Algal blooms and cyanotoxins — advisory numbers, no limit, one certified claim

By Andrew Bates, FounderPublished 2026-09-11Updated 2026-09-11

No cyanotoxin has an enforceable limit. EPA publishes ten-day health advisories instead — microcystins at 0.3 µg/L for bottle-fed infants and 1.6 for adults — and calls them informal technical guidance. Microcystin is the only cyanotoxin with a certified filter claim, held by 48 models.

The numbers exist, the limit does not

A bloom advisory usually arrives with figures attached. It helps to know what kind of figures they are.

EPA has set no maximum contaminant level for any cyanotoxin. What it published, in 2015, are ten-day health advisories.

Cyanotoxin	Bottle-fed infants and pre-school children	School-age children and adults
Microcystins	0.3 µg/L	1.6 µg/L
Cylindrospermopsin	0.7 µg/L	3.0 µg/L

Two things stand out. These are ten-day values, set for a short exposure rather than a lifetime. And EPA splits them by age, which it rarely does.

What a health advisory is

Worth being precise, because the word “advisory” does a lot of work here.

EPA may publish Health Advisories under the Safe Drinking Water Act for contaminants “not subject to any national primary drinking water regulation.”

They describe concentrations at which adverse effects are not anticipated over a stated duration, and contain a margin of safety. EPA’s own description of their status: “informal technical guidance.”

So nobody is in violation for exceeding one. No notification duty attaches, and no utility is judged against it.

→ Manganese: three numbers, none enforceable

The smell is not the toxin

The most useful correction on this page, and it runs both ways.

EPA states blooms “can create taste and odor problems in drinking water, which do not have adverse human health impacts but can create an earthy and musty taste and smell.”

The odour compounds and the toxins are different substances produced by the same organisms. An earthy taste is not evidence a toxin is present. Clean-tasting water during a bloom is not evidence one is absent.

→ Why your water tastes, smells or looks wrong

What the utility is doing

EPA states conventional treatment — coagulation, sedimentation, filtration and chlorination — can generally remove intact cyanobacterial cells and low levels of cyanotoxins.

The qualifier is the part that matters. Systems “may face challenges in providing drinking water during a severe bloom event” when source levels are high.

EPA frames this as planning and operations rather than compliance, because there is no limit to comply with.

→ City water treatment, and the three gaps a utility cannot close

The inversion

Here is something this site has not found before.

Microcystin has no enforceable limit — and it has a certified filter claim. NSF listings returned 52 rows for microcystin reduction under NSF/ANSI 53, resolving to 48 distinct models from 7 companies.

Meanwhile 33 of EPA’s regulated contaminant entries have no NSF claim at all.

Certification follows what manufacturers choose to test, not what EPA chooses to regulate. The two lists overlap; neither contains the other.

→ The regulated contaminants with no certified filter

Only one cyanotoxin, and only one kind of product

The claim is narrower than the problem.

Cylindrospermopsin has no NSF claim. Anatoxin-a has none. Microcystins are a family of variants, and the claim appears under a single name.

Product form	Models certified for microcystin
Plumbed-in to separate tap	31
Counter-top	16
Refrigerator filter	1
Pitcher	0
Faucet mount	0
Whole house	0

So the certified route is a point-of-use device at one tap.

→ Which claims exist in which product form

The claim travels in a package

Nobody buys a microcystin filter as such.

Of the 48 certified models, 47 also hold a lead claim and 42 also hold PFOA and PFOS claims. Twenty-nine also carry cyst reduction.

This is a premium multi-contaminant carbon block set with microcystin added, not a bloom-specific product category.

→ The 615 filters certified for lead

The decision framework

1. During an advisory, follow the notice

It is the only binding instruction in play, and it is specific to the event and the toxin measured.

2. Check which toxin was named

Microcystin is the one with a certified claim. Cylindrospermopsin and anatoxin-a have none, so no product can be verified against them.

3. Do not use taste as the test

EPA says the odour has no adverse health impact of its own. It is neither a warning nor an all-clear.

4. Treat the advisory value as guidance, not a threshold

It is a ten-day figure with a margin of safety, and it binds nobody. A result near it is not a violation; a result under it is not a certificate.

5. If you buy, buy the package rather than the claim

Every certified microcystin model carries other claims worth more of the year. Match those to your report as well.

