How Is Legionnaire's Disease Contracted?

Maybe I’ve been misinformed all these years, but I always heard Legionnaire’s disease is spread via the water of a facility’s air conditioning system. But, this does not make sense because the system (known as a hydronic system) is a closed loop. The public is never exposed to the water in a circulating loop (cold supply, warm return). Googling, I found the same rhetoric which must be the short story. Does the SDope have a factual explanation? Maybe the A/C is not to blame, after all?

I’ve always heard it often was from showers, in systems which hadn’t been in use for some time so the bacteria had had time to grow; it’s the reason given in Spanish “old women’s wisdom” to always let the water run for several minutes before using it if a faucet hasn’t been used for weeks. Note that a/c didn’t become common in Spain until very recently, a lot more recently than showers and public fountains.

According to the Mayo Clinic, one of the possible sources of contaminated water is indeed a/c. Note that many a/c systems aren’t closed-loop for the water: my own a/c system for example condenses water and pours it outside through a tube.

Legionella bacteria are typically transmitted via inhalation of aerosols from contaminated water or soil. AC units on roofs and elsewhere are notorious for dripping condensed water into pools, where it can thrive. Said pool can even aerosolize and get transmitted thru the AC ducts, thus spreading the deadly mist.

I actually treated a patient with legionella back in 1986. He came into my ER with his arms around his pregnant wife. We thought he was bringing her in to deliver the baby, but she was dragging him in. Feverish, delirious, and short of breath, a lung exam quickly indicated pneumonia, and CXR confirmed. But he was young and healthy, and looked too damn sick for a typical community-acquired pneumonia. I dithered a bit about what antibiotic to put him on, given the severity of his symptoms. But then I remembered my Pulmonologist mentor who once told me that erythromycin was good for about any bacterial pneumonia, and if the patient looked sick as hell, give erythro IV even tho it burns like hell via that route. So I empirically put him on about the only drug we had then that would kill Legionella (never suspecting he might have it).

He lived. A few other folks showed up in the community over the next 72 hours sick as hell with pneumonia, and some of them died. Legionella was soon found to be growing in the pooled water of a factory’s air conditioning unit.

I got lucky with my WAG; and he was even luckier.

Haven’t seen a patient with it since.

It is not the water in the closed systems of a HVAC system, but the open systems.

Condensate forms on cooling coils of a AC system, runs down the cooling fins and collects in the pan under the coils. This moist enviorment is a place where the Legionella spores can grow and multiply. In a properly maintained building pads are put in the drain pans to kill the Legionella.

Another source is cooling towers. Legionella can grow on the damp surfaces of the tower. Then when the tower fan starts up the exhaust from the tower can contain Legionella. If the buildings fresh air inlets are near the tower the building’s ventilation can draw some of the Legionella. In a properly maintained tower chemicals should be added to the tower to kill any biological growths. And the chemical company should be testing for Legionella.

But there can be pools of water caused by any number of situations: leaking pipes, rain that doesn’t get drained, sprinkler systems, etc.

What’s so special about the pools of water from air-conditioning systems?

There are many different bacteria classified as Legionella. If it comes in contact with water it can grow and multiply. It can be in pools from leaky pipes, but from there it is hard to become air born. Same with rain water and sprinkler systems. But AC systems will have air moving over and possibility causing the bacteria to be air born and delivered in spaces where humans are breathing the contaminated air.

Its because the condensate from air cons is not only moist, its also nice and gently warm - allows all sort of things to grow in it from algae to legionella.

Interestingly, Legionella seems to require amoebas (and other protozoa) in its environment to survive and proliferate. They have co-evolved with one and other.

A leaking pipe may produce a stagnant pool of water on the floor. That pool is probably in a place that isn’t getting a lot of airborne bacteria (and food for said bacteria) blown over it, and it isn’t getting aerosolized by a high-speed air flow over it. If it’s in an occupied area, it’ll probably get noticed and cleaned up before too long.

Condensate stuck in an HVAC system has the opposite situation: it’s in a place where it may persist unnoticed for long periods, it’s getting plenty of airborne bacteria and particulates blown across it, and it’s likely to get aerosolized at some point and blown directly into occupied areas of the building.

While Legionella can thrive in a lot of places, cooling towers are the biggest culprit in outbreaks, and can spread the bacterium for many kilometers around them. Whereas HVAC units typically pass filtered air through the chiller coils, cooling towers pass large volumes of unfiltered outdoor air through them, pretty much guaranteeing that the water they are cooling will eventually collect, nurture and aerosolize various microbes, spewing clouds of infectious matter throughout the surrounding area.

Thanks for your thoughts.

Isn’t the water coming out of the pipe water that has condensed from the outside air, rather than water from the cooling loop? (Bear in mind I am from the UK, where we rarely encounter air conditioning, other than in cars.)

The condensate drip is water from the inside air (formerly known as the outside air, because it’s air and it goes places.) No air conditioner is using water as a coolant.

Automotive air conditioners typically take outside air, chill it, and pass it through the interior of the car once. In this situation, yes, the condensate is being wrung out of outside air. (you may select a recirculation mode, in which case you’re wringing condensate out of interior air; in the long run, this is just moisture exhaled by the occupants.)

