Submitted by malahchi t3_1118ssa in askscience
Legionella is a clade of bacteria that were discovered living in mechanical ventilation systems. Some countries currently forbid double-flow mechanical ventilation systems because of how much pathogenic microorganism can live in there.
My question is : what do they feed on ? Legionella are cultured in yeast extract agar. What are the sources of food and how can there be enough water in mechanical ventilation systems ?
Yep, the water used in the cooling tower is the perfect place for bugs to grow and many commercial HVAC systems don’t have HEPA/UV treatment for the air handlers. You can treat the cooling tower water with quats to suppress bacterial/viral growth. Also, the condensate that forms in the air handlers is also ideal for bugs to grow.
The movement in HVAC is to have separate air handlers and only one cooling tower.
Anywhere that you cannot afford to shut down the cooling you will have more than one cooling tower so you can continue operation if a tower is out of service. (eg Hospital, Critical computer centre, Critical Process)
I had no idea legionella was named after the outbreak among American legion members and that’s where the legionnaires name comes from. Wow
It was a tragic irony that they'd survived WW1, WW2, Korea, Vietnam and/or whatever else, only to be killed by contaminated water droplets in the USA.
And finding out that Legionella was the main cause was also very lucky as the Hotel had started to clean the ventilation shafts before a proper investigation of them had started.
I'm curious to hear more about the story. Were the US Legionnaires killed from breathing in what is essentially toxic fumes?
https://en.wikipedia.org/wiki/Legionnaires%27_disease#Epidemiology
Elderly men, possibly with a large proportion of smokers or ex-smokers. It had been thought the outbreak was a form of pneumonia.
They inhaled atomized water droplets from the cooling towers that carried the bacteria.
This is why it’s a good habit to breathe through your nose. I swear that’s why it took me so damn long to get Covid. But now now masks and packed bars I got it once.
Did the bacteria enter the hotel's ventilation system? Aren't they sealed off from each other? I'm picturing my home cooling unit and an invasive element would have to penetrate the metal coil to get into the house, and repeat that to access the air.
+1 to this. The cooling tower ought to be in the secondary circuit - a refrigerant is used in the primary circuit to cool the air in the evaporator - the inside unit of the air conditioner, and condensed back to a liquid in the condenser, with the heat being removed by the secondary circuit water. The water gets warmed up and then cooled down again in the cooling tower.
Was the party out on the terrace near the cooling towers where the legionnaires were exposed to the cooling tower spray carried by the wind?
They probably inhaled contaminated water droplets outside.
There was an outbreak in Cumbria, UK, 15 years or more back and it was random people passing through the vicinity who were infected.
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Another thing about Legionella, the case that made it famous, was where it was growing in a cooling system. That involves significant changes in the air temperature, resulting in lots of condensation, making it real easy for the micro-organisms to get access to water.
There is a lot of water in mechanical ventilation circuits due to condensation from air exhalation from the lungs. In fact many ventilators add humidification to decrease this water loss, which further saturates the circuit.
Warm water will turn into an ecosystem if left undisturbed, especially when exposed to oropharyngeal secretions dripping around an endotracheal tube cuff.
Condensation occurs where air meets a surface that is below the dewpoint temperature. This is mainly in winter or in warmer weather on cooling coils when mechanical cooling is needed.
Humidification is mainly used in hospitals and when the outdoor air contains very little moisture. (Winter or Desert). Hospitals have high ventilation rates, sick people and immune compromised people. Without humidification, the humidity in the occupied space can drop far below the optimum for health and comfort of around 50%
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Which section of the mechanical ventilation system? The only places I could even conceive of legionella growing is in cooling towers that are not properly maintained, and immediately downtream of cooling coils where the condensation pans are plugged and there is standing water.
The cooling towers weren't 'properly maintained' by modern standards because the hazard wasn't recognized.
Cooling towers mostly vanished from AC systems in subsequent decades because the maintenance & chemical water treatment regimes are now so onerous and expensive. Air cooled condensers are more usual now.
Similarly, spray humidifiers were replaced with steam humidifiers, etc..
Outbreaks occur in new York still. They passed law in 2015 for water tower maintenance. 50000 violations...
https://www.nytimes.com/2022/11/12/nyregion/nyc-legionnaires-nursing-home.html
What I think most the posts right now are missing that condensation itself isn’t enough to sustain life. Moisture is only one part. There is a lot of dust in the air that gets sucked into the vents. Usually hvac systems blend a mix of inside and outside air. Indoor air has a lot of things like skin cells, fibers from clothing and carpets. Outdoor air has dust air pollution pollen and other stuff.
Commercial hvac systems in theory have filters, but they aren’t perfect filters, and roof mounted systems are notorious for having old dirty filters, missing or not properly placed so they intake a lot of unfiltered air.
All that stuff settles in the vents, add moisture and it’s a perfect combination for bacteria to grow. Especially in poorly maintained systems they might have other mechanical issues, such as condenser coils that don’t run a defrost cycle long enough or often enough to prevent the formation of ice or excess condensation build up. Burnt out uv lamps in some commercial hvac systems.
And dust is composed of a lot of organic materials. Mainly dead skin cells.
