Have you ever hesitated before diving into a pool, trying to remember if it's been at least 30 minutes since you ate? Or not itched a mosquito bite for fear that scratching would only make it worse? Health myths buzz around summer pastimes such as swimming, camping, hiking and picnicking like so many gnats.

People have a hard time letting go of word-of-mouth wisdom, even when faced with good evidence to the contrary.

"Myths stick with us because they make sense to us, on some level," says Indianapolis pediatrician Rachel C. Vreeman. "When you've heard them from your grandmother and mother and important adults in your life, you believe those things."

Vreeman and fellow Indiana University School of Medicine pediatrician Aaron E. Carroll published studies in 2007 and 2008 debunking medical myths that doctors believe. Among them: Hair and nails continue to grow after a person has died. Shaving causes hair to grow back thicker. We use only 10 percent of our brains. Reading in dim light ruins your eyesight.

The studies received so much media attention that the doctors turned them into two books debunking many health myths: "Don't Swallow Your Gum!" and "Don't Cross Your Eyes . . . They'll Get Stuck That Way!"

"It's fair to ask 'Why?' when someone tells you you shouldn't do something, even if that someone is your doctor or your mother," Vreeman says.

Remembering the many warnings that swimming and outdoor activity inspire, we dug into some of the most pervasive summer health myths to find out whether they're true.

Myth: Swallowing water-melon seeds is bad for you.

Swallowing a few watermelon seeds won't do any harm, Washington nutritionist Rebecca Scritchfield says. Our bodies try to digest them but can't, so the seeds pass directly through our system. Stay hydrated and continue to eat normally and everything will work out, so to speak. If someone were to chew up and eat every seed in a watermelon, the only danger would be overdoing fat and calories for the day, Scritchfield says. One cup of seeds contains 602 calories, 31 grams of protein (about the same as a chicken breast) and 51 grams of fat, a day's worth for most people. Watermelon seeds are eaten in other parts of the world, such as Nigeria and China, Scritchfield says.

Small children might imagine that watermelon seeds could sprout in their stomachs (or be tricked into thinking this by a mischievous sibling). There have not been any reports of that happening — and it's unlikely, given how strong digestive acids are — but in 2010, a pea seed did sprout in a man's lung: A seed he had aspirated grew a half-inch-long sprout and had to be surgically removed at Cape Cod Hospital in Massachusetts.

Myth: You can catch poison ivy from someone who has it.

No matter how icky and oozy a poison ivy rash looks, the rash itself is not contagious, Vreeman says. It's the oil from the poison ivy plant that is contagious, not the reaction to it that is the blistery rash you see on someone's skin. "Leaves of three, let them be," as they say.

Poison ivy causes a delayed response; the rash doesn't appear for 24 to 72 hours after contact with the plant oils, which are found on the leaves and on the stems, and it can spread for days even without additional contact with the oil depending on individual reactions and sensitivities, according to the American Academy of Dermatology. By the time the rash is in full force, it's unlikely the person would still have the oil on his or her skin. Washing the exposed area with soap and water for five minutes at the time of exposure may prevent the rash, says dermatologist Ali Hendi.

The blisters cannot spread the rash to other people, nor to other parts of the infected person's body. The oils can stick around on clothes and shoes, though, so be sure to wash everything that might have brushed against the ivy.

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

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 nutritionist 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.