Sargassum does not sting or cause rashes. However, tiny organisms that live in Sargassum (like larvae of jellyfish) may irritate skin if they come in contact with it.
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Most sargassum grows off the shore of the Gulf of Mexico. Like all plants, it needs nitrogen, phosphorous, and sunlight to survive. In recent years, the sargassum has had its fair share of nitrogen, thriving on large amounts of fertilizer from the runoff of the Amazon River. The seaweed can also increase when upwelling in the eastern Atlantic brings cooler water and nutrients from the bottom of the ocean to the surface.
As the sargassum grows, it starts moving along ocean currents, which transport it from the Gulf to the Florida Current via the Loop Current, and then on to the Sargasso Sea, Tropical Atlantic, and the Caribbean. If winds are strong, it can end up near the Sahara desert, the Amazon River, or here on the Florida coast, bringing problems when it arrives en masse.
When washed ashore, Sargassum will decompose (rot). Rotting Sargassum causes the production of hydrogen sulfide gas which smells like rotten eggs.
Hydrogen sulfide can irritate the eyes, nose, and throat. If you have asthma or other breathing illnesses, you will be more sensitive to hydrogen sulfide. You may have trouble breathing after you inhale it.
Hydrogen sulfide is not known to cause cancer in humans. If you are exposed to hydrogen sulfide for a long time in an enclosed space with little air flow (like some work exposures), it can affect your health. However, hydrogen sulfide levels in an area like the beach, where large amounts of air flow can dilute levels, is not expected to harm health.
You should not use Sargassum in cooking because it may contain large amounts of heavy metals like arsenic and cadmium.