Most algae you encounter in daily life, from the nori wrapped around sushi to the spirulina in a smoothie, is harmless. But certain types of algae produce potent toxins that can cause serious illness and, in rare cases, death. The danger comes primarily from harmful algal blooms in freshwater lakes, ponds, and slow-moving rivers, as well as from marine “red tides” along coastlines. These blooms produce toxins that attack the liver, nervous system, and respiratory tract through multiple exposure routes.
Which Types of Algae Are Dangerous
The biggest threat to human health comes from cyanobacteria, commonly called blue-green algae. Despite their name, they’re actually bacteria that behave like plants, and they thrive in warm, nutrient-rich freshwater. An estimated 95% of cyanobacteria genera produce a neurotoxin called BMAA, and many also generate liver-targeting toxins called microcystins. These organisms form visible blooms that can look like green paint, foam, or scum on the water’s surface.
In saltwater, the primary concern is red tide, caused by microscopic organisms called dinoflagellates. These produce brevetoxins that become airborne in sea spray, along with other toxins that accumulate in shellfish. Red tide blooms can stretch for miles along a coastline, and you don’t have to enter the water to be affected.
How You Can Be Exposed
There are three main ways algal toxins get into your body: swallowing contaminated water, skin and eye contact, and breathing in airborne toxins. Swimming, water skiing, or even wading in a lake during an active bloom can expose you through all three routes simultaneously.
Ingestion is the most dangerous route. You can swallow toxins by accidentally gulping lake water, drinking inadequately treated tap water sourced from a contaminated reservoir, or eating contaminated fish and shellfish. Children are especially vulnerable because they’re more likely to swallow water while playing and their smaller body weight means a lower dose can cause harm. The EPA’s health advisory for microcystins in drinking water is set at just 0.3 micrograms per liter for children under 10, compared to 1.6 micrograms per liter for adults.
Inhalation becomes a concern when blooms die off and release gases like hydrogen sulfide, or when wave action and wind turn toxins into fine aerosol droplets. Along Florida’s coast, people living or vacationing near red tide blooms inhale aerosolized brevetoxins simply by being outdoors near the shore.
Symptoms From Freshwater Algal Toxins
Freshwater cyanobacteria produce several categories of toxins, each targeting different organs. Liver-targeting toxins (including microcystins) are the most commonly encountered. They enter liver cells through specific transport channels and trigger cell death, inflammation, and internal bleeding. Symptoms of exposure include nausea, vomiting, diarrhea, abdominal pain, headache, fever, and dark or bloody urine. In severe cases, acute hepatitis and jaundice can develop.
Neurotoxins produced by cyanobacteria are less common but far more immediately dangerous. One called anatoxin-a locks open nerve receptors in a way the body cannot reverse, progressively shutting down muscle function. Symptoms include loss of coordination, muscle twitching, tingling, numbness, difficulty speaking, and convulsions. At high doses, death from respiratory paralysis can occur within minutes to a few hours, depending on the amount ingested.
Direct skin contact with contaminated water typically produces allergic reactions: rashes, itching, blisters, and eye irritation resembling conjunctivitis. These symptoms are uncomfortable but rarely dangerous on their own.
Red Tide and Respiratory Effects
Red tide toxins are unusual because they can harm you without any water contact at all. When waves break during a bloom, brevetoxins become aerosolized and drift inland. Healthy people who breathe these particles commonly report cough, sore throat, nasal congestion, sneezing, and eye irritation. For people with asthma, the effects are more concerning: wheezing, chest tightness, shortness of breath, and full asthma attacks, even at relatively low exposure levels.
The good news is that these respiratory effects appear to be temporary. A study following people with stable asthma who lived in areas with recurring Florida red tides found no evidence of chronic respiratory damage from intermittent exposure over time. Symptoms generally resolve once you move away from the affected area.
Shellfish Poisoning From Algal Toxins
Filter-feeding shellfish like mussels, clams, and oysters concentrate algal toxins in their tissue, creating a separate and significant risk. There are three major types of shellfish poisoning, each caused by different algal toxins.
