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Water in idle buildings can quickly stagnate allowing Legionella, a potentially lethal pathogen, to thrive. Joachim Keck, a Casualty Risk Consultant with AXA XL, outlines the threat as well as the measures companies should consider taking to eliminate this risk.

COVID-19 is retreating across parts of the world, and some semblance of “normal” life is starting to return. However, in the fight against this virus, we may have unwittingly created conditions that enable another potentially lethal pathogen—in this case, a bacterium—to thrive.

I know: You’re thinking, “Great, just when they gave the okay to reopen our doors, now there is something else to worry about?”

Unfortunately, yes. But, while the risk is real, in most cases it can be eliminated through simple measures that typically cost next to nothing.

Stagnant water is bad water

This new threat is from Legionella pneumophila, a common pathogen in many environments that flourishes in water pipes and cooling towers. Especially when the water isn’t flowing. Like during a lockdown when facilities are shuttered.

Legionella are a genus of widespread, rod-shaped bacteria that naturally occur in surface water and groundwater, usually in small numbers. It acquired its name after an outbreak at an American Legion convention (that’s an association of U.S. military veterans) sickened 221 attendees and led to 34 deaths. The causative pathogen was a previously unknown bacterium, subsequently named Legionella.

What’s now called “Legionnaires' disease" is not uncommon. In Germany, an estimated 15-30 thousand people catch the disease each year, while in the U.S. there are approximately 100 thousand cases per year. These figures, however, likely are understated as doctors may not consider Legionellosis when treating patients with pneumonia.

Legionnaires' disease has several similarities with COVID-19, as well as some important differences. In both cases, the illness is caused by inhaling droplets or aerosols containing the pathogen. With COVID-19, the source is an infected person. With Legionellosis, on the other hand, the starting point is water contaminated with high levels of Legionella which are then released into the atmosphere; common sources include showers, humidifiers, hot tubs and even water faucets/taps. Picture this: Someone washing their hands to limit the spread of coronavirus could inadvertently release water droplets containing Legionella into the air.

The incubation period for the illness also is similar; from two to ten days. Furthermore, the risks are greater for people with weakened immune systems as well as for seniors and smokers. It seems to strike men more often than women, and children only rarely.

Like COVID-19, the illness is characterized by cough, chills, headache and high fever. It is, however, more lethal. According to the U.S. Centers for Disease Control and Prevention, the mortality rate is about ten percent. Moreover, for patients who contract it in a healthcare facility, the death rate approaches twenty-five percent.

There is also a lesser form called “Pontiac fever” that results in flu-like symptoms such as fever, malaise, headache and body aches. This milder type is nonfatal, and the disease usually heals itself after a week.

Unlike COVID-19—and this is important to understand—neither Legionnaires' disease nor Pontiac fever are highly contagious. Only rarely will an infected person spread these diseases to someone else.

Uncharted waters

Legionella reproduce best at temperatures between 25°C and 45°C (77-113°F). They die above 60°C (140°F) and hardly reproduce below 20°C (68°F).

While water pipes and cooling towers offer ideal conditions for the bacteria, the risk of it developing or multiplying usually is very low. The constant flow of fresh water, along with the occasional introduction of very hot or very cold water, keeps Legionella in check. So, too, does the addition of chlorine or other disinfectants into the water supply.

However, if water in a building stagnates for too long, the disinfectant disappears and the ecosystem inside the piping changes. The same also applies to idle air conditioning and industrial systems using water as a medium for heat exchange.

That’s the situation the world faces today with countless buildings that have been shuttered for a long time. These include not only office buildings and manufacturing/assembly operations but also hotels, retail establishments, etc.

Neither their water-supply systems nor water-using equipment were designed for these stagnant conditions. Moreover, academic researchers and public health officials report that the consequences of a long standstill are mostly unknown. Said one, “We haven’t really done studies on months long stagnation.” We are truly in—so to speak—uncharted waters.


Companies have an obligation to ensure the safety and well-being of their employees, customers and suppliers. With Legionnaires' disease, that means taking reasonable precautions to ensure that the water in a facility—wherever it appears—does not contain harmful levels of Legionella. The following outlines the conventional practices commonly used to prevent Legionella from breeding.

Fortunately, many buildings won’t have sat completely idle and unattended during the lockdown with security and maintenance personnel onsite at least periodically. In these cases, the building presumably can be opened if:
• the water system was flushed out at least every three days
• the water temperatures at the taps have been checked and reach at least 55°C (131°F)
• microbiological tests performed during the lockdown showed no significant change in the total number of bacteria.

For buildings that were idle for at least seven days—and especially those where the water pipes and relevant water-reliant systems were turned off for more than a month—the water needs to be tested, and, as necessary, flushed and disinfected. At a minimum, test samples should be collected at the following locations:
• flow and return of the water heating units (circulation)
• all endpoints of the hot-water pipes in different buildings or floors
• all tapping points that are conspicuous during the orientation inspection
• cold-water supply and pipe parts with warming above 25°C (77°F)
• all tapping points with stagnant water.

If more than 100 colony-forming units (CFU) of Legionella are found in a 100 ml sample, the water is considered contaminated, and the entire system needs to be thoroughly flushed with hot water (65-70°C / 150-158°F) for at least three minutes. Note, however, that this is not always possible in cold-water systems; in those cases, chemical disinfection is required.

At concentrations above 10,000 CFU @ 100 ml, immediate further countermeasures are required, e.g., by chemical disinfection. Chemical disinfection must cover the entire drinking water system. The installation must be flushed with approved chemicals such as chlorine, chlorine dioxide, calcium or sodium hypochlorite or ozone. This process should be done by a specialized company, and the water is not drinkable during disinfection.

Note that all these provisions—testing, flushing, disinfecting—also apply to cooling towers, air conditioning systems and any other machinery or equipment with water reservoirs.

All things considered, preventing an outbreak of Legionnaires’ disease is far easier and less expensive for businesses to undertake than preventing the spread of coronavirus. Testing for Legionella and, if necessary, flushing out or disinfecting building water should be a fundamental part of all plans for resuming operations. COVID-19 has already claimed enough lives without the unnecessary loss of ancillary victims of this pandemic.

Note: If you have questions about potential impacts of the shutdown to your operations, or about Legionnaires disease specifically, please reach out to our Casualty Risk Consulting Teams to discuss.

Joachim Keck is a biologist who works as a Casualty Risk Consultant specializing in environmental and product safety in a variety of industry sectors including biotechnology, genetic engineering, pharmaceuticals, and food and beverage. He is based in Munich and can be reached at


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