Workers remove massive accumulations of _Sargassum_ seaweed from a Caribbean beach in the Riviera Maya near Cancun, Mexico, in June 2025. elena.tres/ShutterstockSeaweed piling up on a beach might look harmless, but as quantities begin to rot, they can release gases that are toxic to people.In recent years, two people and several animals (including a horse and 36 wild boar) have died after inhaling toxic gases from decomposing seaweed on the Brittany coast in France. And between January and August 2018, thousands of people reported headaches, nausea and eye irritation after inhaling seaweed gas in the Caribbean. Seaweeds are generally a good thing. They provide structure to coastal ecosystems, store carbon from the atmosphere, and they’re a sustainable food source. But when seaweeds grow uncontrollably, they can become a nuisance. Vast patches of seaweed often develop after nutrients are released into rivers and coastal seas from sewage treatment works or run-off from farmland. Warm weather further stimulates their rapid growth. When ocean currents and wind push free-floating patches of seaweed together, seaweed strandings can inundate the coastline with often devastating consequences. These uncontrollable seaweed “blooms” can suffocate marine life. This harms fisheries, tourism industries and coastal communities relying on healthy ocean ecosystems. Collecting seaweed before it accumulates helps avoid the build up of toxic gases. Felix Mizioznikov/Shutterstock The best way to avoid catastrophic consequences like this is to remove seaweed before it piles up and produces a concentrated accumulation of toxic gases. Workers removing the seaweed are at the highest risk of deadly gas inhalation due to their proximity to the source. Low-level exposure to nearby seaweed strandings is also a concern.Once collected, seaweed can be used as a soil additive and fertiliser or for energy generation (anaerobic digestion of seaweed produces methane-rich biogas for heating and electricity).Composition of the culpritsScientists know little about the composition of gases in decaying seaweed. Prime nuisance candidates are hydrogen sulfide, which produces a typical rotten-egg smell, and ammonia, which has a pungent odour. Both are hazardous at high concentrations. Guidelines from the European Agency for Safety and Health at Work suggest that the legal limit of five to 20 parts per million (ppm) of these gases must not be exceeded. When starting to research this subject in 2018, I found little scientific information about the composition of toxic seaweed gases. With the help of a team of students and scientists, I quantified the amount and diversity of gases released from two nuisance seaweed species in the UK: green gut weed (Ulva intestinalis) and brown wireweed (Sargassum muticum). Our initial tests confirmed that hydrogen sulfide is a key contributor to the overall gas bouquet but ammonia was not released during the first eight days of our laboratory experiment. When focusing our analyses on sulfur gases, we identified a range of toxic volatiles (gases and vapour which easily evaporate) that could cause respiratory illness and death. These included high amounts of hydrogen sulfide and dimethyl sulfide, a climate-cooling gas that makes the typical smell of the sea. Others included the highly toxic methane thiol that smells of decayed cabbage, and dimethyl disulfide, which has a garlic smell. Sulfur emissions were significantly higher for Ulva intestinalis than for Sargassum muticum in seawater. This suggests that different seaweed species may differ in their potential to cause illness. Read more: From a salty breeze to the stench of sewage, here’s how smell affects our ocean experience and reflects changing seas Seaweed growth can be stimulated by excessive nutrient-rich run-off, including fertilisers or sewage pollution. But harmful bacteria in sewage could be concentrated by seaweed strandings. When pollutants are released into our rivers, environmental engineers calculate the dilution to keep contaminants below legal limits. This minimises the risks to swimmers, fish and other aquatic life. But even if just a few nasty bugs end up in seaweed strandings, these may proliferate inside the decaying seaweed mass where conditions are warm, humid and anaerobic (oxygen is absent) – similar to the gut of humans and other animals. Here, harmful bacteria could grow and exceed safe limits, dramatically increasing the risk of infection. We found various bacteria in the decaying seaweed that are associated with the human intestine and faeces. These can cause severe illness. Some of the bacteria we found on the surface of decaying seaweed were resistant to antibiotics that they may have been exposed to in our sewage. This suggests that seaweed strandings can propagate infections that are difficult to treat.Climate change will probably increase seaweed strandings as environmental conditions worsen. Further proliferation of nuisance species into new habitats may increase health risks too, so preventing the build-up of seaweed strandings is crucial. Read more: How a toxic seaweed choking Caribbean beaches could become a valuable resource Removal and spray solutionsRemoving the seaweed is only possible if the location is accessible. Employing staff and hiring equipment to rake up tonnes of seaweed can also be expensive. Removal of one of the largest seaweed strandings in China involved more than 10,000 workers and cost upwards of US$100 million (£76 million). Funding that is difficult for small coastal communities. Our experiments showed that adding freshwater to decaying seaweed changes the decomposition process. Soaking in freshwater decreased the amount of sulfate, an abundant dissolved component of seawater that anaerobic bacteria use to produce toxic hydrogen sulfide, resulting in up to a 99% decrease in hazardous gases. Freshwater is not always readily available, particularly on small Caribbean islands that get inundated with sargassum seaweed. But locations with access to reservoirs or rivers could potentially pump-spray a seaweed stranding before it becomes a health hazard – a mitigation strategy that should be explored.Michael Steinke does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.