Understanding your Legionella Risk Assessment

There is often a misconception that a Legionella Risk Assessment is just looking for the presence of Legionella; as if it’s a tick boxing exercise to state whether or not it’s present. In fact, a Legionella Risk Assessment assesses not just whether Legionella is present but what the risks are of the bacteria emerging. It’s about making sure that your site is safe and recommends actions to reduce the risk from Legionella bacteria.

To give you guidance on what’s involved in a Legionella Risk Assessment and how you should use a Legionella Risk Assessment report, we’ve answered some key questions:

What is a Legionella Risk Assessment?

During a Legionella Risk Assessment, we are not looking for Legionella in the system. We look to identify the conditions that encourage Legionella to grow. Following on from our assessment, we recommend steps to eliminate or reduce that risk, making sure the site is as safe as it can be.

What causes Legionella?

There are three key elements that will encourage the growth of Legionella bacteria:

  • Temperature – Legionella likes temperatures between 20°C and 45°C. Therefore the best thing to do is avoid these temperatures. That means keeping cold water below 20°C and hot water above 50°C.
  • Stagnation – the infrequent use of taps or showers, or other devices where water does not flow through will encourage bacterial growth. Most common scenarios are vacant or disused areas of a building.
  • Nutrition – scale, corrosion, biofilm, algae or sediment in water systems all provide food for bacteria. These are the ideal sources of nutrition for Legionella and will help make a nice home for it to develop.

How is it harmful for humans?

The only way of contracting Legionellosis (disease caused by Legionella bacteria) is by breathing in small droplets containing the bacteria. Any form of aerosol poses a risk, should Legionella be present in the system. This could be through spray from a tap or shower, process waters or cooling towers etc.

What will the Legionella Risk Assessment report tell me?

A Legionella Risk Assessment will analyse the areas where the conditions encouraging bacterial proliferation can be found. It will look at the water tanks, water heaters and distribution pipework. It will assess temperatures, system usage and presence of nutrients that could feed bacteria. The assessment will identify areas where bacteria may be growing if it enters the system.

Occupancy levels will be reviewed as well as the occupancy status – whether they are in a high risk group for example. The elderly or those with immune deficiencies are obviously more vulnerable. We review aerosol dissemination and also the management procedures that are currently in place.

The report will make recommendations on any control measures that are necessary to eliminate, reduce or manage the risk.

How detailed is a Legionella Risk Assessment?

A Legionella Risk Assessment analyses a lot of data. Everything from the number of showers to the plant inspection, along with all distribution pipework. It needs to cover all systems leaving no area omitted that could possibly cause harm. Therefore, it is very important that someone familiar with the building is available at the time of the assessment.

The Causal Chain

The job of the Legionella Risk Assessment is to determine whether conditions are right for the chain of events to begin. It will then make recommendations on how to break that chain. To break the chain, water temperatures can be adjusted, scale or corrosion removed or items that are too damaged to clean, replaced. We may also recommend removing showers or other devices that are no longer in use.

Taking Action

The report will list a number of recommendations. It’s not always possible to action them straight away, however responsible people have a legal duty to take reasonable steps to reduce any risks. They must keep records to prove they have been actively trying to do this.

Following the assessment, a score matrix will be produced in the report. This looks through the causal chain and will recommend what needs to be done. In the RM Legionella Risk Assessment, we recommend what needs to be done with a priority rating. We propose a timescale, easily highlighting which issues require urgent attention:

  • Intolerable risk – take immediate action
  • Substantial risk – action within 1 month
  • Moderate risk – action within 3 months
  • Tolerable risk – action within 6 to 12 months
  • Trivial risk – this isn’t a huge issue, but it’s good practice to remedy it

Who needs to take action?

Every site should have staff or key personnel who have the responsibility for going through the Risk Assessment and taking the necessary action. This person or people need to be sufficiently trained. We recommend completing Legionella Awareness training as a minimum, or more advanced training where necessary. They must fully understand the Risk Assessment, what its purpose is, how important it is & how to take action on the results of the report.

The RM Legionella Risk Assessment

At RM, we have some additional elements to our Risk Assessments, and we use them alongside our HASAD system as that allows for the recommendations to be signed off and managed more easily. The responsible person(s) need to keep records, so that they can evidence what has been done. They need to be able to prove that all reasonable steps have been taken to reduce risks.

How often does the Risk Assessment need to take place?

Risk Assessments should be reviewed regularly and should be kept up to date following any changes, according to the Health and Safety Executive (HSE). This could be changes to the building use, system, plant or significant changes to key personnel. In addition, if the building has control issues or someone has contracted Legionella then the Risk Assessment needs to be reviewed.

A Legionella Risk Assessment needs to be completed every 1 to 2 years. This applies even in “low risk” properties and even if there are no major changes.


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