The summer time is here and while the UK weather might be ever changing, it’s safe to say that the temperature has definitely been bumped up a few degrees.

When the temperature rises, keeping cold water below 20°C becomes all that much harder and that makes controlling Legionella all the more difficult.

On the go? You can download a copy of the article to read later

Before we start

A little background information on Legionella and how it operates before we get into it.

Three common factors can heavily influence the growth of Legionella in water systems – temperature, stagnation & nutrients. A combination of these factors allows the bacteria to multiply and spread more easily throughout water systems.

Greater ambient temperatures, like those during the summer months, within buildings can lead to the cold water pipework and more specifically, the dormant bacteria within water in general, heating up and resuming its spread throughout the system.

Now while the weather is warm, we recommend paying close attention to your existing control measures, maybe even implementing some additional checks as necessary.

In an ideal world, you could safely assume that your control measures are keeping you protected from any potential risk and leave it there. However, there’s no accurate way of knowing how effective your measures are without testing the water.

One last thing before we move onto the main article, it’s worth mentioning that there is no requirement to take Legionella samples as part of your water safety programme, but the HSE does provide advice and guidance in both ACoP L8 and HSG 274 for incorporating samples into your regime.

What is a Legionella water sample?

A Legionella sample in simplest terms is a representative microbiological count of the level of Legionella bacteria within a water system.

Depending on the complexity of the water systems in a property, multiple samples may be required in order to fully cover all the elements of the building.

Legionella Sample
A Legionella Sample

How frequently should samples be taken?

As previously mentioned, there’s no absolute requirement for Legionella sampling, but HSG 274 part 2 does offer some advice for when samples should be taken.

In some circumstances, your risk assessment will suggest taking samples as part of the recommended actions and will highlight elements of your system as well as a frequency.

The guidelines do make specific recommendations to a few elements that are more likely to warrant testing, these are:

  • A system that is treated with biocides and water is stored or distributed at lower temperatures

Recommended monthly testing, to be reviewed and continued until there is confidence in the effectiveness of the control measures.

  • Control measures are not being consistently achieved

Frequent testing is recommended, this could be weekly or fortnightly. Once the system has been brought under control, the testing frequency should be reviewed.

  • There is a high-risk area or exposure to people with higher susceptibility

Frequency will vary depending on the specifics identified in the risk assessment, this could equate to infrequent but regular testing as a warning mechanism.

  • A water system that is suspected or confirmed as the source of an outbreak

Usually infrequent testing that will either prove the absence or presence of Legionella, either before or after corrective actions.

How many samples should be taken?

The actual number of samples will be determined by the complexity of the water system. The key factor when deciding the number of samples is ensuring to get a representation of the water flowing through the system and not just downstream of the fittings.

This may mean the number of samples will change as outlets are added or removed, storage tanks are changed or removed, but the end goal will always be the same.

How should samples be taken?

Keeping in line with the above principle, there are some things to consider when selecting sample points.

  • Each hot and cold outlet must be sampled separately and not through mixer taps or downstream of TMVs or showers
  • Samples also have to be labelled with the collection point and if the sample is taken pre or post flushing

In order to get a proper representation of the system and the water beyond the immediate pipework, 1-litre of water is required per Legionella sample.

Taking a Legionella Sample
Taking a Legionella Sample

Can I take my own samples?

Mail order sample kits are a bone of contention within the industry, mainly due to some less than helpful instructions. Some kits advise filling a single sample bottle from multiple points, this will alert you to the presence of Legionella, although it won’t help you identify the source.

Although we air on the side of caution and advise against the “do it yourself” approach to Legionella control. Once the sample points have been identified and providing that steps are taken to avoid contaminating samples, there’s no reason you can’t take your own, but they will still need to be sent to a UKAS accredited lab for testing.

How long do results take?

Most culture tests will take a minimum of 10 days for a negative result, but positive results can be verified sooner depending on how long it takes the bacteria to be observed.

Legionella Culture
A Legionella Culture

I’ve got my results, what next?

The results will have different implications based on factors like where it was taken, the use of the building, current control measures and the strain of bacteria found.

Since there are countless possible outcomes, let’s focus on some of the common ground.

  • Not detected – nothing to worry about, keep on top of your control measures and resample as appropriate.


  • <100 cfu/l – still nothing too concerning, but healthcare premises may wish to examine and alter control measures to minimise risk of exposure.


  • >100 – <1000 cfu/l minority – if the minority of samples are positive, the system should be resampled. If similar results are found again, control measures should be reviewed in conjunction with the risk assessment.


  • >100 – <1000 cfu/l majority – if the majority of samples are positive, immediate review of measures and outstanding remedial actions to root out the cause. You can also consider a system disinfection.


  • >1000 cfu/l – System should be resampled and disinfected. Control measures need to be reviewed with a view to improve and resampling should be carried out and continued until adequate control is resumed.
Example Legionella Sample Report
Example of a Legionella Sample Report - Click to enlarge

Should you take samples?

There’s not a right or wrong choice here, it all comes down to how proactive you wish to be with your water safety programme.

You can choose to incorporate Legionella sampling as a means of early warning or a measure of the effectiveness of your control regime.

The choice is ultimately yours, but an ounce of prevention is worth a pound of cure.