- Paralytic shellfish poisoning (PSP) causes tingling in the face and fingertips, dizziness, double vision, numbness around the mouth, and weakness within 20 minutes to 5 hours of eating contaminated shellfish. In serious cases, it can progress to generalized weakness and respiratory failure.
- Amnesic shellfish poisoning (ASP) starts with gastrointestinal distress but can progress to headache, short-term memory loss, and seizures. A 1987 outbreak in Canada affected dozens of people, causing memory loss in 25% of patients and killing three. Long-term verbal memory decline has been reported in people who regularly consume contaminated razor clams.
- Diarrhetic shellfish poisoning (DSP) is the mildest form, causing nausea, abdominal pain, vomiting, chills, and fever that appear within 30 minutes to 4 hours and can last up to 72 hours.
Public health agencies issue shellfish harvesting advisories during known blooms. Cooking does not destroy these toxins, so the only reliable way to avoid shellfish poisoning is to check local advisories before harvesting wild shellfish.
Possible Links to Long-Term Brain Disease
Beyond acute poisoning, there is growing concern that chronic, low-level exposure to the cyanobacterial toxin BMAA may contribute to neurodegenerative diseases. Researchers have found high concentrations of BMAA in the brains of people who died from ALS, Alzheimer’s disease, and Parkinson’s disease, but not in the brains of age-matched people who died of other causes. Only 5 to 10% of these diseases are caused by inherited genetic mutations, which has fueled interest in environmental triggers.
The proposed mechanism is striking: BMAA is structurally similar enough to a normal amino acid that the body’s protein-building machinery can accidentally incorporate it into proteins. When this happens, the proteins misfold and the affected cells die. BMAA can also overstimulate nerve cell receptors, causing direct neuronal injury. This research is still developing, but it raises the stakes for people who live near frequently contaminated water sources or who regularly consume seafood from bloom-affected areas.
Edible Algae and What’s Actually Safe
Commercial seaweeds like nori, wakame, kombu, and dulse are not the same organisms that produce harmful blooms, and they are generally safe to eat. These are macroalgae (large, visible seaweeds) farmed or harvested under food safety standards, while toxic blooms are caused by microscopic organisms in a completely different biological category.
That said, edible seaweeds carry their own, more modest risks. Some varieties accumulate heavy metals, particularly arsenic, from the water they grow in. Hijiki seaweed, popular in parts of Asia, contains high enough levels of inorganic arsenic that health authorities in Asia, Australia, Europe, and the United States recommend against eating it. Other common varieties like nori, wakame, and kombu contain arsenic at levels well below safety thresholds. Iodine content also varies dramatically between species and can be dangerously high in some kelp products, particularly for people with thyroid conditions.
Spirulina supplements deserve special caution. Spirulina is itself a cyanobacterium, and when harvested from natural water bodies rather than controlled facilities, it can be contaminated with microcystins and other toxins from co-occurring cyanobacteria. If you take spirulina, choosing products from manufacturers that test for cyanotoxin contamination reduces this risk.
How to Protect Yourself
When it comes to recreational water, the simplest rule is visual: if the water looks discolored, has visible scum or foam, or smells musty, stay out. The World Health Organization considers 20,000 cyanobacterial cells per milliliter (roughly the point where water starts looking off) to be the threshold for a low-probability health risk. At 100,000 cells per milliliter, the risk becomes moderate, and visible scum formation in swimming areas is considered high risk, warranting possible prohibition of water contact.
For drinking water, municipal treatment systems are generally effective at removing algal toxins, but private well owners and people drawing water from surface sources near agricultural runoff should be aware that standard filtration may not be sufficient during heavy bloom periods. Keep children and pets away from water that shows any signs of a bloom, since dogs are particularly susceptible and fatalities in pets occur regularly. If you develop gastrointestinal or neurological symptoms after water exposure, mention the possibility of algal toxin contact to your healthcare provider, as it’s easily overlooked in diagnosis.