The regulated contaminant with no number
Legionella appears on EPA’s list of regulated microorganisms. Goal of zero, treatment technique rather than a limit.
Then EPA’s own footnote: “Legionella: No limit.”
It continues: “but EPA believes that if Giardia and viruses are removed/inactivated, according to the treatment techniques in the Surface Water Treatment Rule, Legionella will also be controlled.”
Compliance by belief about a surrogate. Not by measurement.
→ Cryptosporidium and Giardia: what the treatment rules require
Which is why it never appears on your report
No limit means nothing to exceed. No sampling requirement means nothing to publish.
Your Consumer Confidence Report carries a line for every regulated contaminant your utility detected. Legionella is regulated and will not be on it.
→ How to read your water quality report
The surrogate does appear, indirectly. Surface water systems must achieve 3-log (99.9%) removal or inactivation of Giardia and 4-log (99.99%) of viruses.
The route is the whole story
EPA is specific: “People are exposed to Legionella when they inhale water droplets containing the bacteria.”
Inhaled. Not swallowed.
That single sentence disposes of most of the product question. A drinking water filter treats water you drink, and NSF certifies drinking water treatment units against contaminants in water that is drunk.
The growth sites EPA names are consistent with it: hot water heaters, storage tanks, pipes, cooling towers, decorative fountains and hot tubs.
It grows on your side of the meter
EPA’s regulated contaminant table gives the source as “Found naturally in water; multiplies in heating systems.”
And it defines the territory precisely. A premise plumbing system is “the portion of the water distribution system from the water meter to the tap in homes and buildings.”
So the organism multiplies in the stretch the utility does not own, from equipment the utility does not operate, at temperatures the utility does not set.
That is why EPA’s page is about buildings rather than about treatment plants.
No certified claim exists
Searching NSF’s full selectable claim list for legionella, bacteria, virus and microbiological returns exactly one option: bacteriostatic effects under NSF/ANSI 42.
That claim is about the filter resisting bacterial growth inside itself. It is not a removal claim for anything.
Cyst reduction and microcystin reduction both return their own options, so the search works and the absence is real.
→ The regulated contaminants with no certified filter
And shower filters are certified for something else entirely
Worth saying plainly, because the shower is the aerosol route.
NSF/ANSI 177 is titled Shower Filtration Systems — Aesthetic Effects. It offers one selectable claim: free available chlorine reduction.
Aesthetic effects. One claim, about chlorine. Nothing about organisms.
→ Shower filters: what NSF/ANSI 177 actually covers
What the numbers that do exist say
EPA cites CDC estimates that 8,000 to 18,000 people are hospitalised with Legionnaires’ disease each year in the United States.
It also states most healthy people do not become infected after exposure, and names higher-risk groups:
- People 50 years or older
- Current or former smokers
- People with chronic lung disease
- People with a weakened immune system
The hospitalisation figure covers all exposure routes, including cooling towers and hot tubs, not drinking water systems alone.
The decision framework
1. Recognise that no product decision is available
There is no certified claim, so nothing can be verified before purchase. That ends the shopping question rather than opening it.
2. Treat it as a plumbing and building question
The organism multiplies in heating systems inside premise plumbing. That is equipment and temperature, not filtration.
3. If you have tested and found it, go to the health department
EPA’s stated advice is to consult state and county health departments and follow current state, county and city guidelines.
4. Do not substitute a shower filter for the question
Its standard is titled aesthetic effects and offers one chlorine claim. It is not a microbiological device.
5. Where risk factors apply, ask a clinician, not a retailer
EPA names age, smoking history, lung disease and immune status. None of those is a water treatment question.
The decision path
| Your situation | What decides it | Where that lands you |
|---|---|---|
| Looking for it on your report | No limit, no sampling duty | It will not be there |
| Want a certified filter | No NSF claim exists | Nothing to verify or buy |
| Considering a shower filter | NSF/ANSI 177 is aesthetic effects | Chlorine only, not organisms |
| Home test came back positive | EPA directs you to health authorities | State and county health departments |
| Large building or landlord | Premise plumbing is the growth site | Building water management, not filtration |
| In a higher-risk group | EPA names age, smoking, lung, immune status | A clinical conversation |
| On a surface water supply | Control inferred from Giardia and viruses | The utility’s treatment is the control |
| Worried about hot tubs or fountains | Named growth sites, outside drinking water | Different guidance entirely |
Choose to act if
- A test in your building has come back positive, in which case the action is contacting health authorities rather than buying anything.
