Hot weather and Legionella Risk

So we’re all in the midst of a heat wave. First thoughts turn to ice-creams, beer gardens and sun cream BUT don’t forget to take a moment to consider the increased Legionella risk from your water systems.  Longer, warmer and wetter summers are a recognised factor in the increase in cases of legionnaires’ disease.

How does temperature affect the growth of Legionella bacteria?

Legionella bacteria thrives between 20˚C and 45˚C, they are dormant below 20˚C and are being killed off quickly at 60˚C. Temperature is the primary method of controlling legionella growth in our water systems (although other methods using chemicals are also widely used). In hot weather, or where water usage is lower than normal, any stored cold water (in tanks or in pipes) is at increased risk of the temperature creeping up above 20˚C. Where this happens, bacteria can start to grow at an exponential rate.

Remind me about the ‘causal chain’

The main factors that increase Legionella risk in any water system (referred to as the ‘causal chain’) are:

  • CONTAMINATION – Bacteria exists in incoming water and finds comfortable conditions for growth in man-made water systems
  • AMPLIFICATION – Bacteria grows to large numbers
  • AEROSOLISATION – Bacteria is released in an aerosol (e.g. fine spray from taps, showers or hoses)
  • EXPOSURE – Bacteria is then exposed to individual(s) and inhaled into the lungs

How can we break the chain and be compliant?

Temperature is the most common strategy for controlling the risk from Legionella bacteria. One important part of breaking the causal chain is to keep the cold water cold. Don’t forget the five steps to compliance and safe water system operation:

  • Keep hot water hot
  • Keep cold water cold
  • Keep it clean
  • Keep it moving
  • Record all actions

What should we be doing?

So in hot weather, consider increasing your flushing regime – more frequent flushing, for a longer period, across all cold water outlets. And don’t forget to keep up with your usual Legionella control actions. If you’re concerned about the risks, or just want peace of mind, why not also take samples and get your water tested by a UKAS accredited laboratory.  Let us know if you need some help with this. Email us ( or give us a call (01245 362 366 / 01422 208 518) to discuss your requirements.