Legionella in Water: An MCLG of Zero

By SubcontractorHub Editorial Team·Published November 2026

A cooling tower, a decorative fountain, and a hot-water system are not the same machine as a kitchen tap on a private well. Legionella in water is the organism that ties those building systems to a pneumonia. This page is the public row and the CDC description. It is not a medical diagnosis, and it is not a biocide dose.

What Legionella in Water Means on the National Table

EPA's national primary table, fetched the day this page was written, lists Legionella with an MCLG of zero and a treatment technique under footnote 3. The health cell says Legionnaire's Disease, a type of pneumonia. The source cell says the bacteria are found naturally in water and multiply in heating systems. There is no milligram-per-liter MCL on that row. Do not invent one so the quote has a digit.

Footnote 3 is the Surface Water Treatment Rule note. It says systems using surface water, or ground water under the direct influence of surface water, must disinfect and must filter or meet criteria for avoiding filtration. For Legionella, the footnote says there is no limit, but EPA believes that if Giardia and viruses are removed or inactivated according to the treatment techniques in the Surface Water Treatment Rule, Legionella will also be controlled. Giardia on that note is 99.9 percent removal or inactivation. Viruses are 99.99 percent. Those percents belong to Giardia and viruses. They are not a Legionella removal claim.

The rule page that walks that footnote is what is the Surface Water Treatment Rule. The standards apply to public water systems. A single-home well is not named in that opening sentence. Building plumbing can still grow the organism the source cell ties to heating systems. That is a facilities question, not a private-well MCL.

How Legionella in Water Spreads, on the CDC Page

CDC's page on Legionnaires' disease, marked August 6, 2025, says the illness is a serious type of pneumonia caused by Legionella bacteria. It says people can get it by breathing in mist that contains the bacteria, and that in general it is not spread person to person. Symptoms usually develop 2 to 14 days after exposure, and can take longer. The symptom list on that page is cough, fever, headaches, muscle aches, and shortness of breath. Confusion, diarrhea, or nausea can also occur.

The same page separates two illnesses. Legionnaires' disease is the severe pneumonia. Pontiac fever is a milder illness that can include fever, muscle aches, and headaches. Most healthy people exposed to Legionella do not get sick. The page says about 1 out of every 10 people who get sick with Legionnaires' disease will die from complications, and about 1 out of every 4 will die if they got it during a stay in a healthcare facility. Those fractions are CDC's. They are not a prediction for a building you have not tested, and this page is not medical advice.

Treatment, on that page, is certain antibiotics. Most cases can be treated successfully. Identifying and treating the illness early can increase the chances of getting better. Healthy people usually get better, and they often need hospital care. The organism was identified after a 1976 outbreak among people who went to a Philadelphia convention of the American Legion. A dealer does not diagnose any of that from a water sample on a kitchen counter. A healthcare provider or the lab reports a case to the health department.

Water Management, Without a Temperature This Page Did Not Fetch

CDC's overview of water management programs, dated March 15, 2024, says many buildings need a program for the building water system or for specific devices. The seven steps are: establish a team, describe the building water systems, identify where Legionella could grow and spread, decide control measures and how to monitor them, establish interventions when limits are not met, make sure the program runs and works, and document and communicate the activities.

The principles on that page are adequate disinfection, maintaining devices to prevent sediment, scale, corrosion, and biofilm, maintaining water temperatures to limit Legionella growth, and preventing stagnation. Sediment, scale, corrosion, and biofilm provide a habitat and nutrients. The page says anti-scald regulations may limit maximum allowable water temperatures. It does not print a degree number in the text fetched for this article, so this page does not supply one.

Devices the page names, where a program may cover the device rather than the whole building, include cooling towers, decorative fountains, hot tubs, and respiratory equipment intended for nebulization. Whole-building programs are named for hospitals and long-term care, buildings that house or serve a vulnerable population, and large buildings with complex water systems. A cooling-tower contract that never mentions biology is thin for a different reason. The water-balance side of that machine is cooling tower water treatment. This article does not set a biocide residual.

What Should the Legionella in Water Note Say?

Five lines keep a zero goal from being rewritten as a dose or a diagnosis.

1. The organism name, and that the public MCLG is zero with a treatment technique, not a milligram MCL.
2. The Surface Water Treatment Rule sentence: no separate limit, control described through Giardia and viruses.
3. Whether the water is a public system, a building system, or a private well. Do not apply the public row as a house fine.
4. If a building program is in scope, the device: cooling tower, fountain, hot tub, or the plumbing the customer actually has.
5. What you did not claim: no temperature setpoint, no biocide dose, no removal percent, and no statement that a person is sick.

Where the Note Sits on the Job

A water-treatment dealer and an HVAC contractor can both meet this organism. The dealer meets it on a tower or a building loop. The HVAC contractor meets it when a heating system is the place the source cell says the bacteria multiply, or when a tower serves the chillers. Neither meeting is a license to invent a kill claim. Write the sample, the device, and who is responsible for the program. Leave the medicine to the clinician.

The federal frame for public systems is the Safe Drinking Water Act. A customer file that can hold the device and the sample is water treatment contractor software or HVAC contractor software, depending on who owns the job. This article does not say a building is safe.

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