When a jellyfish stings you, it pierces your skin with tiny poison-filled stingers called nematocysts. Treating a sting is a matter of removing these bodies without letting them release the poison, and then using acid or hot water on the areas that absorbed poison. The chemical composition and strength of jellyfish poison varies widely, causing anything from a mild tingle to cardiac arrest.
(Don't try this at home!)
Fortunately, the nematocysts can just barely penetrate the skin. This means poison can be treated topically.
So, what does this have to do with urine?
Depending on its chemical composition, jellyfish poison can be washed away with either a base or an acid. It was once thought urine was acidic enough to wash away the poison, but recent tests have shown that it isn't strong enough to be effective.
If you can't use urine, what do you do?
First, find out what jellyfish poison in your area can be cleaned with. If it's acid-reactive, keep some white vinegar handy. If it's base reactive, you can rinse the wound with sea water. Do not rinse with fresh water: nematocysts will inject poison as soon as they come in contact with it. The box jellyfish in the video has acid-reactive poison, but rinsing it off will probably be the least of your worries.
Put on some gloves to avoid transferring the nematocysts to your hands and wipe the stingers away with a dry towel. The less pressure you apply, the less likely the poison will go into your body.
Once you get to shore, rinse the area with the hottest water you can stand. Fresh water is OK now that the stingers are gone. Ice the area and take an anti-inflamatory like acetaminophen or ibuprofen.
Posted 1820 day ago