Control parameters: the reference
The authoritative UK Legionella numbers in one place, each with its source and a plain-English note. The figures the whole site, and your scheme of control, point back to.
Checked against HSE ACoP L8, HSG274 and the BS standards on 27 June 2026.See what changed ›
Last reviewed 27 June 2026Machine-readable JSON
Indicative guidance only. Every value here must be confirmed against the current edition of the cited document and applied through a competent, site-specific risk assessment. Figures are common UK baselines, not a substitute for professional judgement or the source standard.
Temperature control
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Hot water storage temperature | ≥ 60 °C | Stored in the calorifier / hot water cylinder | Store hot water hot enough that the whole vessel stays at temperature. Confirm the figure and any healthcare variation against current guidance and your risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Hot water temperature guidelines to prevent Legionella, Calorifier maintenance for Legionella: inspection, drain-off and sludge removal |
| Hot water at the outlet | ≥ 50 °C ( ≥ 55 °C healthcare ) within ~1 min | Sentinel hot outlets, time to reach temperature | Distributed hot water should reach the listed temperature quickly at the tap; healthcare uses the higher figure. Balance against scald risk via TMVs.HSE HSG274 Part 2 (2024), “Operation and inspection…”, p.70; healthcare 55 °C per HTM 04-01See: Hot water temperature guidelines to prevent Legionella, TMV maintenance: balancing scald risk and Legionella control |
| Cold water at the outlet | below 20 °C within ~2 min | Sentinel cold outlets, after running | Keep cold water cold. Warming of cold mains in summer or near heat sources is a common failure; confirm the target against your risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Cold water temperature guidelines to prevent Legionella |
| Legionella growth range | 20–45 °C | Water temperature where the bacterium multiplies | Dormant below ~20 °C, multiplies fastest 20–45 °C, killed progressively above ~50 °C and rapidly above ~60 °C. Indicative ranges.HSE HSG274 Part 2 (2024), Introduction; WHO (2011) “Water safety in buildings”See: Pipe Scale, Corrosion and Biofilm: How Surface Condition Drives Legionella Risk, Temperature control basics for Legionella prevention |
| Time to kill at temperature | ~60 °C: minutes · ~50 °C: hours | How quickly heat inactivates Legionella | Higher temperatures kill faster. Used as the basis for thermal disinfection; confirm dwell times against current guidance.HSE HSG274 Part 2 (2024), Introduction; WHO (2011)See: Thermal disinfection: using heat to control Legionella |
Microbiology & action levels
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Legionella result: in control | ≤ 100 CFU/L | Hot & cold water systems (HSG274 Part 2) | System generally considered under control; continue routine monitoring. Indicative band: interpret with your risk assessment and a UKAS lab.HSE HSG274 Part 2 (2024), “Microbiological monitoring” (para 2.123), p.86See: Understanding lab results: CFU counts and what they mean, Legionella sampling 101: how and why to test your water |
| Legionella result: review | > 100 to 1,000 CFU/L | Hot & cold water systems (HSG274 Part 2) | Review control measures and resample; if the majority of samples are positive, act. Indicative band.HSE HSG274 Part 2 (2024), “Microbiological monitoring” (para 2.123), p.86See: What to do if you get a positive Legionella test, Understanding lab results: CFU counts and what they mean |
| Legionella result: immediate action | > 1,000 CFU/L | Hot & cold water systems (HSG274 Part 2) | Investigate, review controls and consider disinfection. Indicative band; your risk assessment and a competent person guide the response.HSE HSG274 Part 2 (2024), “Microbiological monitoring” (para 2.123), p.86See: What to do if you get a positive Legionella test |
| Cooling tower action levels | ≤ 100 · > 100–1,000 · > 1,000 CFU/L | Cooling towers & evaporative condensers (HSG274 Part 1) | Per HSG274 Part 1 Table 1.11: not detected/up to 100; >100 and up to 1,000 (low-level, reassess); >1,000 (act). Interpreted alongside the tower’s biocide and aerobic/dip-slide regime (Table 1.10).HSE HSG274 Part 1 (2024), Table 1.11 “Comments and action levels in response to legionella analysis results”, p.47See: Cooling towers and evaporative condensers: high-risk systems |
Monitoring frequency
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Sentinel outlet temperatures | Monthly | Nearest & furthest hot and cold outlets | Sentinels bracket each service at its extremes. Frequency is risk-assessment led; monthly is the common baseline.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Sentinel outlets: what they are and how to monitor them, Routine monitoring and flushing: the basics |
| Representative outlet temperatures | On rotation (all over a set period) | A representative selection of other outlets | Rotate so a representative spread of outlets is checked over the period set by your risk assessment (often a year).HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Routine monitoring and flushing: the basics |