In homes, the air conditioning system typically chills and recirculates indoor air. Any water on the air-side of the heat exchanger will just be condensate from indoor air (although most homes are porous enough so that eventually you do exchange all the indoor air with fresh outdoor air; this depends on the age of the house, construction quality, wind, how often doors are opened, etc.).

In both of these cases, the refrigerant being passed through the heat exchanger that actually cools the air is usually some kind of CFC or HCFC, not water. OTOH, large commercial buildings typically employ an HCFC-based chiller that chills water, and then that chilled water is piped to various zones in the building where it passes through heat exchangers to chill the local indoor air. As long as the piping system isn’t leaking chilled water, then the only water present on the air-side of those zone heat exchangers is condensate from the indoor air itself.

As noted upthread though, these systems don’t usually have the sort of flows that tend to aerosolize water droplets. If you don’t drain the condensate well you may end up with a mold issue. However, a Legionella colony is unlikely here (though not impossible), not just because of the poor aerosolization, but because they prefer warmer temperatures. If you’re trying to cool indoor air to, say, 75F, then the heat exchanger (and any condensate coming off of it) is going to be quite a bit colder than that; Legionella can survive at those temps, but tends not to grow.

Home and automotive air conditioners typically rely on ambient outdoor air to cool the freshly-compressed refrigerant in the condenser (the hot heat exchanger at the front of the car or outside the home). Commercial water chillers may instead use a second water circuit to take heat out of the condenser, and then use a cooling tower to take the heat out of that hot water and give it to the ambient outdoor air. Those cooling towers rely on direct water-to-air contact, and deliberately create lots of air flow and lots of water surface area for heat transfer and evaporation. Here’s a short educational video about how cooling towers work, and here’s a great video from the inside of a natural updraft cooling tower. In that second video at 0:08, note the vast array of showerheads spraying water into the air, and the steam cloud rising up from there. This is a warm, moist environment, perfect for cultivating Legionella unless preventive measures are taken (e.g. adding chlorine to the process water).

Now I’m doubly confused. I thought that water-cooled HVAC systems were what we were talking about.

Are you thinking of an evaporative cooling system? Where Water is piped over a medium that air is passing through> In this case the water absorbs heat from the air and evaporates lowering the temperature of the air and increasing the humidity of the air. Great breading ground for many things if not treated. But these systems are normally once through and used in areas with extremly dry air, like the desert. Mostly used in homes and not buildings.

Legionnaire’s is more common in buildings where the HVAC systems are not properly maintained.

In a residential air conditioner (or even a window a/c or your car) the hot refrigerant runs through a condenser coil that’s outside. The refrigerant is hotter than the air outside, and that outside air is blown over the coil to force the heat out. It’s just like the radiator for your car’s engine.

In large buildings, they don’t want to be pumping huge volumes of refrigerant at high pressures to the roof of the building because that’s expensive piping and it requires additional pumps and increases the risk of refrigerant leaks. So instead, there’s a heat exchanger in the mechanical room that transfers the heat from the condenser to water. Think of it like putting the coil from your home air conditioner in a bathtub full of water. That water is then pumped to the cooling tower on the roof, requiring much less pressure and energy, which then sprays the water into the air with a fan blowing through it to extract the heat before it’s sent back to the heat exchanger, as Machine Elf described.

So instead of dumping the heat from the refrigerant directly into the air, it’s dumped first into a water loop and then into the air via the cooling tower. That can even be more efficient than direct air transfer because some of the water evaporates, cooling it below the outdoor temperature, sometimes significantly if the relative humidity outside is low. Most of the water still falls back into the reservoir, and just a small amount is needed to keep it topped up. But what you have there is warm to hot water in a box that’s constantly having air and dust blown through it, that’s shielded from the sun and any ultraviolet light, and in a lot of cases droplets of water get blown out the top. Stuff the grows in the pool of water in the cooling tower gets blown out and can fall on people on the sidewalk, or get sucked into fresh air intakes that are also on the roof, or even though buildings next door.

So this has nothing to do with the cold condensate that forms on the interior evaporator coils. That’s usually drained away, or in a window a/c unit it can be splashed over the condenser by the fan, but it tends to dry out between on/off cycles and the water isn’t aerosolized. The “water cooling” via the cooling tower is simply an alternate way to extract the heat from the hot side of a regular refrigeration system using water as an intermediary.

I disagree with your last paragraph. The possibility of legionella being spread through the cooling coils is very small but not “nothing to do with the cold condensate”. This small chance is one of the reasons why chemical pads are put in the condensate pan. The chance is very slim but not impossible so one should not take the chance.

In what sort of system does condensate collect in a pan rather than being just drained away?

Since the OP has been answered, I’d like to point out a water feature in an Hospital that caused legionnaires’ disease.

Here is the link : “An outbreak of Legionnaires disease associated with a decorative water wall fountain in a hospital.”

I suspect the bacteria for this disease is aerobic and likes warm conditions. So pooled water may not provide it the oxygen it can get in fountains or cooling towers.

It collects until it can reach the drain. I have never seen a dry drip pan with the chiller is running.