Q. I used to get poison ivy a lot when I was a kid. As an adult, I can't remember getting it. Do you develop an immunity to poison ivy?

Most people have some level of sensitivity to toxic plants. It's the world's most common allergy.

Sensitivity to poison ivy, oak and sumac tends to decline with age. People who got rashes as children usually see their sensitivity decrease by early adulthood. People who were once allergic to poison plants may even lose their sensitivity entirely later in life.

The irritating substance is the same for each plant, an oily sap called urushiol (u-ROO she-ol). If the saps gets on your skin, a rash will usually begin to appear a day or two later. The skin will become swollen and red. Then, small blisters will begin to form, and the skin will itch. The rash will begin to go away after about a week.

If you contact a poison plant, use soap and water to wash yourself, your clothing and anything else that may have touched the plant such as garden tools, walking sticks, golf clubs or camping gear. Rubbing alcohol is an effective solvent for urushiol, which can remain active for a long time.

To treat an itchy rash, you can use calamine lotion, hydrocortisone creams and oral antihistamines. Cool showers, wet compresses and oatmeal baths relieve symptoms.

Get medical attention if you are feverish, your condition is not improving, the rash is widespread, blisters are oozing pus, or the rash is in your eyes, mouth or on your genitals. Prescription medication may be needed to reduce the swelling and itch.

The best prevention methods are avoiding poison plants, covering up when you know you might be exposed, and applying an over-the-counter product that contains bentoquatum, which helps prevent urushiol from penetrating the skin.

Aluminum chlorohydrate, which is in deodorants, can prevent urushiol from irritating the skin. If you don't have a skin-barrier product with you before going where there might be poison plants, spray deodorant on your arms and legs, but don't get any on your face.

A few misconceptions.

Ÿ Poison plant rashes can't be spread from person to person.

Ÿ The rash will only occur where the plant oil has touched the skin, so a person with poison ivy can't spread it on the body by scratching.

Ÿ The fluid in blisters is not plant oil and cannot spread the rash.

How can you identify these toxic plants?

Poison Ivy

Poison ivy is found throughout the United States except Alaska, Hawaii, and parts of the West Coast. It grows as a vine or shrub. Each leaf on poison ivy has three smaller leaflets with smooth or toothed edges. Leaves are reddish in spring, green in summer, and yellow, orange, or red in fall. Poison ivy may have white berries.

Poison Oak

Poison oak grows as a low shrub in the eastern United States, and in tall clumps or long vines on the Pacific Coast. Poison oak has fuzzy green leaves in clusters of three; they are lobed or deeply toothed with rounded tips. Its leaves resemble the leaves of an oak tree. Poison oak may have yellow-white berries.

Poison sumac

Poison sumac grows as a tall shrub or small tree in bogs or swamps in Northeast, Midwest, and parts of the Southeast. Each leaf has clusters of seven to 13 smooth-edged leaflets. Leaves are orange in spring, green in summer, and yellow, orange, or red in fall. Poison sumac may have yellow-white berries.

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