What's an Itch?

Medically Reviewed by Louise Chang, MD on April 08, 2011

Does the world outside make you itch? Warmer months can bring you in contact with any number of scratchy culprits: poison ivy, bug bites, and sunburn, to name a few.

From a survival perspective, there isn’t a good explanation for why we itch. One theory goes that animals itch in order to get parasites off their skin, and our desire to scratch an itch could be a carryover from that instinct.

Another possible explanation is that pain and itch engage many of the same areas of the brain. When you scratch an itch, you cause yourself enough pain to relieve the itch, temporarily. Regardless, itches are best left unscratched.

Find out what happens to your skin when you develop an itch.

Never Too Old for Poison Ivy

“I’ve had patients in their late 70s come in with their first case of poison ivy,” says Lisa A. Garner, MD, FAAD, clinical professor of dermatology at the University of Texas Southwestern Medical School. “People think if they’ve never had poison ivy, they never will,” she says. But that’s not the case. The CDC estimates that 80% to 90% of adults are allergic to urushiol, the oil in poison ivy -- and poison oak and poison sumac -- that causes the reaction.

It is true that you can come in contact with poison ivy and not develop a rash the first time. Below the surface, however, your immune system could be arming itself for the next exposure. The next time you are exposed to urushiol, you’re likely to have a reaction.

Some, but not all people, may see their sensitivity to poison ivy decline as they get older, Garner says.

Dealing With Poison Ivy’s Rash

Depending on how sensitive you are and how much poison ivy gets on your skin, an itchy red rash will start to develop anywhere from 24 to 72 hours after contact. As your immune system battles with urushiol, the area may become inflamed and very itchy. The rash often turns into oozing blisters.

It may seem almost impossible not to scratch the rash, but Pamela Scheinman, MD, director of the Contact Dermatitis and Occupational Dermatology Unit at Tufts Medical Center, advises against it. Vigorous scratching can break the skin. “If the skin barrier is not intact, the risk of bacterial infection goes up,” Scheinman says.

Usually, poison ivy rash is at its worst one week after exposure. Unless you have a severe case of poison ivy, the rash should go away within one to two weeks.

To relieve itching, try calamine lotion, an over-the-counter hydrocortisone cream, or antihistamine.

How Poison Ivy Does and Does Not Spread

The idea that poison ivy rash spreads from one part of your body to another is a common but understandable myth.

People often think it has spread because different areas of skin can break out at different times after a single exposure. But the rash only breaks out where urushiol has had direct contact with the skin -- touching the rash or blisters won’t spread it. Your skin may absorb the oil at different rates, Scheinman tells WebMD. Areas where your skin is the thinnest may break out first: wrists, ankles, neck, and face.

By the time the poison ivy rash appears, the urushiol that triggered it has probably been washed off. But the oil does have staying power on clothes, shoes, and other items, however. Scheinman has seen people who were exposed through touching garden tools or sporting equipment with traces of urushiol on them.

Insect Bites

Insects pose another itchy outdoor hazard. Some bug bites, like those from mosquitoes, biting flies, and ants, can make the surrounding skin turn red, swell, and itch. Unlike poison ivy, the reaction to insect bites is almost instantaneous. The swelling, redness, and itching is caused by histamine, which your immune system releases as part of the allergic response.

Most of the time, bug bites cause a mild reaction that goes away in a day or two. Try calamine lotion, an over-the-counter hydrocortisone cream, or antihistamine to relieve itching.

Be sure to read and follow the label directions on any over-the-counter medications. Icing the area can help reduce swelling.

If you feel your throat swelling, find it hard to breathe, or become dizzy or nauseous after an insect bite, seek emergency care.


Make no mistake about it: A sunburn is an injury, says Garner. She tells a story of one young patient whose back was covered in blisters. “It was like someone poured scalding water all over his back,” she says. Although some sunburns are more serious than others, any change in skin color is a sign of damage, Garner says.

Usually, a sunburn will start out red and then may swell and blister. A few days later, the redness may give way to peeling, itching skin. The itch is part of the healing process that happens as your body sloughs off injured skin.

What helps relieve sunburn discomfort? A cool shower or bath and a moisturizer or an over-the-counter hydrocortisone cream may help. If you get blisters, don’t break them.

Of course, it’s better to avoid a sunburn in the first place. Use a sunscreen with an SPF of 30 or higher, and limit your time in the sun, especially between the hours of 10 a.m. and 2 p.m. Also, wear protective clothing, such as long-sleeved shirts, pants, and a wide-brimmed hat to shield you from the sun.

Enjoy Warm Weather Safely

Spring and summer are a great time to get outdoors and shake off the winter blues. As you enjoy the great outdoors, however, keep safety in mind. Educate yourself about poison ivy, bee stings, and sunburn so you can enjoy every moment.

Show Sources


Lisa A. Garner, MD, FAAD, Clinical Professor of Dermatology, University of Texas Southwestern Medical School; fellow of the American Academy of Dermatology.

Centers for Disease Control, “Poisonous Plants - NIOSH Workplace Safety and Health Topic.”

American Academy of Dermatology, “Poison Ivy, Oak & Sumac,” “Sunscreens.”

Asthma and Allergy Foundation of America, “Poison Plants,” “Contact Dermatitis.”

U.S. Food and Drug Administration, “Consumer Updates: Outsmarting Poison Ivy and Other Poisonous Plants.”

Pamela Scheinman, MD, director, Contact Dermatitis and Occupational Dermatology Unit, Tufts Medical Center, Boston.

American Academy of Family Physicians, “Poison Ivy, Oak and Sumac Contact Dermatitis.”

American Academy of Allergy Asthma & Immunology, “What Makes Us Itch? Allergy & Asthma Issues: Fall 2009.”

Ikoma, A. “The neurobiology of itch.” Nature Reviews Neuroscience, 2006: vol 7 535-547.  

Rush University Medical Center, “Sunburn.”


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