Can brevetoxins from algal blooms make me sick? A toxicologist explains

For about a year, an algal bloom in South Australian waters has had devastating effects on marine life. At my local beach, walks were a sad parade of dead sea life. But what of the health effects of these algal blooms on humans? And what do a class of compounds called brevetoxins have to do with it? I’m a toxicologist. Here’s what the evidence says.

What are algal blooms? Algal blooms are a rapid, explosive growth of algae. These blooms are complex and often contain a variety of algal species, some of which produce toxins. The mix of algal species and amount of toxins produced can vary considerably. A bewildering variety of toxins can cause effects from nausea to liver failure in humans. Australia’s algal bloom catastrophe has left more than 87,000 animals dead. What will happen this summer?

How about in South Australia? For a long time, it was thought the major species in South Australia’s algal bloom was Karenia mikimotoi. This does not produce toxins but can suffocate fish by the algae (or fragments of the algae) directly damaging their gills. This species can irritate our skin and eyes. It can also worsen asthma symptoms, possibly by the same mechanisms that produce gill damage in fish. Another species – K. brevis – does produce brevetoxins. But this species was not found. Then in November 2025 the same group that initially identified K. mikimotoi found K. cristata was the dominant species in some areas. Surprisingly, but importantly, the group showed for the first time that K. cristata produces brevetoxins. Mystery solved: our tests reveal the tiny algae killing fish and harming surfers on SA beaches

So what exactly are brevetoxins? Brevetoxins are part of a large family of fat-soluble toxins. These are similar in structure to ciguatoxins, which cause shellfish poisoning in humans. Brevetoxins and the related ciguatoxins act by indiscriminately stimulating nerves, causing nerve over-activity. For brevetoxins this results in gut illness, muscle cramps, seizures and paralysis. This nerve activation also results in the smooth muscle in the trachea (wind pipe) to contract. The processes that lead to inflammation are also stimulated, worsening symptoms in people with asthma. Concentrations of brevetoxins in sea water are too low to produce direct poisoning. However, as brevetoxins are fat-soluble, they can accumulate in the food chain. High concentrations have been linked to food poisoning after eating shellfish, but not fish. How about aerosol droplets? While the levels of brevetoxins in sea water do not cause direct poisoning, these toxins can still have a significant impact. Wave action can produce aerosol droplets containing the brevetoxins, which people can inhale. As brevetoxins cause tracheal contraction and inflammation, this can make people’s asthma worse.

We do not have information about brevetoxins from K. cristata and its effect on people with asthma. But we do have related evidence from the United States, specifically the effect of K. brevis blooms in Florida. People with asthma who spent an hour on a beach during K. brevis blooms there said they had more asthma symptoms (wheezing, chest tightness and shortness of breath). We also have evidence from exposing animals and humans to known concentrations of aerosolised K. brevis. These studies showed increased levels of asthma symptoms even at levels lower than you’d expect for brevetoxin poisoning. It’s difficult to compare the US study with the South Australian one that identified K. cristata, as the latter didn’t measure aerosol levels of brevetoxin. But sea water levels of algae are comparable between the two studies. This makes it likely aerosols containing K. cristata would trigger asthma symptoms. All sites tested in South Australia by the authors of the study that discovered K. cristata also had substantial levels of K. mikimotoi, which can also exacerbate asthma. What to do? Health advice includes avoiding discoloured water and areas of the beach with foam. Try to avoid exposure to aerosolised algae or their fragments. People with asthma are advised to carry their reliever medication (puffer) with them while on beaches, especially when there is abnormally thick foam or discoloured water. They’re also advised to take their preventive medication as prescribed and check their asthma management plan is up-to-date. And what about long-term effects? While exposure to aerosolised brevetoxins can exacerbate asthmatic symptoms during the bloom, there does not appear to be evidence for any long-term effects of exposure to these aerosols when the blooms are over. A study of asthmatic people exposed to K. brevis blooms over a seven-year period found no chronic respiratory effects. However, further studies would be worthwhile.

Ian Musgrave collaborated with SA Water to study saxitoxin and its implications for water quality. He has received funding from the National Health and Medical Research Council to study adverse reactions to herbal medicines. He has previously been funded by the Australian Research Council to study potential natural product treatments for Alzheimer's disease. He is currently a member of one of the Therapeutic Goods Administration's statutory councils.