Tourists to the Caribbean may not realize that a brown drifting seaweed that's been piling up on beaches in recent years is dangerous, researchers say. Travelers and doctors alike should be aware that prolonged contact with the Sargassum weed, or inhaling the hydrogen sulfide gas it gives off as it decomposes on the beach, can cause heart palpitations, shortness of breath, dizziness, vertigo, headache and skin rashes, the authors write in the Journal of Travel Medicine.