Jellyfish aren’t aggressive and don’t attack humans, but accidental contact with their tentacles can cause mild to severe reactions, depending on the type of jellyfish.
If you are stung by a jellyfish, forget the old wives’ tale about urinating on the sting—it won’t help and may actually make the pain worse, says Jay Bradley, curator of the National Aquarium in Washington, D.C. Instead, immediately flush the area with seawater (don’t use fresh water—it might exacerbate the pain) to wash away the toxins that caused the sting. (Vinegar works too, though your chances of having it at the beach are probably slim.) Once home, submerge the affected area in hot water for up to 90 minutes in order to neutralize any stinging tentacles that may be lingering. Tentacles that are still attached can be removed carefully with tweezers.
To prevent a jellyfish sting, leave beached jellyfish alone; if you know you’ll be sharing waters with jellyfish, wear protective swimwear, such as a wet suit or a rash guard. In the United States, most jellyfish are not very dangerous and their stings cause only mild discomfort. But reactions to stings can be more severe if you are allergic; if you are experiencing swelling, welts, or respiratory distress, seek immediate medical attention.