Personal Health

Jane Brody on health and aging.

I was once among those who claim, “I could walk through a field of poison ivy and not get it.” One day I learned otherwise: On a hike, I needed to relieve myself in the great outdoors and ended up with an impossibly itchy, blistering rash on a most delicate body part.

Too late I learned that you can develop an allergic reaction to poison ivy after previously uneventful exposures, which induce a sensitivity to the plant’s oily sap, urushiol. Once sensitized, your skin is likely to react to every subsequent exposure. Even people repeatedly exposed who appear to be immune may react to high concentrations of the toxin.

I also learned not to rely on the popular warning, “Leaves of three, let them be,” to alert me to the presence of Toxicodendron radicans, as the poison ivy plant is aptly called by botanists. It is not just the leaves that can provoke a reaction; the stems, roots, flowers and berries all contain urushiol.

Touching or brushing against any of these plant parts, even if they are dead, can cause a reaction. The sap is hardy and can cause a rash in the dead of winter, or even a year after contaminating clothing or shoes that are not thoroughly cleaned.

iStockPoison oak.

Zoran Ivanovich/The New York TimesPoison sumac.

iStockPoison ivy.

Urushiol shows up elsewhere, including in the skin of mangoes (and the leaves and bark of the mango tree), as I discovered when I ate a mango still in the rind and ended up with a blistering rash on my mouth. Cashew shells also have the toxin, which is why cashews are sold shelled and processed (either roasted or in the case of “raw” cashews, steamed) at a temperature high enough to destroy urushiol. Poison ivy is not the only problem plant one might encounter while hiking, camping or simply strolling in the countryside. T. radicans has two relatives, poison oak and poison sumac, that don’t always form the classic clusters but are equally toxic troublemakers.

Myths and misconceptions abound about these three plants and the reactions they can cause. Knowing the facts can help to spare you and your family considerable distress.

First, learn to recognize the plants in their various growth patterns. While poison ivy is most often encountered as a small ground plant, it also grows as a shrub and vine. The vines, which turn bright red in fall, were once used to adorn buildings in England.

Poison oak, which has compound leaves made up of three (or sometimes five) leaflets, usually grows as a shrub, but will form a vine in the Western states.

Poison sumac, which grows as a tall shrub or small tree, produces leaves with rows of paired leaflets and a single leaflet at the end. It likes a wet habitat, growing in peat bogs in the Northeast and Midwest and swamps in the Southeast.

Urushiol can penetrate cloth. Although long sleeves, pants and gloves can reduce the risk of exposure, they cannot guarantee protection. Even rubber gloves can be breached. If you must handle the plants or are likely to contact poison ivy when gardening, wear vinyl gloves.

You don’t have to touch the plant directly to react to urushiol. Gardening tools, sporting equipment, even a pet that has been in a patch of poison ivy — all can cause a reaction. My brother, who has been sensitive to urushiol since childhood, once developed the rash on his arm after retrieving a baseball that had rolled through poison ivy.

Before possible exposure, use an over-the-counter skin-care product containing bentoquatam (IvyBlock) to prevent or reduce absorption of urushiol. The combination of this barrier product and protective clothing is your best defense against an inadvertent encounter.

But there is no scientific evidence that jewelweed, feverfew, plantain or other herbal remedies prevent or cure a urushiol-induced rash.

Nor do you become immune to urushiol through repeated exposures to small amounts. Quite the opposite. There is no way to desensitize a person to urushiol as there is with pollen and peanut allergies. Eating mangoes or cashews will not work.

Contrary to popular belief, a poison ivy rash is not contagious. It cannot be spread by oozing blisters, or by scratching or touching the rash. Only direct contact with urushiol causes a reaction. (Scratching can result in an infection, however.)

The rash can appear on different parts of the body at various times. This may happen because the parts were exposed at different times, or because areas with thicker skin are less easily penetrated by the oil. The delicate skin of the genital and perianal areas, for example, is more easily breached than tougher skin on the hands.

Repeated tilling or mowing can eventually kill poison ivy plants, as can repeated applications of an herbicide like Roundup. The latter should be applied with serious caution and only on a warm, sunny day with little or no wind when the plants are actively growing.

If you are highly allergic, or wary of applying herbicides, clearing your property of the plants may be best left to a professional. While goats are said to have a hearty appetite for poison ivy plants, they may eat everything else in the yard, too.

Never try to burn a poison plant. Burning releases the toxin, which may land on skin or, worse, be inhaled and cause a serious internal reaction.

Should you contact a urushiol-containing plant, the American Academy of Dermatology recommends washing your skin immediately. Lukewarm, soapy water is best, but even plain water can limit exposure to the sap. Take care in removing contaminated clothing, and wash it separately as soon as possible.

You can relieve a rash by applying cool compresses with an astringent like Burow’s solution, soaking the affected area in colloidal oatmeal, or using calamine lotion; all are sold over the counter. Do not apply products containing a topical antihistamine, like Benadryl, which can cause a sensitivity reaction that makes matters worse.

Severe reactions may require medically prescribed treatment with an oral corticosteroid like prednisone.

A version of this article appears in print on 06/17/2014, on page D7 of the NewYork edition with the headline: Steering Clear of Poison Ivy.