→ How to read your water quality report

The decision path

Your situation	What decides it	Where that lands you
Active bloom advisory	The notice is the binding instruction	Follow it exactly
Microcystin named, want a filter	48 certified models exist	Point-of-use, claim-checked
Cylindrospermopsin named	No NSF claim exists	Nothing certified to buy
Anatoxin-a named	No NSF claim exists	Nothing certified to buy
Earthy or musty taste, no advisory	Odour compounds carry no health impact	Aesthetic, not a toxin signal
On a groundwater system	Blooms are a surface water phenomenon	Very low priority
Want whole-house protection	No point-of-entry microcystin claim	Point-of-use only
Infant in the house	Advisory value is lower for that group	Note which figure applies

Choose to act if

Your utility named microcystin in an advisory and you want a certified route at one tap.
You already wanted lead or PFAS reduction, in which case the certified microcystin models cover those too.

Choose to wait if

Your system draws from groundwater, where blooms are not the exposure route.
The only symptom is an earthy taste with no advisory attached to it.

Choose to do nothing if

No advisory has been issued and your report shows nothing. There is no limit to exceed and no result to act on.

→ Testing city water: what your utility already measured

Mistakes that cost the most money

Treating the advisory number as a legal limit. It binds nobody, and EPA calls it informal technical guidance.

Using smell as the safety test. EPA states the bloom odour carries no adverse health impact, so it is neither warning nor reassurance.

Buying for cylindrospermopsin or anatoxin-a. Neither has an NSF claim, so nothing on the market carries listed performance against them.

Buying a pitcher for it. No pitcher holds the microcystin claim. Nor does any faucet mount or whole-house unit.

Ignoring the age split. The microcystin advisory for bottle-fed infants is 0.3 µg/L against 1.6 for adults, a factor of more than five.

Assuming a quiet year means a quiet source. Blooms are episodic, and conventional treatment handles low levels routinely without anyone noticing.

Three situations, and what changes

A lake-supplied city issues a bloom advisory naming microcystin. The notice governs. If you want a lasting certified option afterwards, 48 models hold the claim, all point-of-use, and most also cover lead and PFAS — which matter every week rather than during one bloom.

An earthy taste in late summer, no advisory. Consistent with EPA’s description of bloom taste and odour compounds, which it states have no adverse human health impact. Unpleasant, not a toxin reading, and nothing certified targets the odour compounds specifically.

A groundwater system with an infant in the house. Cyanotoxins are a surface water problem, so this is largely the wrong worry. The lower advisory figure for bottle-fed infants matters only where a bloom-affected surface supply is involved.

The objections worth answering

“If EPA published a number, it must be enforced.” It publishes health advisories precisely for contaminants not subject to any national regulation, and describes them as informal technical guidance.

“Algae is natural, so it is harmless.” EPA states cyanobacteria occur naturally and that some blooms produce toxins posing health risks through drinking and recreational exposure. Natural and harmless are different claims.

“My water smells fine, so the bloom has passed.” The odour compounds and the toxins are different substances. Neither presence nor absence of smell measures the other.

“Surely a good filter handles all of it.” One cyanotoxin has a certified claim. For the other two named by EPA, no certified household product exists.

“The utility would have to tell me.” It must follow whatever its state requires and will normally issue a notice, but no federal violation or notification duty attaches to a cyanotoxin figure, because there is no limit.

Who this is for

Households on a surface-water public supply that has had, or may have, a harmful algal bloom, and anyone trying to read an advisory number and work out what weight it carries.

It is also for anyone who has been told an earthy taste in late summer means their water is unsafe.

Groundwater systems are largely outside this problem.

What to settle first

Which toxin was actually named, and is the figure I was given a limit or an advisory?

Only one of the three has a certified product behind it. And none of them has a limit, which changes what the number is for.

→ Unregulated contaminants: the candidate list and the monitoring rule → Why your water tastes, smells or looks wrong → The regulated contaminants with no certified filter → Which claims exist in which product form → How to read your water quality report → Radon, the other unregulated contaminant with a certified claim

The bottom line

Cyanotoxins have advisory numbers and no enforceable limit, so nobody is in violation for exceeding one. Follow the utility's instruction during a bloom, because that is the only binding thing in play. Do not use taste or smell as the signal — EPA says the earthy odour a bloom produces carries no health impact of its own. If you want a certified product, microcystin is the only cyanotoxin with a claim: 48 models, all point-of-use, and 47 of them also certified for lead.

