Legionella risk is not created by one single fault within a water system. In most cases, risk increases when several conditions occur together: water is stored or allowed to stagnate, temperatures support bacterial growth, nutrients are available within the system and contaminated water can be released as an aerosol that people may inhale.
The Health and Safety Executive (HSE) identifies a reasonably foreseeable Legionella risk where water is stored or recirculated, temperatures may fall between 20°C and 45°C, deposits such as rust, sludge, scale or organic matter are present, water droplets can be produced and people may be exposed to them.
For commercial premises, understanding these individual risk factors is essential. Offices, schools, hotels, healthcare facilities, manufacturing sites and other buildings can contain increasingly complex hot and cold water systems, and seemingly minor changes to their condition or use can affect how well Legionella risk is controlled.
This guide looks at the main Legionella risk factors duty holders should consider, how they can be identified and the measures used to manage and document them.
Legionella bacteria occur naturally in environmental water sources. The presence of bacteria alone does not necessarily mean that people are at significant risk.
Problems arise when a water system provides conditions in which bacteria can multiply and there is a route through which contaminated water droplets can be inhaled.
In commercial buildings, several factors can contribute to this.
Temperature is one of the most important factors in controlling Legionella.
Legionella can grow where water temperatures are between approximately 20°C and 45°C. Maintaining hot and cold water outside this range, where reasonably practicable, is therefore a fundamental control measure.
Where temperature is being used as the primary control strategy, HSE guidance states that:
A single acceptable temperature reading does not necessarily prove that the whole system is under control. Problems can occur locally, particularly on long pipe runs, poorly balanced circulation systems and sections with low water movement.
Temperature monitoring should therefore provide a representative picture of the system rather than focusing repeatedly on the easiest outlets to access.
Stagnant water creates particularly favourable conditions for microbial growth.
Commercial buildings frequently contain outlets that are rarely used: spare offices, accessible toilets, unused showers, vacant rooms, seasonal areas or facilities retained even though the way a building is used has changed.
HSG274 specifically identifies dead legs, capped pipes or dead ends, infrequently used outlets and areas with poor circulation as locations where stagnation and conditions favourable to microbial growth can occur.
This is why low use should not simply be treated as an operational inconvenience.
Where an outlet is genuinely unnecessary, removing redundant pipework back as close as reasonably practicable to the circulating or regularly used supply is generally preferable to managing an unnecessary section indefinitely.
Where outlets must remain but are infrequently used, an appropriate flushing regime may be required as part of the written control scheme.
Water storage is sometimes necessary, particularly within larger commercial properties. However, storing substantially more water than the building actually uses can increase stagnation and reduce turnover.
An oversized cold water storage tank may leave areas of water relatively static for prolonged periods.
HSE guidance recommends keeping stored water volumes to a minimum and indicates that storage should generally be sufficient for approximately one day's water use.
Changes in building occupancy can make this particularly important.
A water system originally designed for several hundred occupants may subsequently serve only a fraction of that number. The infrastructure has not changed, but the rate at which water moves through it has.
Duty holders should therefore consider whether storage capacity remains appropriate for the building's current use, not simply whether it was appropriate when originally installed.
The design and configuration of a water system can significantly affect Legionella risk.
Problem areas can include:
Commercial buildings are particularly vulnerable to this because systems often evolve with the property.
A building may have undergone refurbishments, changes of tenant, changes in room use or repeated plumbing alterations. What appears to be a relatively straightforward system on paper may contain redundant or poorly configured pipework hidden within ceilings, risers and service voids.
HSE guidance specifically notes that subsequent modifications to buildings can introduce foreseeable Legionella risks if water systems are incorrectly designed.
An effective Legionella risk assessment should therefore consider the actual installed system, rather than relying solely on historic drawings.
Legionella does not exist independently of the wider microbiological condition of a water system.
Deposits including:
can provide nutrients and environments that support microbial growth.
These conditions may develop within calorifiers, cold water storage tanks, pipework, shower hoses, strainers and other water-containing components.
Scale and deposits can also reduce the effectiveness of control measures. For example, accumulated sediment within a vessel can create localised areas in which temperature or disinfectant control is less effective.
The physical condition of the system is therefore an important part of Legionella control.
Cold water storage tanks should be inspected for contamination, corrosion, debris, condition of lids, screens and insulation. HSE guidance recommends annual inspection of cold water storage tanks, with cleaning, disinfection or remedial work undertaken where findings indicate it is necessary.
It is not enough for cold water to enter a building below 20°C.
Once inside the building, poorly located or inadequately insulated pipework can gain heat from:
Water can then remain within the Legionella growth range for prolonged periods, particularly where this is combined with low use.
HSG274 recommends minimising thermal gain through measures including appropriate insulation and separation of cold water services from hot water and heating systems.
Persistent high cold water temperatures should therefore prompt investigation of the cause rather than simply repeated temperature recording.
Large commercial hot water systems commonly rely on secondary circulation to maintain suitable temperatures around the building.
If circulation is poorly balanced or fails within individual loops, sections of the system may cool into temperatures favourable for bacterial growth.
This can occur even when the calorifier itself is achieving the required temperature.
For that reason, monitoring should consider the performance of the wider distribution system, including principal and subordinate return loops and representative outlets. HSG274 sets out monitoring arrangements designed to build a temperature profile across the system rather than relying solely on the calorifier temperature.
