A hazard, in Legionella terms, is not dirt and it is not age. It is any point in your system where the bacteria can multiply and then reach someone’s lungs. Find those two conditions wherever they occur together, and a confusing building of tanks and pipework turns into a short, ranked list you can act on.

The skill is less about chemistry than about reading a system the way the water reads it: where does flow slow down, warm up and sit, and where does that water then break into a breathable mist? Most real hazards are mundane. A tank in a hot loft. A capped-off branch left after a refurbishment. A shower in a room nobody books. The point is to notice them before an assessor does, or before an outbreak does it for you.

What actually counts as a hazard

It helps to keep two words apart. A hazard is the potential to cause harm: conditions that let Legionella grow, plus a way for contaminated water to be inhaled. The risk is how likely that harm is, given who uses the building and how. You identify hazards first; the risk assessment then weighs them. HSE guidance approaches this systematically, asking you to understand the system, find where the bacteria could grow or spread, and judge who could be exposed [1].

Two conditions do almost all the work. The first is water that sits in roughly the 20-45 °C band long enough for the bacteria to multiply in the biofilm coating pipes and fittings; below about 20 °C they stay largely dormant, and above around 60 °C they are progressively killed off [2]. The second is aerosol, a spray fine enough to breathe. Drinking the water is not the route. Inhaling the mist is [3]. Hold those two filters against every part of the system and the hazards start to announce themselves.

Walking the system: where the hazards hide

Take the journey in the order the water does, from where it enters and is stored to where it finally leaves.

Storage and water heating. Cold water storage tanks are a classic find. A tank in a warm roof space, oversized for the building so the water barely turns over, with a loose or missing lid letting in warmth and debris, is several hazards in one place. On the hot side, calorifiers and cylinders can stratify, leaving a cooler layer near the base that never reaches a controlling temperature. Check that stored hot water actually leaves hot and that the cold genuinely stays cold; Hot water storage: preventing Legionella in tanks and cylinders covers storage in detail.

Distribution and dead legs. Pipework is where most hidden hazards live. A dead leg, a length of pipe with little or no flow, holds warm, still water indefinitely, and the commonest example is a branch to a fitting that was removed but never cut back. Long horizontal runs to a distant wing shed heat on the way and arrive tepid. Redundant pipework left in place after alterations is easy to miss precisely because nothing about it looks wrong. Stagnation sits behind a large share of failures, which is why Neglected water systems: the danger of stagnation treats it on its own.

Outlets and the last metre. The end of the system is where water becomes breathable. Showers and spray taps make aerosol by design, and the less they are used, the more the water behind them stagnates. Flexible hoses and scaled shower heads give biofilm somewhere to thrive. Thermostatic mixing valves matter here too: they blend hot and cold to a safe delivery temperature for scald protection, which leaves a short length of tepid water in and just upstream of the valve. The single hazard you most want to find is the low-use outlet: the shower in the rarely-booked room, the tap in the disused cleaner’s cupboard, the eyewash nobody tests.

Higher-risk systems that need their own look

Some systems sit outside the ordinary hot-and-cold picture and carry enough risk to be assessed in their own right. Cooling towers and evaporative condensers throw aerosol outside the building and have been behind some of the largest outbreaks; if you have them, they are also notifiable to your local authority, so confirm that has actually been done [4]. Spa and hydrotherapy pools fall under separate, more demanding guidance in HSG282 rather than the general rules [5]. Other incidental aerosol sources slip past a first pass: decorative fountains, vehicle and plant washes, misting units, and emergency safety showers. HSE keeps a useful summary of the systems most likely to create risk, and it makes a good cross-check against your own list [6].

A hazard-spotting walk-round

You do not need lab kit for the first pass. A clipboard, the system schematic if one exists, and a methodical route will surface most hazards. Record what you find, not just that you looked.

Storage and generation:

  • Locate every cold water storage tank; note its temperature, whether the lid seals, and any debris, scale or sludge.
  • Confirm stored hot water reaches and holds a controlling temperature, including near the base of calorifiers.
  • Flag any tank that looks oversized for current demand, so turnover is low.

Distribution:

  • Trace the pipework against the schematic and mark every dead leg, blind end and capped branch.
  • Identify long runs where hot water could arrive cool, or cold water could warm up.
  • Note redundant pipework left from past alterations.

Outlets and use:

  • List every outlet that produces a spray: showers, spray taps, hoses.
  • Mark outlets used infrequently or only seasonally.
  • Inspect shower heads and hoses for scale and condition.
  • Record the location of each thermostatic mixing valve and the blended section beyond it.

Special systems:

  • Record any cooling tower, evaporative condenser or spa pool, and confirm the correct separate regime and notifications are in place.
  • Note incidental aerosol sources: fountains, washes, misting, safety showers.

Write a temperature, a location and a condition against each line. A sheet that only ticks “done” proves attendance, not control.

Turning what you found into ranked actions

A list of hazards is raw material, not a plan. Converting it into decisions is the real work. As you go, separate two things: immediate defects (a missing tank lid, a redundant dead leg, a shower running tepid) and underlying management weaknesses (no flushing schedule for void rooms, no schematic, no record of who checks what). Defects get fixed; weaknesses get designed out.

Then rank. Weigh each hazard by how likely growth is and by how many people are exposed and how vulnerable they are. A tepid, low-use shower in a care setting outranks a slightly warm tank feeding a tap that runs all day. Give every prioritised item an owner, an acceptable result, and a trigger for review. Recording the reasoning, not just the action, is what turns a survey into a managed programme, and it is exactly what the step-by-step risk assessment you feed this into will expect to see.

Where spotting stops and assessing begins

A walk-round identifies candidates; it does not settle them. Whether a given branch is truly a dead leg, what temperature is acceptable at a particular outlet, and which remedy is proportionate all turn on your specific system and how it is used. That judgement belongs in a competent, site-specific risk assessment built to recognised practice such as BS 8580-1, not to a generic rule of thumb [7]. Treat each hazard you find as a question to answer rather than a verdict, and never stand a control down on the strength of one clear inspection or a single clean sample. This is general guidance to sharpen your eye, not legal, engineering or clinical direction for your building.

Common questions

What is the difference between a Legionella hazard and a Legionella risk?

The hazard is the source of potential harm: the bacteria, and the conditions that let them cause illness, meaning warm, stagnant water plus a way to inhale it. The risk is the likelihood that someone is actually exposed and harmed, given how your building is used. You identify hazards first, then the risk assessment weighs them against who is exposed [1].

When does a quiet pipe count as a dead leg?

There is no single length that flips a pipe into a dead leg; what matters is flow. A branch that rarely or never sees water moving through it holds warm, still water and should be treated as a hazard, whether it is a capped stub from an old fitting or a tap used once a year. If you cannot say when it last flowed, assume it has stagnated and investigate [2].

Do I need a schematic drawing to find hazards?

It is hard to be confident without one. An up-to-date schematic lets you trace every branch and catch the dead legs and redundant runs that are invisible on a quick look. If no drawing exists, sketching one as you walk the system is often the most valuable single output of the exercise.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice 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] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [4] HSE, “Other duties: RIDDOR and notification of cooling towers or evaporative condensers”. https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm [5] HSE, “Control of legionella and other infectious agents in spa-pool systems (HSG282)”. https://www.hse.gov.uk/pubns/books/hsg282.htm [6] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [7] 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