RESIDENTIAL
Here we have the normal 365 day/year homeowner and the "snow birds" from the northern latitudes in the USA. The 365 homeowner is at risk because the water temperature in the H/W heater is too low, perfect for the amplification of the opportunistic LB. Under the right conditions, the homeowner will get the "flu" and perhaps recover - perhaps not. It depends on how fast the victim seeks treatment from a knowledgeable physician. If the victim presents symptoms of tuberculosis, the odds for recovery are not favorable. Also, sometimes the treatment, cure, is worst than the illness due to toxic drugs used to address the illness. Long term kidney and liver damage is the reality.
The next scenario is the "snow bird", the retiree that flee's the snow and heads for warmer climates. Perhaps the retiree will have the house winterized, H/W heater and pipes drained, - perhaps not. If it is "winterized", has the plumber or person doing the process done it correctly?
If not, then consider the potential on that person's return several months later. Turns up the heat on the H/W heater to "normal" and proceeds to shower in order to remove the travel dirt.
SO WHAT DOES A HOMEOWNER DO?
To address the debris upon which Legionella hitches a ride, one should consider filtration of the water entering the home. A 5- micron pleated filter should remove most of the cats and dogs. Now that we have a clean supply, what next? To ask a homeowner to purchase an Ultraviolet (Uv) system, unproven for Legionella control, and then expect the necessary maintenance is asking for failure. Although, filtration systems with built-in Uv lamps forces one to visit and inspect as the pressure drops. Remember, unlike commercial buildings the homeowner is purchasing with after-tax dollars - so continued expense is not looked upon favorably and will be neglected. For the same reason Ozone is cost prohibitive and rechlorination is not an option due to TriHaloMethane (THM) scare that undermines chlorine application and the lack of expertise on the part of the homeowner. Since the HOT WATER TANK is the likely amplifier, the Legionella incubator, one should focus attention in that direction. The American Society of Professional Engineers (ASPE) has already gone on record as recommending temperatures greater than 140F/60C to address the issue. So, we have our answer, raise the temperature to maximum on the H/W heater and leave it there. Mine is at 160F! This will cook the bacteria and provide a safe water supply.
THE MANUFACTURER OF MY WATER HEATER WILL VOID THE WARRANTY IF 120F IS EXCEEDED!
Write a letter to the manufacturer with a Return Receipt Requested card (in the USA) - explaining your concerns - and hold them liable for ills. Here's an example:
"Att: C.E.O.
Ref: MODEL yyyyyyy WATER HEATER Ser#1234567890
Dear Sir/Madam
The warranty notice claims that if we exceed the XXXF maximum temperature stated therein all warranty claims will be voided.
Since the ASPE has gone on record as recommending greater than 140F, and since all health authorities agree that temperatures below 120F provide a breeding ground for Legionella Bacteria (LB) - we are placing your firm, dealer and installer on notice that we will seek redress in the courts for illnesses than can be attributed to your equipment."
The manufacturer could counter with claims of "ubiquitous" for LB. However, a DNA test will settle that issue. Also, it has been reported that LB are not as "ubiquitous" as some would have us believe. A researcher at the VA Medical Center in Pittsburgh has found that less than 50% of hospitals tested had LB in the domestic hot water systems - the same could be said of residents. If the DNA of the LB in the H/W heater is the same as the DNA from your lungs, then case closed - assuming you do not also own a whirlpool bath!
MY CHILDREN AND I WILL BE SCALDED TO DEATH!
What about scalds? OK, what about them? How many people die of scalds each year? Does it compare to the greater than 20,000 that fall victim to Legionellosis in the USA? Should we place thousands of people at-risk to death and a lifetime of misery to prevent a scalding injury to a few? One can heal 100% from a scald, one never recovers 100% from a bout of LD. If scalding is an issue, then address it with thermostatic shut-off pinch-valves that fit over the faucet proper. At less than $10.00(US) they are inexpensive compared to the alternative. I grant they are an inconvenience, but you LIVE with the inconvenience rather than suffer (if you survive) the alternative. As for showers, a good plumber should be able to retrofit with a safety system to shut the water off in event shower temperature exceeds a set point. What about the tub? Except for the "toe test" method, install a thermo sensor with alarm and light. I do not ascribe to the "it can't be done" or "it's too dangerous" school of thought. People are dieing.
 HEALTH CARE FACILITIES, HOMES FOR ASSISTED LIVING, HOTELS & APARTMENT BUILDINGS
These facilities pose a different perspective, the objective is the same but the road to the goal must be altered. The primary defense remains filtration. The cats and dogs must be eliminated. As with a residence, we should filter at the entering main. After the filtration system the technology starts to get interesting due to turf wars on the part of suppliers.
MONOCHLORAMINE
Some researchers are of the opinion that where monochloramine [MCA] (formed by adding ammonia and chlorine gas into a municipal water supply) is used, the incidence of nosocomial LD outbreaks are less frequent than where it is not. It has also been suggested that use of MCA at an outbreak site can be efficacious in decontamination at 1.5 ppm as it is long lived. For the homeowner this may be something to consider as MCA is safer to use than bleach (5% Sodium Hypochlorite) where water is drawn from wells that contain methane gas. This approach would require a residential water treatment firm with above normal level of expertise. A call to your water supplier should let you know if MCA is used by them. If they do, good... If not, then lobby for it.
RECHLORINATION
An option endorsed by health authorities - but must be controlled and addressed correctly. Unfortunately, without pH control chlorination is an "iffy" proposition as "hypochlorous acid" will not be produced. The potable cold supply is easy. However, the hot water loop poses difficulty as the chlorine residual will be reduced by the heat and increased reactivity of the chlorine on system components. Must be carefully engineered so as not to endanger those we seek to protect. A plus, in the event of a positive swab culture of the piping the equipment is already in place to superchlorinate. Also, recall that unless we address the pH issue we will not form the hypochlorous acid, the killing agent. Cost, worldwide availability, perceived ease of use and endorsement by health authorities are motivating factors going for this technology.
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