in intensive care, or undergoing certain treatments. Water serving augmented care areas needs tighter control, monitoring and sometimes point-of-use filtration.; Point-of-use water filters for Legionella in high-risk and augmented-care settings Legionella in healthcare: HTM 04-01 and NHS guidance Legionella prevention in hospitals and healthcare facilities
Biofilm: A thin, slimy layer of micro-organisms that forms on the inside surfaces of pipes, tanks and fittings. Biofilm shelters Legionella from heat and disinfectants and supplies nutrients, which is why it is central to colonisation and so difficult to eradicate.; Biofilms: how Legionella hides in plumbing systems Legionella in biofilms: why eradication is difficult
Blind end: A capped, dead-ended section of pipe with no outlet at all, often left behind when equipment is removed or a system is altered. Because nothing ever draws water through it the water is permanently stagnant; unlike a dead leg, which still feeds a little-used outlet, a blind end serves nothing and should generally be cut out.; Dead legs and blind ends: how to find, assess and remove them Neglected water systems: the danger of stagnation
BS 8580-1: The British Standard that gives a framework for carrying out a Legionella risk assessment, including how to judge assessor competence and how to structure the assessment. It is widely used as the benchmark for what a good risk assessment should contain.; BS 8580-1: what a good Legionella risk assessment should include
Calorifier: A vessel that heats and stores hot water, usually by passing it over a heating coil; in commercial systems the term is broadly interchangeable with a hot water cylinder. Sludge and scale collecting at the base can shelter Legionella, so calorifiers need temperature checks, periodic inspection and draining off.; Calorifier maintenance for Legionella: inspection, drain-off and sludge removal Hot water storage: preventing Legionella in tanks and cylinders Temperature stratification in hot water cylinders
CFU per litre: Colony-forming units per litre, the unit a laboratory uses to report how many live, culturable Legionella bacteria it grew from a water sample. UK guidance for hot and cold water systems commonly treats counts above around 100 CFU/L as needing review and resampling, with higher counts (commonly above about 1,000 CFU/L) prompting more urgent action — but your risk assessment and lab set the exact response.; Understanding lab results: CFU counts and what they mean Interpreting Legionella test results and counts
Chlorine dioxide: A disinfectant dosed continuously as a secondary control in some water systems, valued for working across a range of pH values and for penetrating biofilm. It is generated on site and needs careful dosing and monitoring to keep the residual within safe limits.; Chlorine dioxide for Legionella control: how it works, dosing and when to specify it Secondary disinfection for Legionella compared: chlorine dioxide vs copper-silver vs UV vs monochloramine
Competent person: Someone with the training, experience, knowledge and other qualities needed to carry out a particular Legionella task correctly, whether assessing risk or doing remedial work. Competence is about demonstrable ability for the specific task, not simply holding a certificate.; What is a 'competent person' for Legionella, and how do you prove it?Who is qualified to perform a Legionella risk assessment?
Cooling tower: Equipment that rejects heat from air-conditioning or industrial processes by evaporating water, generating large volumes of aerosol as it does so. Because they can disperse contaminated droplets over a wide area, cooling towers are among the highest-risk systems and in the UK must be notified to the local authority.; Cooling towers and evaporative condensers: high-risk systems HSG274 Part 1 explained: controlling Legionella in cooling towers and evaporative condensers Cooling tower notification duties and local authority registers
Copper-silver ionisation: A secondary disinfection method that releases small quantities of copper and silver ions into the water to suppress Legionella. It needs electrode upkeep, regular monitoring of ion levels and attention to pH, and its effectiveness can fall if it is not properly managed.; Copper-silver ionisation for Legionella: effectiveness, electrode upkeep and cost Emerging treatments: UV, copper-silver ionisation and more
Dead leg: A length of pipework serving an outlet or fitting that is used rarely or not at all, so water sits in it and can drift into the temperatures where Legionella multiplies. Dead legs are one of the most common risk-assessment findings and are usually best removed back to the circulating main, or kept flushed if removal is not practical.; Dead legs and blind ends: how to find, assess and remove them Expansion vessels: the hidden dead-leg that fails Legionella risk assessments Neglected water systems: the danger of stagnation
Duty holder: The person or organisation with legal responsibility for managing Legionella risk, usually whoever is in control of the premises or water system — for example an employer, building owner or landlord. The duty holder must make sure risks are assessed and controlled, though they can appoint competent people to carry out the work.; Duty Holder and Responsible Person roles in Legionella management Managing agent or landlord: who actually holds the Legionella duty?