Who this is not for

This covers cyanotoxins in drinking water on a public surface-water system. It does not cover recreational exposure, which has its own guidance, or pets and livestock, which are affected at different levels. It is not a diagnosis of any bloom event, and during an active advisory the instruction from your utility or health department governs over anything written here.

Questions

Is there a legal limit for cyanotoxins in drinking water?

No. EPA has set no maximum contaminant level for any cyanotoxin. It published ten-day health advisories in 2015 for microcystins and cylindrospermopsin, and states health advisories serve as informal technical guidance rather than enforceable standards.

What are the cyanotoxin advisory numbers?

For microcystins, 0.3 µg/L for bottle-fed infants and pre-school children and 1.6 µg/L for school-age children and adults. For cylindrospermopsin, 0.7 µg/L and 3.0 µg/L for the same two groups. All four are ten-day values, not lifetime limits.

Why are there two numbers for each toxin?

EPA splits the advisory by age group. The lower figure covers bottle-fed infants and pre-school children, the higher one school-age children and adults. That split is unusual — most drinking water figures are a single number for everyone.

Can I smell or taste a bloom in my water?

Often, but it tells you nothing about toxins. EPA states blooms can create an earthy and musty taste and smell which does not have adverse human health impacts. The odour compounds and the toxins are different substances.

Does normal treatment remove cyanotoxins?

Generally, at low levels. EPA states conventional treatment — coagulation, sedimentation, filtration and chlorination — can remove intact cells and low levels of cyanotoxins, but that systems may face challenges during a severe bloom when source levels are high.

Is there a certified filter for cyanotoxins?

For microcystin only. NSF listings show 48 distinct models certified for microcystin reduction under NSF/ANSI 53, from 7 companies, on 10 September 2026. No claim exists for cylindrospermopsin or for anatoxin-a under any NSF standard, so nothing on the market carries listed performance against those two.

What kind of filter holds the microcystin claim?

Point-of-use only. Of the 48 certified models, 31 are plumbed in to a separate tap, 16 are counter-top and one is a refrigerator filter. No pitcher, faucet mount or whole-house product holds it.

Should I boil water during a bloom advisory?

Follow the notice you were given rather than a general rule. Boiling concentrates dissolved substances rather than removing them, which is why boil notices address microbes. Your utility's instruction governs during an active advisory.