- You manage a large building with hot water storage, where premise plumbing water management is an established field with its own guidance.
Choose to do nothing about filtration if
- You are shopping for a household filter. There is no certified claim, so no product can be matched to this contaminant at all.
Choose to look elsewhere if
- Your concern is the shower specifically. The exposure route is real but the certified shower standard covers aesthetics, and the control is water heater and plumbing management.
→ Which claims exist at all, and how populated each is
Mistakes that cost the most money
Buying a filter for it. No NSF standard offers a Legionella claim, so nothing on the market carries listed performance against it.
Reading a bacteriostatic claim as a removal claim. It describes the filter resisting growth inside itself, not treating the water.
Expecting it on the annual report. It is regulated and unmeasured, which is an unusual combination and the reason no line appears.
Assuming the utility has it covered. Control is inferred from Giardia and virus treatment at the plant. The organism multiplies after the meter.
Buying a shower filter as a microbiological control. Its standard is titled aesthetic effects and lists one claim, for chlorine.
Treating a drinking water product as protection against an inhaled exposure. EPA describes the route as inhaled droplets.
Three situations, and what changes
A household reading about an outbreak in the news. Nothing on the annual report will address it, because no figure is collected. EPA’s framing points at buildings with hot water storage and cooling towers rather than at single homes, and most healthy people do not become infected after exposure.
A landlord or building manager with hot water storage. This is the case EPA actually writes for. Premise plumbing is defined as meter-to-tap, the growth sites are named, and water management in the building is the recognised field. Filtration is not the lever.
Someone in a higher-risk group worried about their shower. The concern is reasonable and the product answer is empty: no certified claim exists for Legionella, and the shower filter standard covers chlorine and aesthetics. EPA’s route for a positive test is the health department.
The objections worth answering
“If it is regulated there must be a limit.” EPA’s own footnote says there is no limit. It is listed with a goal of zero and a treatment technique, and control is inferred from other organisms.
“Some filter must be certified for bacteria.” The only related option in NSF’s claim list is bacteriostatic effects, which is about the filter not becoming a growth site. Cyst and microcystin claims exist, so the search works — this absence is real.
“Boiling kills it, so boiling is the answer.” Boiling addresses water you drink. EPA describes the exposure as inhaled droplets, which is a different route and a different control.
“My utility should be testing for it.” No federal requirement makes them. EPA’s position is that Surface Water Treatment Rule compliance controls it, and that is the whole of the regulatory mechanism.
“A UV system would handle it.” NSF/ANSI 55 covers ultraviolet microbiological treatment systems, but its two selectable options are Disinfection Performance Class A and Class B — performance classes, not claims against any named organism — and the exposure happens at fixtures downstream of any point-of-use device.
Who this is for
Anyone who has seen Legionella on a list of regulated contaminants and wondered why no number ever appears beside it, and anyone weighing whether a household product addresses it.
It is also for landlords and building managers, for whom the premise plumbing framing is the relevant one.
Anyone with a positive test or a health concern should be talking to their state or county health department, which is EPA’s own advice.
What to settle first
Is this a water purchase question at all?
By EPA’s own description — inhaled exposure, growth in heating systems, no limit, no sampling, no certified claim — it is a building and plumbing question with a public health route attached.
→ Cryptosporidium and Giardia: what the treatment rules require → The regulated contaminants with no certified filter → Shower filters: what NSF/ANSI 177 actually covers → How to read your water quality report → City water treatment, and the three gaps a utility cannot close → Coliform and E. coli, the microbial results that are measured → Premise plumbing: the other thing that happens after the meter → Stagnant water: what EPA says sitting water does in premise plumbing