Harmful Algal Blooms and Cyanotoxins in Drinking Water

Health note: This page is general educational information about harmful algal blooms and cyanotoxins, not medical advice. If you think you or an animal has been exposed to a bloom, contact a doctor, a veterinarian, or poison control, and follow any advisory your water utility or health department issues.

During an active bloom advisory: If your utility or health department issues a do not drink notice for cyanotoxins, do not boil the water. Boiling does not remove cyanotoxins and can concentrate them. Use bottled water or the alternative source named in the advisory until it is lifted.

In August 2014, the city of Toledo, Ohio told roughly half a million people not to drink their tap water. A harmful algal bloom over the Lake Erie intake had pushed microcystin in the finished water past the safety threshold, and the do not drink order held for about three days. It was a municipal system with real treatment, and the bloom still got through. That is the situation this page is about.

What a Harmful Algal Bloom Actually Is

A harmful algal bloom, often shortened to HAB, is a rapid overgrowth of cyanobacteria, the organisms people call blue-green algae. They are not true algae, but the name has stuck. Blooms show up on warm, slow-moving, nutrient-rich water: lakes, ponds, reservoirs, and the slow stretches of rivers. Fertilizer runoff and warm temperatures feed them, which is why bloom season runs through late summer and early fall in most of the country.

Not every bloom is toxic. The trouble is you cannot tell by looking. Some cyanobacteria strains produce cyanotoxins and some do not, and a harmless-looking green sheen can carry toxin while a thick mat does not. Testing is the only way to know, which is why agencies treat a visible bloom as a reason for caution rather than a verdict.

The two cyanotoxins that matter most for drinking water are microcystins, which target the liver, and cylindrospermopsin, which affects the liver and kidneys. A bloom that smells musty or earthy is releasing the same compounds that cause taste-and-odor complaints. That musty note comes from geosmin and MIB, which are byproducts of the same organisms. Those two compounds are not toxic on their own, so a musty smell is a flag worth investigating, not proof of a toxin. The musty or earthy smell page covers that taste-and-odor side in more detail.

Why You Must Not Boil

This is the part that catches people, because boiling is the right answer to so many other water problems.

Cyanotoxins are stable molecules. They do not break apart at the temperature of a rolling boil. When you boil a pot of water carrying microcystin, the water leaves as steam and the toxin stays behind in a smaller volume, so the concentration goes up. The EPA’s own guidance to utilities is to issue do not drink, do not boil advisories during a cyanotoxin event, which is the opposite of a standard boil water advisory for bacteria.

The reflex to boil during any water scare is exactly why this needs saying plainly. Boiling addresses biology. A cyanotoxin is a chemical. Our guide to what boiling does and does not remove lays out the same logic for lead, nitrates, and PFAS, and cyanotoxins belong on that same list of things boiling cannot fix.

The EPA Advisory Levels

In 2015 the EPA published 10-day drinking water health advisories for both toxins. These are the levels at which adverse health effects are not anticipated over a 10-day exposure, with safety margins built in. They are guidance for states and utilities, not enforceable maximum contaminant levels.

The split by age is deliberate. The lower numbers apply to bottle-fed infants and young children because they take in more water relative to body weight, so a given concentration is a larger dose for them. When an advisory is in force for a household with an infant, the alternative water source matters more, not less.

Because these are advisory levels rather than a regulated limit, what triggers a public notice can vary by state and by how a utility has set up its response plan. The figures above are the federal reference points most plans are built around.

Where the Real Risk Lands

For people on a treated municipal system that draws from surface water, the routine risk is low. Those utilities test for cyanotoxins through bloom season and treat for them, and most of the time the finished water stays well under the advisory levels. The danger shows up when a bloom is severe enough to overwhelm treatment, which is the Toledo scenario, and when the source is not treated at all.

That second case is the one to watch. If you draw from an untreated surface source, a shallow well close to an affected lake or pond, or an unmaintained cistern, you do not have a treatment plant standing between you and the bloom. Standard sediment and taste-and-odor filtration will not address cyanotoxins. Private surface and shallow-well users near bloom-prone water should treat a visible bloom as a reason to switch to a known-safe source and to test before going back. Florida’s warm, shallow waters make this a recurring issue, which is why the Florida water quality guide treats blooms as a regional concern.

Pets and Livestock Are the Most Likely Victims

Dogs die from harmful algal blooms more often than people do, and the reason is behavioral. A dog wades into a pond, drinks straight from it, then licks the dried scum off its coat. Livestock do the same at a stock pond. The CDC notes that animals can get very sick or die within hours of contact with a toxic bloom, a far faster and more severe course than what is typical for an exposed adult human.

The rule for pets is simple and worth repeating to anyone with a dog near open water. Keep animals out of water that looks scummy, discolored, painted-on-green, or that smells bad. Do not let them drink it. If a pet does get in, rinse it with clean water before it can groom, and call a veterinarian if it shows vomiting, weakness, drooling, seizures, or collapse. Our water safety guide for dogs and cats covers the broader picture of what is and is not a real pet water risk.

What Treatment Actually Helps

During an active do not drink advisory, the answer is not a home filter. It is the alternative water source the advisory names, usually bottled water, until testing clears the supply. No point-of-use device should be treated as a workaround for an official advisory.

Outside of an advisory, for a private source with a documented cyanotoxin problem, two treatment approaches have certified reduction. Some point-of-use carbon filters are certified to NSF Protocol P477, which verifies microcystin reduction below the EPA infant advisory level (0.3 ppb), and reverse osmosis certified to NSF/ANSI 58 reduces cyanotoxins more broadly. These reduce the toxins, they do not eliminate them, and the right setup depends on which toxin and at what level. A taste-and-odor pitcher or refrigerator filter is built for chlorine and flavor, not for this.

Test your water before choosing treatment. Cyanotoxin levels swing with the bloom and with the season, so a single reading is a snapshot, and a private surface or shallow-well source near bloom-prone water is worth testing during and after a visible bloom rather than guessing. If your water also runs musty between blooms, that is the taste-and-odor side, and treating for it is a different decision than treating for the toxins themselves.

The Short Version

A harmful algal bloom is the one water emergency where the instinct to boil is actively wrong. Boiling concentrates cyanotoxins instead of removing them, so during a bloom advisory you switch to bottled or another named source and wait it out. The EPA’s 2015 advisory levels are lowest for infants and young children, treated city water usually stays under them even during a bloom, and the highest day-to-day risk is to dogs and livestock that drink straight from the water. If you are on an untreated surface source or a shallow well near a bloom-prone lake or pond, test before you trust it.