Sources — 10 claims

EPA — Drinking Water Health Advisories for Cyanotoxins
EPA published ten-day drinking water health advisories for two cyanotoxins in 2015. For microcystins the values are 0.3 µg/L for bottle-fed infants and pre-school children and 1.6 µg/L for school-age children and adults. For cylindrospermopsin they are 0.7 µg/L and 3.0 µg/L for the same two groups. (captured 2026-09-11)
Limits: Ten-day values, not lifetime limits, and not enforceable. They are set for a short exposure such as a bloom event. EPA also produced a health effects support document for anatoxin-a but published no health advisory value for it.
EPA — Managing Cyanotoxins in Public Drinking Water Systems (page last updated 2026-02-06)
EPA states cyanobacteria, formerly referred to as blue-green algae, occur naturally in lakes, rivers, ponds and other surface waters, and that in warm water with abundant nutrients they can rapidly form harmful algal blooms, some of which produce cyanotoxins. EPA states such blooms can also create an earthy and musty taste and smell in drinking water which does not have adverse human health impacts. (captured 2026-09-11)
Limits: A source-water phenomenon affecting surface supplies. A groundwater system is not exposed in the same way. The taste and odour compounds are separate from the toxins, so an earthy taste is not itself evidence of a toxin, nor its absence evidence of safety.
EPA — Drinking Water Health Advisories for Cyanotoxins
EPA states that under the Safe Drinking Water Act it may publish Health Advisories for contaminants not subject to any national primary drinking water regulation, that these describe concentrations at which adverse health effects are not anticipated over specific exposure durations such as one day, ten days, several years and a lifetime, that they contain a margin of safety, and that they serve as informal technical guidance to assist officials and water system managers. (captured 2026-09-11)
Limits: A health advisory binds nobody. No utility is judged against one, no violation follows from exceeding one, and no notification duty attaches. It is the same class of figure as the manganese health advisory already recorded on this site.
EPA — Managing Cyanotoxins in Public Drinking Water Systems (page last updated 2026-02-06)
EPA states that conventional water treatment — coagulation, sedimentation, filtration and chlorination — can generally remove intact cyanobacterial cells and low levels of cyanotoxins from source waters, but that water systems may face challenges in providing drinking water during a severe bloom event when levels of cyanobacteria and cyanotoxins in source water are high. (captured 2026-09-11)
Limits: A statement about routine treatment capability, not a guarantee during a bloom. EPA frames cyanotoxin management as planning and operations rather than as compliance, because there is no maximum contaminant level for any cyanotoxin to comply with.
NSF Official Listings — Drinking Water Treatment Units, NSF/ANSI 53 microcystin reduction claim search (listings current as of Thursday, September 10, 2026, 12:15 a.m. Eastern Time)
NSF listings returned 52 rows for microcystin reduction under NSF/ANSI 53 on 2026-09-10, resolving to 48 distinct models from 7 companies: 31 plumbed in to a separate tap, 16 counter-top and 1 refrigerator filter. No pitcher, faucet mount or point-of-entry product holds the claim. (captured 2026-09-11)
Limits: Microcystin is the only cyanotoxin with an NSF reduction claim. Cylindrospermopsin and anatoxin-a have none, and microcystins are a family of variants while the claim appears under a single name.
NSF Official Listings — Drinking Water Treatment Units, NSF/ANSI 53 microcystin reduction claim search (listings current as of Thursday, September 10, 2026, 12:15 a.m. Eastern Time)
The microcystin claim travels with a wider package rather than alone: of the 48 certified models, 47 also hold lead reduction and 42 also hold PFOA and PFOS reduction. Microcystin is a contaminant with a health advisory and no enforceable limit, yet it has a certified claim, while several regulated contaminants have none. (captured 2026-09-11)
Limits: Co-held claims describe what these particular models were tested for, not a relationship between the contaminants. A certified claim existing for an unregulated contaminant is a commercial decision by manufacturers, not a regulatory one.
EPA — Contaminant Candidate List 5 (CCL 5), page last updated 2026-02-27
EPA defines the Contaminant Candidate List as contaminants that are not subject to any proposed or promulgated national primary drinking water regulation but are known or anticipated to occur in public water systems, and which may require future regulation. CCL 5, published 14 November 2022, contains 66 chemicals, three chemical groups — PFAS, cyanotoxins and disinfection byproducts — and 12 microbes. (captured 2026-09-11)
Limits: A candidate list, not a regulation and not a warning. Listing means EPA considers a contaminant a priority for regulatory decision-making and information collection. It sets no limit, obliges no utility to do anything, and implies nothing about concentration in any particular supply.
EPA — Secondary Drinking Water Standards: Guidance for Nuisance Chemicals
EPA has established National Secondary Drinking Water Standards setting non-mandatory levels for 15 contaminants, states it does not enforce them, and states that because they are not health threatening at the secondary maximum contaminant level, public water systems only need to test for them on a voluntary basis. (captured 2026-09-11)
Limits: Federal position only. States may adopt secondary standards as enforceable requirements of their own, and many public water systems monitor them voluntarily. A figure absent from a Consumer Confidence Report may simply never have been measured.
Crosswalk of EPA National Primary Drinking Water Regulations against the NSF reduction-claim taxonomy (both captured 2026-09-10)
Crossing EPA 94 regulated contaminant entries against NSF 101 selectable reduction claims on 2026-09-10 leaves 33 regulated contaminants with no NSF reduction claim of that name: 19 organic chemicals, 4 microorganisms, 4 inorganic chemicals, 3 disinfection byproducts and 3 radionuclides. (captured 2026-09-10)
Limits: The absence of a claim by name is not evidence that no product treats the contaminant. Reverse osmosis and distillation remove many of these; NSF offers no named claim to certify them against. NSF offers no claims at all under its distillation standard, so distillation is invisible to this crosswalk, and microorganisms are certified through NSF/ANSI 55 disinfection performance classes rather than by contaminant.
40 CFR §141.62 — Maximum contaminant levels for inorganic contaminants (2024 edition, 40 CFR Ch. I, 7-1-24)
Reverse osmosis is the Best Available Technology EPA names most often for inorganic contaminants, appearing for 12 of the 14 listed, followed by ion exchange at 10 and lime softening at 8. Granular activated carbon appears exactly once, for mercury, and only where influent mercury is at or below 10 micrograms per litre. (captured 2026-09-11)
Limits: Counted across the inorganic table only. Granular activated carbon is separately named as a best available technology for all six regulated PFAS and is widely used for organic contaminants under §141.61, so its single appearance here bounds its role among inorganics rather than overall.