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Legionella risks during the coronavirus pandemic

Employers, the self-employed and people in control of premises, such as landlords, have a duty to protect people by identifying and controlling risks associated with legionella.

If your building was closed or has reduced occupancy during the coronavirus (COVID-19) pandemic, water system stagnation can occur due to lack of use, increasing the risks of Legionnaires' disease.

You should review your risk assessment and and manage the legionella risks when you:

  • reinstate a water system or start using it again

  • restart some types of air co

nditioning units

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

You can find out what Legionnaires' disease is, where it comes from, how people get it and symptoms and treatment by reading our guidance What is Legionnaires' disease?.

Hot and cold water systems

If hot and cold water outlets are used infrequently, flush them weekly to prevent water stagnation. If you cannot do this, work with your competent person or people to ensure systems are cleaned (if required) and disinfected before the building is occupied.

For further guidance read:

  • Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8 ACOP)

  • Legionnaires' disease - Technical guidance (HSG274 Part 2) (PDF)

Cooling towers and evaporative condensers

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

  • adequately trained personnel are available to carry out essential checks and monitoring

  • chemical supplies are maintained and dosed appropriately

Speak to your water treatment company for help and if you need to stop operation of any systems.

If cooling towers and evaporative condensers are likely to be out of operation for:

  • up to a month - isolate fans, but circulate biocidally-treated water around the system for at least an hour each week

  • more than a month - drain down the systems and clean and disinfect them. Clean and disinfect the systems again before refilling and returning to operation

For further guidance read:

  • Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8 ACOP)

  • Legionnaires' disease - Technical guidance (HSG274 Part 1) (PDF)

Air conditioning units

If your workplace has been closed for an extended period and has air conditioning units that have a source of water that can generate aerosol, you will need to assess the risks of legionella being present within them before restarting.

Small wall or ceiling-mounted units with closed cooling systems should not present a risk.

Larger units may present a risk if they have improperly drained condensate trays, or humidifier or evaporative cooling sections where water can stagnate, becoming a reservoir for bacteria to grow.

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 must maintain the existing control regimes

  • not being used, you should drain, clean and disinfect them. You should also clean and disinfect them before reinstatement

For further guidance read:

  • Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8 ACOP)

  • Control of legionella and other infectious agents in spa-pool systems (HSG282)

Methods of control


Temperature control is the main form of control used in hot and cold water systems. 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 do an equally effective job. Your supplier should be able to confirm which products you can use. If you're still unsure you can email HSE at:

In addition to biocides, there may be 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 as an alternative to scale inhibitors, but this may not be reasonably practicable on larger systems.

Physical methods

You can use physical methods for cooling tower control such as hydrodynamic cavitation, ultrasonic cavitation, TiO2 Advanced Oxidation Process. However, the uptake on those systems has been low to date and they may not suit all systems.


You may be able 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. However, there may be a significant capital investment to pay for this and there may also be a long lead time.

Changing control methods: risks to operators

If you change your control methods or strategy, operators may be exposed to additional or different risks. For example, moving from an oxidising biocide to isothiazoline may introduce a new skin sensitisation risk.

HSE would expect you to:

  • review your risk assessments (for legionella and COSHH) and controls

  • increase the level of monitoring during the commissioning of any new controls

Personal protective equipment (PPE) required for cleaning water systems

If you need to clean water systems it's likely that respiratory protective equipment (RPE) will be needed.

RPE must:

  • be adequate

  • be suitable

  • provide an assigned protection factor of at least 20

Disposable RPE, such as FFP3 respirators used by health and care workers, may be in short supply. If your usual types of RPE are unavailable, you can source alternatives as long as your risk assessment demonstrates they are suitable and adequate for workers and the task.

An alternative could be:

  • a reusable half-mask or full-face respirator fitted with a P3 filter

  • a powered respirator and hood class TH2 or 3

  • a powered respirator and close-fitting full-face mask class TM3

  • an air-fed hood or full-face mask supplied with breathing quality air

You can find more information in our guide Respiratory protective equipment at work (HSG53) (PDF) .

Getting specialist help

STN can support you in the advice you may need from a competent person and/or specialist to help you identify and implement suitable controls for legionella.

HSE recognises that getting specialist help may be difficult during the coronavirus pandemic. If you're unable to appoint someone with the appropriate authority and competence to oversee the risk controls of water systems, you must consider stopping operation of the systems.

HSE Page last reviewed: 31 March 2021


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