Evaporative condenser: A close relative of the cooling tower that cools a refrigerant or process fluid inside a coil while spraying water over it and drawing air through. Like cooling towers it produces aerosol, ranks as a high-risk system and is notifiable to the local authority.; Cooling towers and evaporative condensers: high-risk systems HSG274 Part 1 explained: controlling Legionella in cooling towers and evaporative condensers
Expansion vessel: A pressure vessel with a flexible diaphragm that absorbs the expansion of heated water in sealed systems. Because water can sit largely undisturbed behind the diaphragm, an expansion vessel acts as a hidden dead leg and is a frequent risk-assessment failing unless it is the right type and properly maintained.; Expansion vessels: the hidden dead-leg that fails Legionella risk assessments Dead legs and blind ends: how to find, assess and remove them
HSG274: The HSE’s technical guidance that supports ACoP L8, published in three parts covering cooling towers and evaporative condensers, hot and cold water systems, and other risk systems. It provides the practical detail on how to assess and control Legionella.; HSG274 guidance explained: practical Legionella control HSG274 Part 2 explained: controlling Legionella in hot and cold water systems L8 vs HSG274: what's the difference and how they fit together
HTM 04-01: The Health Technical Memorandum on safe water in healthcare premises, setting out water-safety requirements for the NHS and similar settings. It goes beyond general guidance to address higher-risk patients and complex hospital water systems.; Legionella in healthcare: HTM 04-01 and NHS guidance Healthcare water safety roles explained: AE(W), AP, CP and RP under HTM 04-01
Hyperchlorination (shock dosing): A one-off disinfection that doses a water system with a high concentration of chlorine for a set contact period to kill bacteria, typically after contamination, major work or commissioning. It is a corrective measure, not a substitute for ongoing temperature or treatment control.; How to shock-chlorinate (disinfect) a water system: a step-by-step procedure Cleaning and disinfection after remedial work: thermal and chemical approaches
Legionella pneumophila: The species of Legionella responsible for most cases of Legionnaires’ disease and the main focus of water testing. It is one of many Legionella species, but its serogroup 1 in particular causes the largest share of reported illness.; Legionella species and serogroups beyond L. pneumophila What is Legionella? An introduction to the bacteria and disease
Legionellosis: The umbrella term for any illness caused by Legionella bacteria, covering both the serious pneumonia of Legionnaires’ disease and the milder Pontiac fever.; Legionnaires disease and Pontiac fever: key differences Legionella vs Legionnaires' disease: what's the difference?
Monochloramine: A more stable chlorine-based disinfectant, formed by combining chlorine with ammonia, dosed into some larger water systems for its persistence and ability to penetrate biofilm. It is less common in UK building systems and requires specialist control and monitoring.; Secondary disinfection for Legionella compared: chlorine dioxide vs copper-silver vs UV vs monochloramine Chlorine tolerance and disinfectant limits in Legionella control
Pasteurisation (thermal disinfection): Raising water temperature throughout a system high enough to kill Legionella, then running it through every outlet for a sustained period. Often used after a positive result or as periodic control, it carries a genuine scald risk while it is being carried out and must be managed carefully.; Thermal disinfection: using heat to control Legionella What temperature kills Legionella?
Point-of-use filter: A fine filter (commonly rated at around 0.2 micron) fitted at a tap or shower that physically removes Legionella and other bacteria from the water leaving that outlet. Used as short-term or supplementary protection, especially in augmented care, the filters have a limited lifespan and must be changed on schedule.; Point-of-use water filters for Legionella in high-risk and augmented-care settings
Pontiac fever: A mild, flu-like form of legionellosis that does not involve pneumonia and usually clears up on its own within a few days. It shares a cause with Legionnaires’ disease but is far less serious because it does not produce lung infection.; Pontiac fever: symptoms, recovery and how it differs from Legionnaires' disease Legionnaires disease and Pontiac fever: key differences
Pre-flush vs post-flush sample: Two ways of taking a water sample: a pre-flush (first-draw) sample is collected the moment the outlet is opened, to capture what is sitting at the tap, while a post-flush sample is taken after running the water, to test the incoming supply. Which one to use depends on the question you are trying to answer.; Pre-flush vs post-flush sampling: which to use when BS 7592 sampling: how to plan representative Legionella water samples
Representative outlet: An outlet selected for monitoring or sampling because it stands in for a wider group of similar fittings, used alongside the fixed sentinel outlets. Representative points are usually rotated so that, over time, a cross-section of the whole system is checked rather than only the extremes.; Monitoring water temperatures in a Legionella control programme Sentinel outlets: what they are and how to monitor them BS 7592 sampling: how to plan representative Legionella water samples
Responsible person: A competent individual appointed by the duty holder to take day-to-day charge of managing Legionella control, with enough authority, knowledge and training to do the job. The role, and any deputies, should be named in writing so accountability is clear.; Duty Holder and Responsible Person roles in Legionella management What is a 'competent person' for Legionella, and how do you prove it?
