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Diagrams & figures

Our original Legionella diagrams, in one place. They are accurate, accessible and free to reuse with attribution, for training packs, toolbox talks, reports and slides.

Free to use, with credit. Reuse these figures if you credit “Legionella.io” with a link to this page. The diagrams are vector (SVG): they scale cleanly and screenshot at any size. Figures are illustrative, confirm every figure against your risk assessment and current guidance.

Legionella & water temperature

Legionella and water temperatureIndicative water-temperature observation bands: below 20 degrees Celsius for cold-water monitoring, a 20 to 45 degree growth range, a 45 to 50 degree transition, and hot-water observations from 50 degrees. A reading alone does not show system control.Cold-water benchmarkGrowth rangeHot-water observations< 20 °C20–45 °C50 °C +20 °C45 °C50 °C60 °CA reading is one observation, not proof that a water system is controlled.
Indicative HSG274 Part 2 benchmarks. Measurement point, elapsed time, storage/distribution context and the site’s written scheme determine what the reading means. Manage scald risk separately where people may be vulnerable.

A typical hot & cold water system

A typical hot and cold water systemThe cold water main feeds a cold water storage tank and a calorifier that heats the hot water. Water is distributed to outlets; the furthest is the sentinel outlet, and capped pipework with no flow forms a dead leg where water stagnates.Cold water mainCold water storagetank (CWST)Calorifier(heats hot water)Cold outletSentinel outletfurthest on the circuitCold distributionHot distribution (≥ 50 °C)Dead leg (stagnant)
A simplified layout. Real systems vary, but the principles are constant: keep cold water cold and hot water hot, monitor the sentinel (furthest and nearest) outlets, and remove dead legs where water sits and warms.

The cycle of control

The cycle of Legionella controlLegionella control is a continuous cycle: assess the risk, set out the control scheme in writing, monitor and record the controls, then review and act on what you find, and repeat.Repeat, and whenever the system or its use changes1Assess the riskRisk assessment2Write the schemeControls in writing3Monitor & recordTemperatures, flushing4Review & actClose out findings
Control is never "done". The duty is a continuous loop: assess, document the scheme, monitor and record, then review and act, repeating whenever the water system or how it is used changes.

How Legionella reaches people

How Legionella reaches peopleLegionella multiplies in warm water between 20 and 45 degrees Celsius. When that water is broken into an aerosol by showers, taps, sprays or cooling towers, the fine droplets can be inhaled and reach the lungs, where they can cause Legionnaires' disease.Warm water20–45 °C, multipliesAerosol releasedShowers, taps, spraysDroplets inhaledDrawn into the airwayReaches the lungsLegionnaires’ diseaseBreak any link, control the temperature, reduce stagnation, limit aerosols, and the risk drops.
Infection needs warm water, an aerosol and someone to breathe it in. Most control measures work by breaking one of those links, which is why temperature control and reducing stagnation matter so much.

The course of Legionnaires’ disease

Legionnaires': exposure to recoveryThe typical course of Legionnaires' disease, from inhaled aerosol through the incubation window to recovery.Exposureinhale contaminatedaerosolIncubationapprox 2-10 days, nosymptomsSymptom onsetfever, cough, pneumoniaDiagnosisurine antigen, chestX-rayTreatmentantibiotics, hospitalcareRecoveryweeks to months
The typical course of Legionnaires' disease, from inhaled aerosol through the incubation window to recovery.

What Legionella needs to multiply

What Legionella needs to multiplyLegionella multiplies only where warmth, stagnation, biofilm nutrients and host amoebae coincide.Legionella growthall four together amplify itWarmth 20-45 Cdormant below 20, dies 60+ CStagnationstill water, dead legsBiofilm & nutrientsscale, rust, sedimentHost amoebaeprotozoa shelter and feed it
Legionella multiplies only where warmth, stagnation, biofilm nutrients and host amoebae coincide.

The risk-assessment process

The Legionella risk assessment processA Legionella risk assessment as five stages, ending in a ranked, recorded scheme of control.Survey systemoutlets, tanks,calorifiersIdentify hazardsdead legs, stagnation,20-45CAssess riskgrowth x exposure x usersRank actionsprioritise remedial workRecord & controlwritten scheme + reviewdate
A Legionella risk assessment as five stages, ending in a ranked, recorded scheme of control.

Legionella action levels (CFU/L)

Legionella action levels (CFU/L)Indicative HSG274 response bands for Legionella in hot and cold water systems; confirm exact figures against current HSG274.Under ~100 CFU/LSystem considered in control~100-1,000 CFU/LReview controls, resampleOver ~1,000 CFU/LImmediate action, investigate
Indicative HSG274 response bands for Legionella in hot and cold water systems; confirm exact figures against current HSG274.

These figures appear in context across the articles; the numbers behind them live in the control-parameters reference.

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"Legionella figures & diagrams." Legionella.io. https://legionella.io/figures/

Free to reuse under CC BY 4.0: quote, adapt or republish with attribution and a link back to the original.

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