Approximately 85% of the population will develop an allergic reaction if exposed to poison ivy, oak or sumac, according to the American Academy of Dermatology. Nearly one-third of forestry workers and firefighters who battle forest fires in California, Oregon and Washington develop rashes or lung irritations from contact with poison oak, which is the most common of the three in those states.
Usually, people develop a sensitivity to poison ivy, oak or sumac only after several encounters with the plants, sometimes over many years. However, sensitivity may occur after only one exposure.
The cause of the rash, blisters, and infamous itch is urushiol (pronounced oo-roo-shee-ohl), a chemical in the sap of poison ivy, oak and sumac plants. Because urushiol is inside the plant, brushing against an intact plant will not cause a reaction. But undamaged plants are rare.
"Poison oak, ivy and sumac are very fragile plants," says William L. Epstein, M.D., professor of dermatology, University of California, San Francisco. Stems or leaves broken by the wind or animals, and even the tiny holes made by chewing insects, can release urushiol.
Reactions, treatments and preventive measures are the same for all three poison plants. Avoiding direct contact with the plants reduces the risk but doesn't guarantee against a reaction. Urushiol can stick to pets, garden tools, balls, or anything it comes in contact with. If the urushiol isn't washed off those objects or animals, just touching them— for example, picking up a ball or petting a dog— could cause a reaction in a susceptible person. (Animals, except for a few higher primates, are not sensitive to urushiol.)
Urushiol that's rubbed off the plants onto other things can remain potent for years, depending on the environment. If the contaminated object is in a dry environment, the potency of the urushiol can last for decades, says Epstein. Even if the environment is warm and moist, the urushiol could still cause a reaction a year later.
"One of the stories I tell people is of the hunter who gets poison oak on his hunting coat," says Epstein. "He puts it on a year later to go hunting and gets a rash (from the urushiol still on the coat)."
Almost all parts of the body are vulnerable to the sticky urushiol, producing the characteristic linear (in a line) rash. Because the urushiol must penetrate the skin to cause a reaction, places where the skin is thick, such as the soles of the feet and the palms of the hands, are less sensitive to the sap than areas where the skin is thinner. The severity of the reaction may also depend on how big a dose of urushiol the person got.
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Poison ivy is a member of the Cashew Family (Anacardiaceae), and I've seen it listed as Toxicodendron radicans and in more recent books as Rhus radicans. It can appear as a ground cover, a shrub, or as a vine growing up a tree. Older vines are covered in fibrous roots resembling hair that grow into the supporting tree. It has dull or glossy compound leaves on a long stem that are divided into 3 leaflets, each 2" — 4" long. The leaflets can be slightly lobed, and are a dark waxy green, above, and light, fuzzier beneath. A short stem sets off the end leaf.
Poison ivy flowers May through July in yellowish-white (possibly green) clusters, 1" — 3" long at the leaf axils. The flowers are 1/8" wide. In fall, poison ivy leaves turn red. Between August and November, poison ivy develops white or gray hairless fruit to 1/4" wide, in clusters that stay through winter and into spring, when not eaten by birds. Dead leaves and stems may have black stains on them, due to the oxidation of the urushiol oil.
Poison ivy grows throughout eastern North America, while its counterpart, poison oak, grows in western North America and has hairy fruit. Poison ivy and poison oak grow in open woods, thickets, fence rows, stone walls, roadsides, and waste places. On roadsides, it tends to be ground cover, and in sandy coastal areas, it tends to be an erect shrub. In woods, you'll mostly see the vines on trees.
POISON IVY CONTROL can be done at any time of the year, but is best achieved May through July while the plants are flowering. Poison ivy should be accurately identified before you attempt any control measures. Spraying is recommended over burning because poison ivy oil vaporizes when hot, carries in smoke and can cause a severe rash.
Poison ivy foliage within reach can be sprayed with glyphosate (sold under the trade names Roundup, Kleenup and others) according to label directions. When using this or any herbicide, always read and follow label directions carefully. Take care to avoid other plants and do not spray so heavily the herbicide drips off the leaves. Glyphosate is a nonselective herbicide and will kill any vegetation it contacts.
To kill poison ivy that climbs high into trees, cut the vine off 6 inches above ground level. Treat the stump with glyphosate (according to label directions) immediately after cutting to kill the roots and prevent sprouting. If re-sprouting does occur, treat the leaves with glyphosate.
Poison ivy can be very persistent, so you may have to spray the vines two or more times for complete control. Poison ivy can spread along fence or hedge rows and under trees by birds dispersing the seeds. Treating young seedlings with glyphosate will kill them and limit the spread of poison ivy.
Read also: If you've been exposed to poison ivy