What Is Legionella? Where It Grows and Why Water Systems Are at Risk
What is Legionella? Legionella is a group of bacteria found naturally in water, which can multiply to dangerous levels in man-made water systems and cause Legionnaires' disease, a potentially fatal form of pneumonia, when people breathe in contaminated water droplets.
The bacteria are not a problem in a river or a pond. The risk comes when they find their way into a building's pipework, tanks or cooling equipment, where warm water, stagnation and nutrient deposits give them the chance to grow. Understanding those conditions is the starting point for every sensible control regime, and it explains why the law expects dutyholders to assess and manage the risk.
What is Legionella? The bacteria explained
Legionella is a genus of bacteria that lives in fresh water. There are many species, but Legionella pneumophila is the one most often linked to human illness. The Health and Safety Executive (HSE) uses the collective term legionellosis for the illnesses the bacteria cause, the most serious being Legionnaires' disease. Milder, flu-like illnesses Pontiac fever and Lochgoilhead fever also sit under that umbrella.
HSE describes Legionella as widespread in natural water systems such as rivers and ponds. In those environments the bacteria are usually present in low numbers and rarely cause harm. Small numbers can also pass into mains supplies and from there into buildings. That is why there is no such thing as a "Legionella-free" system to rely on: the aim is to stop the bacteria multiplying, not to assume they can never arrive.
Where Legionella grows in building water systems
Once in a building, Legionella will multiply wherever conditions suit it. HSE and its technical guidance, HSG274, point to a consistent set of factors.
Temperature
Temperature is the single most important factor. HSE guidance gives the following reference points:
Below 20°C: the bacteria are dormant and do not multiply significantly.
20–45°C: the range in which they can multiply, where nutrients are available.
Above 70°C: the bacteria do not survive.
Warm water is therefore the enemy of good control. Cold water that has picked up heat from a warm plant room or ceiling void, or hot water that has cooled in long pipe runs, can sit in the growth range for long periods without anyone noticing. Our article on the risks of elevated cold water temperatures looks at this problem in more detail.
Stagnation and low use
Water that stands still warms up or cools down towards the temperature of its surroundings, and any disinfectant in it is used up. Stagnation commonly occurs in:
little-used outlets, such as guest toilets and showers and cleaners' sinks
dead legs, which HSE describes as pipes leading to outlets that are unused or rarely used
blind ends, which are redundant lengths of pipe closed at one end
oversized storage tanks with a slow turnover of water
buildings during holidays, shutdowns or periods of low occupancy
Biofilm, scale and sediment
HSE lists rust, sludge, scale, organic matter and biofilms as sources of nutrients that can support growth. Biofilm, the slimy layer that forms on the inside of pipes, tanks and fittings, is especially important. It gives Legionella food and shelter, and it protects the bacteria from heat and chemical treatment. Hard water areas, where scale builds up quickly on showerheads, heat exchangers and calorifiers, can provide extra surfaces for biofilm to form.
How people are exposed
People do not catch Legionnaires' disease from drinking water. HSE explains that people contract it by inhaling small droplets of water, known as aerosols, suspended in the air, which contain Legionella bacteria. The disease is not known to spread from person to person.
This is why the presence of bacteria alone is not the whole story. HSE says the risk increases where:
water is stored or recirculated
the water temperature in all or part of the system may be between 20°C and 45°C
there are deposits that can provide nutrients for the bacteria
it is possible for breathable water droplets to be created and dispersed
Showers, spray taps, cooling towers, spa pools and some industrial processes all create aerosols. A system that grows Legionella and also produces a spray is the combination that causes cases and outbreaks.
HSE stresses that everyone is susceptible to infection, but some people are at higher risk, including people over 45, smokers and heavy drinkers, people with chronic respiratory or kidney disease, diabetes, lung or heart disease, and anyone with an impaired immune system. This matters when you think about who uses a building: a care home, hospital or hotel will often have more vulnerable occupants than a typical office.
Which water systems are most at risk?
HSE identifies the systems most likely to create a Legionella risk. Each is covered by its own guidance document.
Hot and cold water systems (HSG274 Part 2): found in almost every building, from offices and schools to rented homes and hospitals. Showers, taps, storage tanks, calorifiers and long pipe runs are the usual areas of concern.
Evaporative cooling systems (HSG274 Part 1): cooling towers and evaporative condensers. They operate at warm temperatures, recirculate water and release aerosols into the open air, so they have been linked to some of the largest outbreaks.
Spa pools and hot tubs (HSG282): water held at warm temperatures, with bather contamination and air jets that create a fine spray right where people breathe.
Other risk systems (HSG274 Part 3): for example humidifiers, decorative fountains, vehicle washes and some industrial processes.
Even a modest hot and cold water system can present a foreseeable risk if water is allowed to stagnate or sit at the wrong temperature. Size is not the deciding factor; conditions are.
Why the law requires you to control the risk
Employers and people in control of premises have duties under the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health Regulations 2002 (COSHH), which treat Legionella as a biological hazardous substance. The Approved Code of Practice (ACOP) L8 sets out how to meet those duties. It has special legal status: following it is normally enough to comply with the law on the matters it covers.
In practice, that means identifying and assessing the sources of risk, preparing a written scheme to prevent or control it, putting that scheme into practice, appointing a competent person to manage it and keeping records. For hot and cold water, the main control is usually temperature. HSE guidance is that:
hot water should be stored at 60°C or higher
hot water should reach at least 50°C at outlets within one minute (55°C in healthcare premises)
cold water should be kept below 20°C, where possible
Alongside temperature, routine measures such as correct flushing of little-used outlets, cleaning and descaling, removing dead legs and regular temperature monitoring keep systems out of the conditions Legionella needs.
Key points
Legionella bacteria are found naturally in water and usually enter buildings in low numbers.
They multiply between 20°C and 45°C, are dormant below 20°C and do not survive above 70°C.
Stagnation, biofilm, scale and sediment all help the bacteria grow.
Infection comes from breathing in contaminated aerosols, not from person-to-person spread.
Hot and cold water systems, cooling towers, spa pools and other aerosol-producing systems are the main risk systems.
Dutyholders must assess and control the risk, with ACOP L8 and HSG274 setting out how.
Frequently asked questions
Can Legionella be found in a normal household or office water system?
Yes. Small numbers of Legionella can enter any building through the mains supply. Whether they become a problem depends on conditions inside the system, particularly water temperature, stagnation and the build-up of nutrient deposits.
Can you get Legionnaires' disease from drinking water?
No. HSE explains that people contract Legionnaires' disease by inhaling small droplets of contaminated water. Aerosols from showers, taps, cooling towers and spa pools are the main routes of exposure.
At what temperature is Legionella killed?
HSE guidance states that the bacteria do not survive above 70°C and is killed rapidly at 60°C. This is why hot water should be stored at 60°C or higher and distributed so that it reaches at least 50°C at outlets within one minute (55°C in healthcare premises).
Do small buildings need to worry about Legionella?
Yes. The legal duty to assess and control the risk applies regardless of the size of the building. For a small, simple system the assessment may be straightforward, but it still needs to be done and any risks managed. Our frequently asked questions page covers some common queries.
References and further reading
Legionella will always find its way into building water systems in small numbers. What decides whether it becomes a risk is how the system is designed, used and maintained. Keep water moving, keep it at the right temperature and keep it clean, and the conditions the bacteria need simply are not there.
Andrew Arnold
Co‑founder & Director
Andrew has over 20 years' experience in Legionella management and control. As a senior member of the Water Management Society and educated to masters level Andrew has extensive experience in legionella risk assessment of numerous systems and sectors.