There is a lot of stuff floating around in the air like human skin,pollen and fungal spores among many other things. Also, unless humidity is at 0%, there will be water in the air. Humidity is high in many ventilation systems because of the aerosols they use. All that stuff can stick to the walls of the ventilation system, creating biofilms where amoeba and other microbes can feed on.
Legionella lives in some of these microbes, absorbing the nutrients they get when they "eat". Legionela needs Iron to survive, so they can easilly get it from the corrotion ocurring from the "weathering" of the ventilation system.
Unless an environment is being constantly sterilized, microbes will eventually grow. They are everywhere, some are also floating in the air around you right now.
FYI just to clarify some of the things you've said:
>All that stuff can stick to the walls of the ventilation system, creating biofilms where amoeba and other microbes can feed on
Biofilms are formed by bacteria, not the other way around. They do contribute to persistence and survival as many of the individual cells within a biofilm are not actively metabolizing, making them need fewer nutrients and making them very hard to kill
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>Legionella lives in some of these microbes
Legionella is a microbe (gram-negative bacterium), and it can live within other organisms (like amoebae) as an intracellular parasite, but it does not necessarily have to
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>Legionela needs Iron to survive, so they can easilly get it from the corrotion ocurring from the "weathering" of the ventilation system
Almost all bacteria need small amounts of iron (and other metals) to survive, but they don't need to be on a surface made of iron to acquire it. They can grow on surfaces made of many different types of materials
Frikkin skin cells and then the dogs too SOO many and then all the rest pollen The condensate on HVAC in high humid areas like Florida is a thriving jelly menagerie in an endless river of water from the air.
What if I told you, you were MOSTLY bacteria (in terms of cell numbers)?
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How do you clean inaccessible ducts? I know of plenty that are one and done.
Usually shut down the system I imagine. Or reverse the flow and add disinfectants? I have experience inspecting them for mould growth but not any experience actually cleaning or maintaining systems
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This informative and seems to be a well informed response. On the other hand this sounds exactly like what ChatGPT3 would say when prompted with "How do you clean inaccessible ducts?" Which is maybe a compliment.
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The post below was something I posted on a UK motoring forum 20 years ago, so I've just copied and pasted it.
https://www.honestjohn.co.uk/forum/post/index.htm?t=6781
I also mentioned car/truck windscreen washer bottles as a legionella risk, which was 8 years before the UK's HSE issued a warning about using screen wash to kill the bacteria.
https://www.bbc.co.uk/news/10293519
"The wet cooling towers are the problem, as stated by many others.
The heat removed by the cooling process is rejected at the condenser, where the refrigerant vapour is condensed into a liquid and it's latent heat of condensation is released.
In wet AC systems the condensers are cooled by water. The water is cooled at the cooling tower and recirculated. The cooling tower ponds tend to collect all the particles in the air, limescale, leaves, insects, etc, and are often at lukewarm temperatures, providing ideal conditions for the growth of many organisms. The water is intentionally sprayed into the air to cause some of it to evaporate, creating an aerosol of water droplets containing any organisms that have been cultivated in the water.
The legionella bacteria is present in water everywhere and will multiply if given favourable conditions.
The risk of infection is much greater for the elderly or infirm, so the disease has affected many hospitals, in the past. The risk wasn?t recognized until a group of elderly ex-servicemen, attending an American Legion Convention were felled by a hotel cooling tower in Philadelphia in 1976. The disease had existed prior to that, but had been regarded as yet another strain of pneumonia.
The (UK) HSE's Code of Practice on Legionnaires' disease, L8, states that a foreseeable risk exists in;
a) water systems incorporating a cooling tower;
b) water systems incorporating an evaporative condenser;
c) hot and cold water systems;
d) other systems containing water at which is likely to exceed 20 degC and which may release a spray or aerosol."
There was an incident in Quebec City where a wet cooling AC was spraying a mist of infected water outside and a lot of citizens became ill.
It was my understanding that most Legionella that lives within such environments are actually inside of free-living amoebas, like Acanthamoeba. These cells can phagocytose other bacteria for sustenance, and can ingest nutrients from their surroundings as well. They are much tougher than the more fastidious Legionella bacteria.
Ventilators often run with a heated humidification system. The circuit will collect enough water that it has to be drained. Patients are coughing all sorts of gross crud into the tubing. The circuit also gets broken from time to time, as stuff gets soiled or clogged, parts need to get changed out. So there's often opportunity for bugs to get in there.
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Onetap1 t1_j8et0de wrote
They don't live in mechanical ventilation. They live in tepid water which has nutrients. Anything with warm water is hazardous.
The first case, where it was identified, killed a lot of US Legionnaires (armed forces veterans) at a convention at a hotel in Philadelphia. The source was an evaporative cooling tower, which uses evaporating water to discard waste heat from the air conditioning systems.
The thing about cooling towers is that the water is recycled from a pond in the base of the tower. It collects insects, dust leaves, limescale, etc, nutrients for bacteria. And the water is warm. The bacteria was distributed on the water droplets leaving the tower.
https://en.wikipedia.org/wiki/Legionnaires%27_disease#Epidemiology