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Mats of seaweed born in the Atlantic off the coast of Africa are drifting west toward the Gulf of Mexico and some of the world’s most pristine beaches in the world, scientists say.

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The Atlantic sargassum belt, which stretches more than 5,000 miles, is expected to make it onshore in Florida and possibly other areas along the Gulf of Mexico, soon, causing pollution and threatening the health of those with respiratory issues.

While not an unusual event, it happens annually, researchers say that the amount of this year’s sargassum “bloom” is much larger than it has been in recent years.

Here’s what we know now about the mound of seaweed that has already been seen in the Florida Keys.

What is the sargassum?

Sargassum is a type of algae that bunches up in clumps and floats in oceans and gulfs. The clumps are generally several feet thick.

The algae are rootless and leafy, can soak up carbon dioxide and serves as a critical habitat for fish, crabs, shrimp, turtles and birds.

Eventually, the mats of sargassum become heavy and sink to the bottom of the ocean or gulf. There it breaks up and becomes food for marine life.

Under the right wind and wave conditions, sargassum makes it to the shore and onto the beaches.

How big is the sargassum belt this year?

The sargassum belt is more than double the width of the contiguous U.S. this year.

The size of the seaweed patches changes over the months, with mats becoming their largest in the summer months.

What scientists are seeing is that the mats have become larger in recent years.

“The low season of the cycle is now higher than the high point of the cycle five or six years ago,” Brian Barnes, a researcher with the University of South Florida’s College of Marine Science, told NPR.

“What we thought was just a massive bloom has only gotten bigger and bigger and bigger each year,” Barnes adds.

What happens when it gets onshore?

When sargassum gets close to shore, it can alter the pH balance of the water and affect the local ecosystems.

When it gets onshore, it can affect tourism and constrict fishing. It piles up on beaches and begins to rot after about two days. It smells like rotten eggs.

As it rots, it gives off an irritant that can cause respiratory issues or worsen breathing problems for people with asthma, COPD, or other lung issues.

In 2018, doctors on the islands of Guadeloupe and Martinique reported more than 11,000 cases of “acute sargassum toxicity” as sargassum built up over eight months, Reuters reported.

The Journal of Travel Medicine warned travelers to the island at the time that prolonged contact with the sargassum weed, or inhaling the hydrogen sulfide gas it gives off as it decomposes onshore, can cause heart palpitations, shortness of breath, dizziness, vertigo, headache and skin rashes.