What factors affect legionella risk assessment frequency?

There is no single statutory interval that applies to every legionella risk assessment. Frequency should be proportionate to the level of risk and supported by documented evidence. The main factors are the findings of the current assessment, the reliability of control measures, the nature of the water systems, the people exposed, monitoring results, and any event that could alter the risk profile.

The legal and guidance framework affects how often an assessment is revisited. Under the Health and Safety at Work etc. Act and the Control of Substances Hazardous to Health Regulations, dutyholders must assess and control risks arising from legionella. The HSE’s Approved Code of Practice L8 and associated guidance require the assessment to be suitable, sufficient and kept under review. They do not impose one universal calendar interval for every premises. A written procedure should therefore explain why the chosen review frequency is appropriate.

The findings of the previous assessment are an important starting point. A system with clear records, stable operating conditions and consistently effective controls may support a longer planned interval than one with unresolved defects or incomplete information. Outstanding actions should not simply be carried forward to the next scheduled review. Where an assessment identifies a material risk or a control weakness, the responsible person may need a focused reassessment once corrective work has been completed.

The design of the water system can make an assessment more complex and can increase the need for close review. Relevant features include:

• stored hot or cold water systems;
• long pipe runs, infrequently used outlets and areas that may permit stagnation;
• showers, spray-producing outlets, spa facilities or other equipment that can generate aerosols;
• temperature control arrangements, thermostatic mixing valves, calorifiers and secondary circulation;
• multiple buildings, risers or separate water systems managed under one site plan; and
• specialist equipment or processes that introduce additional maintenance requirements.

These features do not automatically dictate a particular interval, but they affect the number of assumptions that must be checked and the potential consequences of control failure. A complex system may justify more frequent review than a straightforward installation, particularly where its operation is not fully understood.

Operational performance and records provide evidence of whether the written control scheme works in practice. Review the quality and consistency of temperature checks, flushing records, cleaning and disinfection records, inspection findings, sampling results and corrective-action logs. Repeated excursions, missed checks, unexplained test results or poor record-keeping may indicate that an earlier assessment is warranted, even where no confirmed case of legionnaires’ disease has occurred.

It is important to distinguish a risk assessment from routine control monitoring. Daily, weekly or other scheduled checks may be required by the written scheme, but completing those checks does not replace a formal assessment review. Conversely, a stable set of monitoring results can help the responsible person justify the planned frequency of the wider assessment.

Maintenance history and management arrangements also influence frequency. Consider whether planned maintenance is completed as specified, whether contractors understand their responsibilities, and whether the responsible person has sufficient authority, competence and resources to act on findings. Changes in contractors, staffing, ownership, water management procedures or record systems can create a need to confirm that responsibilities and controls remain clear.

Occupancy and the consequences of exposure should be considered in proportion to the people using the premises. Particular care is needed where occupants may be more susceptible to serious illness, or where a building provides healthcare, residential, care or other services involving vulnerable individuals. The likely severity of harm may justify a more cautious review approach, even if the physical system appears relatively stable.

Incidents and external information can require an immediate review rather than waiting for the next planned date. Examples include a suspected or confirmed case of legionnaires’ disease, a positive or concerning laboratory result, an outbreak investigation, a prolonged shutdown, loss of water services, flooding, major maintenance, or evidence that water has been outside its intended control parameters. The review should establish what happened, whether the original assessment remains valid and what further action is needed.

Seasonal or intermittent use may also affect the timing of an assessment. Premises that close, operate at a lower occupancy or use parts of their water system only occasionally can experience different patterns of stagnation and recommissioning risk. The assessment should explain how shutdown, reopening, flushing and verification arrangements affect the overall control strategy.

A defensible frequency decision should record the factors considered, the evidence reviewed, the date of the next planned review, the person responsible and the events that would trigger an earlier assessment. This creates a risk-based audit trail rather than relying on an arbitrary recurring date. Where the scope or risk is uncertain, a competent legionella risk assessor can help determine whether the existing assessment remains suitable and whether the review interval is justified.