This distinction is important: producing sufficiently hot water does not automatically mean sufficiently hot water is reaching every relevant part of the system.
For Legionella bacteria to cause Legionnaires' disease there must also be a route of exposure.
The disease is normally contracted by inhaling small contaminated water droplets or aerosols.
Potential aerosol-producing equipment can include:
The significance of an aerosol source depends upon the system, how it is operated and who could be exposed.
For commercial duty holders, it is important that the Legionella assessment considers all relevant water systems, not simply the domestic hot and cold water services. HSG274 is divided into separate guidance covering evaporative cooling systems, hot and cold water services and other risk systems.
A water system can move from relatively low risk to significantly more difficult to control without any physical failure occurring.
Changes in occupancy are a common example.
These can include:
Reduced use means reduced water turnover, increasing the potential for stagnation.
HSG274 specifically highlights the risk of stagnation where buildings are not immediately fully occupied or are only intermittently used.
A suitable management system should therefore respond to changes in occupancy rather than assuming the normal control regime remains appropriate indefinitely.
Even a well-designed water system can become higher risk if its control regime begins to fail.
Examples might include:
Recording a failed control parameter is only the first stage.
Where monitoring identifies a problem, the reason should be investigated and appropriate corrective action taken. HSE guidance requires control measures to be monitored and records maintained of checks and resulting corrective actions.
Repeatedly documenting the same failure without resolving its cause does not demonstrate effective control.
Risk assessment should also consider who may be exposed.
Not everyone has the same susceptibility to Legionnaires' disease. This becomes particularly important in healthcare, care and other environments occupied by vulnerable people.
A water system that might represent one level of risk in a low-occupancy office may require more stringent precautions where occupants have increased susceptibility.
HSG274 therefore includes additional requirements and recommendations for healthcare and care settings, including higher hot water distribution temperatures where temperature control is used.
The building, water system and exposed population must all be considered together.
The starting point is a suitable and sufficient Legionella risk assessment undertaken by somebody competent to assess the system.
Under UK health and safety law, duty holders include employers and people in control of premises. They are required to identify and assess Legionella risks and take suitable precautions to prevent or control exposure.
A thorough assessment should consider areas including:
HSE guidance also expects the assessment to include management responsibilities, a description of the system, identified risks, existing controls, monitoring and maintenance arrangements and provisions for review.
For larger and more complex premises, understanding the system is difficult without an accurate schematic.
The schematic should allow those managing Legionella risk to understand how water travels through the building and identify important assets and sections of the system.
This is particularly valuable when investigating problems such as poor temperatures or persistent stagnation. A temperature failure at an outlet, for example, may actually originate from a circulation problem much further upstream.
Schematics should also be kept up to date when alterations are made.
Where significant risks are identified, the findings should lead to an appropriate written scheme of control.
The scheme should describe:
Depending upon the system, controls may include temperature management, maintaining water movement, reducing excessive storage, removing redundant pipework, cleaning and maintenance, water treatment and appropriate inspection and monitoring.
The objective is not simply to complete a checklist. It is to establish controls that address the risks identified within that particular building.
Not by itself.
For standard hot and cold water systems, routine microbiological monitoring for Legionella is not normally required where the system is demonstrably under control.
HSE advises that sampling may be appropriate where there is doubt about the effectiveness of the control regime, where required temperatures or disinfectant levels are not being consistently achieved, or in certain higher-risk situations.
A negative Legionella result is therefore not a replacement for an effective risk assessment, monitoring programme or written scheme.
Similarly, where sampling identifies Legionella, the result should be considered alongside the system condition, risk assessment and effectiveness of existing controls so that the underlying cause can be addressed.
Effective Legionella management also depends on being able to demonstrate what has been identified, what controls are required and whether they are working.
Where an organisation has five or more employees, significant findings from the risk assessment must be recorded.
Records should include details such as:
Current records should be retained for the period they remain current and for at least two years afterwards. Monitoring and inspection records should generally be retained for at least five years.
Good records provide more than an audit trail. They allow trends and recurring problems to be identified before they develop into more significant failures.
There is no single review interval that is appropriate for every commercial property.
The important requirement is that the assessment remains valid and reflects the building and water system as they actually exist.
A review may be necessary following:
HSE requires arrangements to review Legionella risk assessments regularly, particularly where there is reason to believe an assessment is no longer valid.
This is why treating a Legionella risk assessment purely as a document with an expiry date can be misleading. The priority is ensuring that it continues to represent the actual risk.
The most significant Legionella problems in commercial premises rarely come down to one isolated temperature reading or one piece of equipment.
They often develop through a combination of factors: an oversized tank, reduced occupancy, low-use outlets, ageing pipework, poor circulation and maintenance issues may collectively create conditions that allow bacteria to multiply.
Effective Legionella management therefore starts with understanding the whole water system.
A suitable risk assessment should identify where favourable conditions exist, establish who could be exposed and provide clear recommendations for eliminating or controlling those risks. The resulting written scheme, monitoring programme and records should then demonstrate that those controls continue to work in practice.
For duty holders, this provides a much stronger approach than responding to problems individually: it allows Legionella risk to be managed systematically across the life of the building.
Need to understand the Legionella risks within your commercial water system?
BrodexTrident provides Legionella risk assessments for commercial premises across the UK, helping duty holders identify potential sources of risk, understand existing controls and establish the actions required to manage their water systems effectively.
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