Summer myths, such as one shouldn’t swim for 30 minutes after eating, are bunk

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Myth: If a jellyfish stings you, urinate on the wound.

(Peter + Maria Hoey/TWP) - Poison Ivy is contagious but the rash itself is not.

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Urinating on a jellyfish sting can make it worse, according to Jennifer Ping, an emergency medicine physician at Straub Clinic and Hospital in Honolulu, who has studied the most effective treatments for dealing with jellyfish stings. About 15 people per year check in to her hospital’s emergency room after being stung by jellyfish.

Jellyfish stings are caused by contact with a jellyfish tentacle, which can trigger millions of stinging cells (nematocytes) to pierce the skin and inject venom, Ping says.

The first line of treatment for all species of jellyfish stings is to get out of the water. Then, remove the tentacles with an object other than your fingers. Deactivate the nematocytes with an acidic compound such as vinegar, either by pouring it directly onto the wound or applying a vinegar-soaked cloth. Once the nematocytes are deactivated, scrape them off with a credit card or other flat object. A paste of vinegar and meat tenderizer also works; scrape it off within 20 minutes or the tenderizer will irritate the skin.

Urine has a different pH than vinegar and, like water, it can cause the nematocytes to swell and release more venom, thus worsening the sting, Ping says. However, if the nematocytes have been deactivated and washed away, warm urine might soothe the sting based on its warmth alone. Warm water or heat packs would also work, as would ice packs. “I think [the myth] gets perpetuated because it’s something that is funny, yet believable,” Ping says.

Myth: Scratching a bug bite makes it worse.

This one is true. If you scratch a mosquito (or other bug) bite vigorously enough to break the skin, the bacteria from underneath your fingernails could cause a skin infection, according to Vreeman.

You’ll know that the mosquito bite is infected because it will look worse, rather than better, as the days go on. An infected bite might also itch more than an uninfected one. Treat the bite with Neosporin or another antibiotic ointment.

Scratching will probably make the bite look worse even if it doesn’t get infected, Vreeman says. The degree of swelling depends on your body’s inflammatory response to the bite, but scratching will temporarily inflame the bite further. Itching can cause the body to release more histamines, and this leads to more redness, swelling and itching, Vreeman says.

And that old summer-camp myth that mosquitoes like people with “sweet blood”? It turns out that it probably has more to do with their breath than with anything in their blood, Vreeman says. Mosquitoes are attracted to carbon dioxide, heat and lactic acid in the breath.

“They think it’s primarily related to the balance of gases and maybe the scent of people’s breath, but it’s not clear,” she says.

Myth: Don’t swim for 30 minutes after eating.

If you have a big meal and then go for a swim, the worst thing that could happen is you’d feel uncomfortable or get a cramp, not drown, according to Washington nutritionist Rebecca Scritchfield. (There are no documented cases of drowning or near drowning attributed to eating, according to Vreeman and Carroll.) It’s unlikely that a food-related cramp would disable you, Scritchfield says.

After a meal, the body directs blood to the stomach to help digest the food. If you’re swimming, some of the blood might move to your muscles instead, potentially causing the food to move through the gut more slowly, according to Scritchfield. You might cramp up as a result, in which case you should just get out of the water and rest. In general, it’s wise to swim where you’re able to exit the water fairly quickly, such as in a swimming pool or along a shoreline.

“There’s no magic to the 30-minute number,” Scritchfield says. “Nothing dangerous is going to happen before that. It’s really how you feel.”

Children (or those with overactive imaginations) might also fear that food will not move thround their digestive system if their body is floating in the water. This is also impossible, according to Scritchfield, because the involuntary actions the body takes to move food through the body are so strong. After swallowing, the body takes over completely.

“Astronauts still digest their food,” she points out.

Saslow is a former Washington Post staff writer.