That Seaweed “Blob” May Hurt Your Health, So We Asked Experts How Concerned You Should Be This Summer

a woman wades into the ocean through a tangle of sargassum seaweed

Sargassum seaweed on Miami Beach in 2020

Pedro Portal / Miami Herald / Tribune News Service via Getty Images

Sparkling blue waters, warm sand, palm trees — and a giant mass of seaweed that smells like rotten eggs. This uninviting picture may be the reality along the Caribbean and Florida’s east coast as the headline-grabbing seaweed “blob” (which experts say isn’t a blob at all) continues to grow in the Atlantic and is expected to eventually wash ashore sometime this summer.

The foul-smelling seaweed, which is actually a type of floating brown algae called sargassum, provides shelter for marine critters, protects shorelines from erosion, and acts as biomass for food, fuel, and pharmaceuticals. But it also attracts insects, hurts tourism, smothers sea turtle nests, and could put your health in harm's way.

This particular patch of seaweed is making the news because of its size; it’s estimated to be 5,000 miles wide, or about double the width of the US. It’s about the same size or bigger than the one that hit in 2018, which was the largest ever and affected shores from West Africa to the Gulf of Mexico.

When piled up and left untouched on sand, sargassum can decompose, releasing hydrogen sulfide, methane, ammonia, and other harmful gasses into the air that can cause respiratory problems. The seaweed contains heavy metals like cadmium and arsenic that can be toxic to the eyes, skin, liver, kidneys, and lungs. In large quantities, rotting sargassum can also support the growth of disease-causing germs, such as fecal bacteria.

The seaweed is a common sight on beaches, particularly in the summer. It generally travels not in a massive blob but rather in patches that can stretch for miles, hooking together like chains or velcro and moving along with the help of winds and ocean currents.

It usually accumulates in the Sargasso Sea in the western North Atlantic, with some wandering into the Gulf of Mexico and Caribbean Sea. But ever since 2011, the seaweed has been growing in size and more frequently floating into other coastal areas, including Florida, Puerto Rico, and the US Virgin Islands. It’s not clear why sargassum is growing in volume, but an increase in seawater temperature and nutrients from runoff, as well as changes in ocean currents and wind patterns may be to blame.

It’s difficult to track sargassum, given its complex motion, growth, and decay, but researchers with the National Oceanic and Atmospheric Administration say “there is a strong chance” that it may reach the Florida coast this summer.

Some reports show parts of the patch have already made its way to the northern coast of Cuba and the east coast of Florida.

It sounds like a major summer bummer, so we asked experts whether these health risks are serious enough to warrant canceling your beach trips or just avoiding the affected coasts altogether.

How serious are the health risks associated with sargassum? 

“It's not the end of the world, and Florida is not going to be buried in seaweed. But you have to assess the risks and what you're comfortable with,” said Mike Parsons, a marine science professor at the Florida Gulf Coast University who also studies how the environment influences human health.

“When you see these things and people start canceling flights and everything, is it a little premature? I don’t know, but if you're exposed to it, pay attention to what your body is telling you,” Parsons told BuzzFeed News. “Are you getting headaches? Do you feel nauseous? If you do, get away and get some fresh air.”

Although sargassum releases a host of toxic gasses, exposure to hydrogen sulfide — a colorless gas with a strong rotten egg smell — is your biggest concern. The risks of hydrogen sulfide depend heavily on how much you’ve been exposed to and for how long. It can irritate your eyes and respiratory system, causing dizziness, headaches, difficulty breathing, confusion, weakness, nausea, irritability, stomach upset, and vomiting.

At the highest doses of exposure, the gas can lead to coma, seizures, heart attack, and unconsciousness, but that’s unlikely to happen from spending time near seaweed at the beach, according to Helena Solo-Gabriele, an engineering professor at the University of Miami who studies the environment’s impact on human health.

Strong coastal winds will likely dilute most of the toxic gasses, leaving only small concentrations that will have little to no effect on your health, Solo-Gabriele told BuzzFeed News.

What’s more, the chances you actually run into giant patches of sargassum at a beach when vacationing this summer are slim, Solo-Gabriele said, because local communities, resorts, or hotels will likely sweep as much as they can away. (Toxic exposure usually happens about two days after the seaweed has been decomposing on the shore, so in theory it should be safe for people to remove it.)

However, not all coastal communities have access to resources that enable them to remove sargassum from all beaches. For example, Caribbean countries such as Barbados, Guadeloupe, Martinique, Puerto Rico, Saint Lucia, and Saint Martin have been dealing with sargassum “invasions” in recent years.

Between January and August 2018, doctors in Guadeloupe reported more than 3,340 cases of acute exposure to hydrogen sulfide, and those in Martinique reported more than 8,060 cases, including three patients who required intensive care, according to a correspondence published in the Lancet. Researchers say the number of medical consultations related to chronic exposure to the gas is also increasing in local populations in these countries.

Who’s most at risk of experiencing health issues from sargassum? 

People with respiratory conditions such as asthma may be more sensitive to hydrogen sulfide spewing from rotting sargassum, Parsons said, so you may want to consider avoiding beaches with lots of buildup or make an extra effort to stay away from seaweed on the shore.

That said, the risk to the general population is low. But people who live or work on or near affected beaches, including lifeguards, restaurant employees, and hotel guests, may be more likely to develop headaches or feel nauseous if sargassum is left to rot on the shore.

Is it safe to swim in water with a lot of sargassum floating around?

The short answer is yes — but…would you really want to?

Parsons said the only real risk for people swimming in sargassum-infested waters is that they could get tangled in the seaweed, panic, and potentially drown.

Otherwise, you may experience some skin irritation if it brushes your body. Sargassum has tiny thorns that Parsons said could scratch you. The seaweed doesn’t necessarily sting or cause rashes, but little critters that live in it could irritate and bite you.

“It’s basically a little city,” Parsons said. “You’ll find all kinds of golden shrimp, sargassum shrimp, and crabs.”

Sargassum also contains heavy metals that are toxic to humans and animals and do, to an extent, disperse in seawater, Parsons said. But the risk is quite literally a drop in the bucket given how vast the ocean is. Heavy metals in sargassum become a problem if people use the seaweed for other purposes like compost because the toxins can seep into vegetables or other foods that are grown in the soil.

It’s also worth noting that rotting sargassum can support the growth of fecal bacteria, which feeds on the seaweed and helps break it down. They are naturally found in the large intestines of warm-blooded animals, like humans. Sometimes beaches close because high levels of fecal bacteria like E. coli are detected in the water following sewage contamination or other events, making it dangerous to swim in.

This doesn't necessarily mean that there’s a bunch of poop in the ocean, but this type of bacteria “is associated enough with fecal matter that when you see high abundances of it, you conservatively should assume that there’s some fecal source nearby,” Parsons said. “But on the other hand, there's always going to be some around, and so if there is another food source like decaying seaweed, they're going to take advantage of that, which means they're going to multiply.”

But again, the likelihood you get sick from sargassum-munching bacteria is low because the seaweed would likely be raked away periodically, which will help prevent its growth.

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