| Calorifier flow & return temperature | Monthly | Hot water storage vessel | Record both flow and return; a low return can indicate stratification or scale.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Calorifier maintenance for Legionella: inspection, drain-off and sludge removal |
| Flushing little-used outlets | Weekly (or as risk-assessed) | Outlets not used regularly | Run little-used outlets to replace stagnant water; weekly is the common baseline, set yours by risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: How to implement a flushing programme for Legionella control, Water Age Explained: Why Residence Time Drives Legionella Risk |
Inspection & maintenance
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Cold water storage tank inspection | Annually (typical) | CWST condition, lid, screens, sediment | Inspect condition and cleanliness; clean/disinfect if needed. Interval is risk-assessment led.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Water tank inspections: checking for Legionella risk |
| Calorifier inspection / drain-off | Annually (typical) | Internal inspection and/or drain to check for sludge | Check for sludge/scale at the base; frequency depends on findings and risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Calorifier maintenance for Legionella: inspection, drain-off and sludge removal |
| TMV service & fail-safe test | Per manufacturer (commonly 6–12 monthly) | Thermostatic mixing valves | Service, descale and fail-safe test; healthcare (HTM 04-01) and manufacturer schedules may be more frequent.HSE HSG274 Part 2 (2024), “Operation and inspection…”, p.70; plus TMV manufacturer instructionsSee: TMV maintenance: balancing scald risk and Legionella control |
| Showerhead clean & descale | Quarterly (typical) | Showerheads and hoses | Dismantle, clean and descale; more often where scaling is heavy. Risk-assessment led.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Showerhead cleaning and descaling schedules |
| Risk assessment review | No fixed expiry: review on triggers | The Legionella risk assessment itself | Review regularly and whenever the system, its use, or the people exposed change. The "every two years" figure is a common prompt, not a legal expiry.HSE ACoP L8 (2013), “Carrying out a risk assessment”, p.12; BS 8580-1:2019, clause 10 (risk review and reassessment)See: Reviewing and updating your Legionella risk assessment regularly |
Disinfection
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Shock chlorination | 50 mg/L for ≥ 1 h, with ≥ 30 mg/L free residual at the end | Chemical disinfection of a water system | HSG274 para 2.135: dose to 50 mg/L (ppm) for a minimum 1 hour contact, with at least 30 mg/L free residual chlorine remaining at the end; lower concentrations over longer contact times are acceptable per BS 8558. High-hazard work under COSHH; confirm against current guidance.HSE HSG274 Part 2 (2024), “Cleaning and disinfection” (para 2.135), p.88; lower concentrations / longer contact times per BS 8558See: How to shock-chlorinate (disinfect) a water system: a step-by-step procedure |
| Thermal disinfection (pasteurisation) | ≥ 60 °C at every outlet, defined dwell | Heat disinfection of a hot water system | Raise temperature and run each outlet for the defined period; manage scald risk. Confirm temperature/dwell against current guidance.HSE HSG274 Part 2 (2024), “Cleaning and disinfection”, p.87See: Thermal disinfection: using heat to control Legionella |
Legal & duties
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| General duty of care | HSWA 1974, s.2 & s.3 | Employer duties to employees and others | The overarching duty under which Legionella control sits.Health and Safety at Work etc. Act 1974See: UK Legionella compliance 101: laws and responsibilities |
| Duty to assess the risk | COSHH Regulation 6 | Risk assessment for hazardous substances (incl. Legionella) | The legal hook for a suitable and sufficient Legionella risk assessment.COSHH Regulations 2002See: Legionella risk assessment basics: what it is and why you need it |
| Duty to prevent or control exposure | COSHH Regulations 7–8 | Control measures and their use | Underpins the written scheme of control and its maintenance.COSHH Regulations 2002See: Written scheme of control template and worked example (COSHH & ACoP L8) |
| Notify cooling towers to the local authority | Required (Notification Regulations 1992) | Wet cooling towers & evaporative condensers | Notifiable devices must be registered with the local authority (and de-registered when removed).Notification of Cooling Towers and Evaporative Condensers Regs 1992 |
| Report a workplace-linked case | RIDDOR (where reportable) | Reporting Legionnaires’ disease linked to work | Certain work-related cases/exposures are reportable; check the RIDDOR criteria and public-health duties.RIDDOR 2013 |
Need these as a printable card? Each row also feeds our record templates and calculators. For the documents behind the numbers, see standards & guidance.
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"Control parameters: the reference." Legionella.io, last reviewed 27 June 2026. https://legionella.io/reference/
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