Water hygiene has its own dialect, and most of it lands on your desk uninvited the day you become responsible for it. This A-Z explains the terms you will actually meet on a risk assessment, a logbook page or a contractor’s quote, in plain English.

Skim it, or jump to the word that tripped you up. Each entry says what the term means and why it matters to you.

A to F

ACoP L8 is the Approved Code of Practice from the Health and Safety Executive on controlling Legionella bacteria in water systems [1]. “Approved Code of Practice” is the key phrase: follow it and you are doing enough, depart from it and you must show your alternative is at least as safe. It is the document your duty holder responsibilities trace back to.

Asset register is the list of everything in your water system that can grow or spread bacteria, from the cold water tank to the last shower head. No register, no risk assessment worth the name, because you cannot control plant you have not written down.

Biofilm is the slimy layer of microorganisms that builds up on the inside of pipes, tanks and fittings. Legionella shelters inside it, which is why it survives flushing and shrugs off a quick dose of disinfectant; the biofilm protects it. We cover how this happens in Biofilms: how Legionella hides in plumbing systems.

BS 8580 is the British Standard giving a code of practice for Legionella risk assessment [4]. When a contractor says their assessment is “to BS 8580”, they mean it follows a recognised method rather than a back-of-an-envelope walk-round.

Calorifier is the proper name for a hot water storage cylinder heated indirectly, usually by a coil. A cool layer can sit at the bottom where sediment collects, so its base temperature and drain-off get checked.

Competent person is someone with the training, knowledge and experience to do the water hygiene task safely. The HSE expects the duty holder to appoint competent people rather than assume goodwill covers it [1].

Dead leg is a length of pipe leading to an outlet that is rarely or never used, so water stagnates and warms in it. Prime Legionella territory; the usual fix is to remove the redundant pipework rather than flush it forever.

Duty holder is the person or organisation in legal control of the premises and therefore of the water system risk, often an employer, landlord or building manager. The duty sits with you whether or not you have heard the word before [3].

Flushing is running little-used outlets to draw fresh water through and stop it stagnating, commonly weekly. A control measure, not a cure, and it only works if someone records it.

G to N

HSG274 is the HSE’s technical guidance that sits under ACoP L8, split into parts for evaporative cooling systems, hot and cold water systems, and other risk systems [2]. When L8 tells you what to achieve, HSG274 is where the practical detail lives.

HTM 04-01 is the Health Technical Memorandum on safe water in healthcare premises [5]. Hospitals work to it because vulnerable patients raise the stakes; it is stricter than the baseline for a typical office.

Legionella is the bacterium itself, found naturally in rivers and lakes at low levels and a problem when it multiplies in building water systems. The introduction in What is Legionella? An introduction to the bacteria and disease is the place to start if the basics are new.

Legionnaires’ disease is the serious, sometimes fatal pneumonia caught by inhaling contaminated water droplets, not by drinking water [6]. It is the reason any of this paperwork exists.

Legionella Control Association (LCA) is the membership body whose registered service providers sign up to a code of conduct [7]. Checking LCA registration is a quick sanity check when you hire a contractor.

Nominated (responsible) person is the named individual the duty holder appoints to manage the control scheme day to day. One named person, with deputies, beats a vague “facilities will sort it”.

O to Z

Pasteurisation (thermal disinfection) is deliberately raising hot water temperature to kill bacteria across the system, often after a failure or as planned maintenance. It works, but it carries a scald risk, so it needs care and a method.

Proliferation temperature range describes the band in which Legionella multiplies happily, generally taken as roughly 20C to 45C [2]. The whole temperature regime, hot kept hot and cold kept cold, is built around keeping water out of that band; treat the figure as guidance your risk assessment confirms.

RIDDOR is the regulations requiring certain work-related cases of Legionnaires’ disease to be reported to the authorities [8]. You may never use it, but it matters the day a case is linked to your site.

Risk assessment is the written, site-specific judgement of where Legionella could grow and spread, and what you will do about it. The foundation document; everything else is delivery against it, and it needs reviewing when the system or its use changes.

Sentinel outlet is a tap or shower chosen for routine temperature monitoring because it sits at an extreme of the system, the nearest and furthest points. Watch a few and you read the whole system; Sentinel outlets: what they are and how to monitor them walks through choosing and checking them.

TMV (thermostatic mixing valve) blends hot and cold water to deliver a safe temperature at the tap and prevent scalding, common on showers and basins used by vulnerable people. The catch: it creates a warm, mixed micro-zone that needs its own attention, and it can mask the true hot temperature behind it.

UKHSA is the UK Health Security Agency, which investigates cases and publishes the surveillance data on how many people fall ill each year [9]. It joins the dots when an outbreak crosses several sites.

Water safety plan is a structured, risk-based scheme for managing water safety across a building, drawing the controls, roles and records together in one place. Larger or higher-risk sites increasingly work this way rather than from a folder of separate documents.

The first ten terms to learn, and where each one lives

If the whole alphabet is too much at once, these are the terms that unlock most paperwork, paired with the document you will find them in. Tick them off as you can define each without looking back:

  • Duty holder and responsible person — your risk assessment cover page and policy
  • ACoP L8 and HSG274 — the legal and technical backbone, referenced throughout
  • Risk assessment — the master document everything else answers to
  • Asset register — the appendix listing tanks, calorifiers and outlets
  • Sentinel outlet — your monthly temperature monitoring sheets
  • Dead leg — the remedial actions list
  • Calorifier — hot water plant checks
  • TMV — outlet records and the planned maintenance schedule
  • Flushing — the weekly task log
  • Biofilm — the “why we do this” behind cleaning and disinfection

Define those ten and most of a logbook stops being a foreign language.

FAQ

Is there an official Legionella dictionary I should use?

No single dictionary, but the terms here trace to a small set of authoritative documents: ACoP L8 and HSG274 from the HSE define most operational language [1][2]. When a word’s exact meaning matters legally, check it against the source rather than a vendor’s brochure.

What is the difference between L8 and HSG274?

L8 is the Approved Code of Practice, the “what you must achieve and your legal standing” layer [1]. HSG274 is the technical guidance underneath it, the “how to actually do it” detail [2]. You are measured against L8; you deliver using HSG274.

Do I really need to know all this jargon as a small landlord?

You need to understand the handful that touch your duties, mainly duty holder, risk assessment and the basic temperature and flushing controls [3]. The rest you can look up as it appears, which is exactly what a glossary like this is for.

A note on using these definitions

These are plain-English explanations to help you read your own records, not a substitute for the source documents or for professional judgement. Where a term carries a specific legal or technical meaning on your site, your competent person and your site-specific risk assessment decide how it applies, not a generic definition.

Your next step today: open your current risk assessment, highlight every term you could not confidently explain to a colleague, and find each one above. If your records still live in a paper folder or scattered spreadsheets, that exercise is also the moment to picture those terms as searchable fields in a digital logbook, where an overdue “sentinel” check is one filter away rather than buried in a binder.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [4] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1 [5] NHS England, “Health Technical Memorandum 04-01: Safe water in healthcare premises”. https://www.england.nhs.uk/publication/safe-water-in-healthcare-premises-htm-04-01/ [6] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [7] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/ [8] HSE, “RIDDOR - Reporting of Injuries, Diseases and Dangerous Occurrences Regulations”. https://www.hse.gov.uk/riddor/ [9] UKHSA, “Legionnaires’ disease: guidance, data and analysis”. https://www.gov.uk/government/collections/legionnaires-disease-guidance-data-and-analysis