09th Jun, 2020 Read time 5 minutes

Legionella risks during the coronavirus outbreak

Employers, the self-employed and landlords in control of premises have a duty to identify and control risks associated with Legionella. Legionella is a form of bacteria that can be found in natural water systems such as ponds. However, it can also grow in purpose-built systems where water is maintained at a temperature high enough to encourage growth and if the water is left to go stagnant.

If your building was closed or has reduced occupancy during the coronavirus outbreak, water system stagnation can occur due to lack of use which increases the risks of Legionnaires’ disease, a potentially fatal form of pneumonia.

You should review your risk assessment and manage the legionella risks when you reinstate a water system or start using it again, and when you restart some types of air conditioning units.

If the water system is still used regularly, you are still required to maintain the appropriate measures to prevent legionella growth.

Areas to check when assessing risk for Legionella

There are common purpose-built systems that can harbour the Legionella bacteria. These should be checked and assessed for risk before use, and steps should be taken to prevent or resolve risk.


Hot and cold water systems

If hot and cold water outlets are not being used or are used infrequently, flush them weekly to prevent water stagnation and to lower risk.

If you cannot do this, ensure systems are cleaned (if required) and disinfected before the building is occupied again.


Cooling towers and evaporative condensers

You should have existing plans in place to ensure safe systems of work continue during any shutdown. This includes ensuring that:

  • adequately trained staff are available to carry out essential checks and monitoring
  • chemical supplies are maintained properly even during downtime

If cooling towers and evaporative condensers are expected to be out of operation for up to a month then you should isolate fans but continue to circulate biocidically treated water around the system for at least an hour each week. If they are expected to be out of operation for longer than this time, then the systems should be drained, cleaned and disinfected after draining and again before refilling.

If you need to stop the operation of any systems you should consult your water treatment company for help before doing so.


Air conditioning units

If your workplace has been closed for an extended period and has air conditioning units, you will need to assess the risks of Legionella being present in them before restarting the units.

Smaller wall or ceiling fans units with closed cooling systems should not present a risk, however, larger units may present a risk if they have trays or humidifier or evaporative cooling sections where water can get stagnant and be the perfect place for bacteria.

When you review your risk assessment, decide what the risks are for your units and if you need to clean them safely, before they are turned on.


Commercial spa pools and hot tubs

If commercial spa pools and hot tubs are being used you should maintain existing control routines. If they are not being used, then they should be drained, cleaned and disinfected upon draining and again before re-use.


Methods of control to prevent Legionella


Temperature control is the main form of control used in hot and cold water systems and is effective in preventing Legionella. However, you may also use biocides such as chlorine dioxide and copper/silver systems.


Biocides and other chemicals

Biocides such as sodium hypochlorite, bromine donors or non-oxidising biocides are typically used in cooling towers or evaporative condensers.

If you’re unable to source certain biocides, there are existing authorised alternatives you can use that are equally as good. Your supplier should be able to confirm which products are suitable alternatives. However, if you’re still unsure you can email the Health and Safety Executive at: [email protected] for some more guidance.

In addition to biocides there may be other chemicals used such as corrosion inhibitors, scale inhibitors, flocculents, biodispersants, anti-foams, algaecides and other chemicals in use in an effective water treatment programme. Scale can be controlled by water softening rather than using inhibitors, but this is sometimes not practical on larger systems.



It is possible to replace smaller cooling towers and evaporative condensers with dry coolers or dry/wet coolers, which are likely to need no or very little chemicals to safely run. There is added cost to consider here, and so this is not usually the first option.

If you change your control methods operators may be exposed to additional or different risks and so you should review your risk assessments should this happen.


Physical methods

You can use physical methods for cooling tower control such as hydrodynamic cavitation, ultrasonic cavitation and TiO2 Advanced Oxidation Process. However, these methods are not as commonly used and do not suit all systems.


This is an overview of Legionella and the risk posed, however it is possible that you may require further assistance from professionals to completely eradicate risk. More information and guidance can be found from the Health and Safety Executive here.

Please not that if you need to clean water systems, it is likely that respiratory protective equipment (RPE) will be required to protect workers.

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