RIDDOR: The Reporting of Injuries, Diseases and Dangerous Occurrences Regulations, under which certain work-related cases of Legionnaires’ disease must be reported to the HSE. These reporting duties sit alongside, not instead of, public-health notification.; Reporting Legionnaires disease: RIDDOR, public health and incident escalation
Sentinel outlet: The outlets chosen to represent the worst-case temperatures on a water circuit — typically the nearest and furthest taps from a calorifier on the hot side, and the nearest tap to the storage tank plus the furthest one on the cold side. They are checked routinely (commonly monthly) so a problem with hot water staying hot and cold water staying cold shows up early.; Sentinel outlets: what they are and how to monitor them Monitoring water temperatures in a Legionella control programme How often should you flush and test for Legionella?
Serogroup: A subdivision of a Legionella species based on the antigens on the cell surface, used to classify strains. L. pneumophila serogroup 1 is the most clinically important, and the standard urinary antigen test mainly detects this group.; Legionella species and serogroups beyond L. pneumophila How is Legionnaires' disease diagnosed? The urine antigen test and chest X-ray
Spa pool: A pool of warm, recirculated, aerated water — a hot tub, whirlpool or similar — kept at temperatures that suit Legionella while continuously producing aerosol. Spa pools need rigorous continuous disinfection and frequent monitoring, and in the UK are covered by HSE guidance HSG282.; Safe operation of spa pools and hot tubs under HSG282
Stagnation: Water sitting still in a system instead of being regularly drawn off and replaced. Stagnant water loses any disinfectant residual and tends to drift into the warm temperatures Legionella favours, which is why turnover and flushing of little-used outlets matter.; Neglected water systems: the danger of stagnation Flushing little-used outlets: best practices Avoiding stagnation: design tips for consistent water temperatures
Thermostatic mixing valve (TMV): A valve that blends hot and cold water to deliver a safe, scald-limited temperature at a tap or shower, used mainly where vulnerable people could be burned. Because it holds warm, mixed water close to the outlet, a TMV can itself become a Legionella risk and needs regular cleaning, descaling and servicing.; Thermostatic mixing valves and Legionella risk TMV maintenance: balancing scald risk and Legionella control
Viable but non-culturable (VBNC): A dormant state in which Legionella is still alive and potentially able to cause infection but will not grow on a standard laboratory culture plate. It means a “no growth” culture result does not always prove that water is free of risk.; Lab testing limitations: false negatives and viable-but-non-culturable cells Detecting Legionella: culture and PCR testing methods
Water safety group: A multidisciplinary team, used mainly in healthcare and other complex settings, that oversees water safety and delivers the water safety plan. It typically brings together estates, infection prevention and control, clinical staff and specialist advisers.; Forming a Water Safety Group: roles and benefits Healthcare water safety roles explained: AE(W), AP, CP and RP under HTM 04-01
Water safety plan: A risk-based management plan that pulls water-system hazards, controls and responsibilities together into a single living document, going beyond a one-off risk assessment. It is regarded as best practice generally and is expected in higher-risk settings such as healthcare, rather than being a universal legal requirement for every building.; Developing a comprehensive Water Safety Plan Water safety plan vs Legionella risk assessment: what's the difference?BS 8680 water safety plans: turning controls into a management system
Written scheme of control: The document, expected under ACoP L8, that sets out how Legionella risk will be prevented or controlled in a specific building. It describes the water system, the control measures, who is responsible, what monitoring is done and what to do if something goes wrong.; Documenting a written scheme of control for Legionella compliance Written scheme of control template and worked example (Regulation 8 / ACoP L8)