Despite the fact that poison ivy and poison oak are different species, there is no distinction in the effect these plants have on humans. The culprit is the same for both: urushiol. Urushiol is an oily, resinlike substance found in all sections of the poison ivy and poison oak plants, except in the pollen, anther, epidermis, and xylem tissue found inside the stem. The oil releases when the fragile plant is bruised. This makes it easier to develop allergic contact dermatitis, the rash from poison ivy and oak, in the spring and summer when leaves are tender. Although the plants have a dormant season, Dave Leonard, a consulting arborist from Kentucky, says, "They are still poisonous in winter, even though they are less obvious." In addition to direct transfer from the poison ivy and poison oak plants, urushiol can be deposited on the skin when someone touches contaminated objects, such as clothing and tools.

Arborists and those who enjoy hiking, biking, and any number of other outdoor activities should look for poison ivy and poison oak near fences, forests, stream banks, and rocky canyons. Paul Markworth of Wachtel Tree Science and Service of Merton, Wisconsin, says, "The most common place we find poison ivy around here is in heavily wooded lots, and especially near old fencerows."

Reaction

Urushiol oil is absorbed into the skin within ten minutes of contact with poison ivy or poison oak. After 12 to 48 hours, a burning, itching rash appears in linear streaks, mirroring how the plant brushed against the skin during transfer. This irritation of the skin is usually coupled with a reddish inflammation of small bumps. The appearance of a rash marks the bond of the urushiol oil to the skin. Occasionally, oozing blisters develop as part of the body’s natural reaction to the allergens in the plants.

Despite the common misconception, liquid from the blisters does not spread the rash. In fact, scratching or rubbing the affected area will not spread poison ivy or poison oak to other parts of the body or to other people. The rash spreads only if the urushiol oil contacts the skin. Irritation that occurs a few days after the primary sores appear is caused by contact between less-sensitized areas and places where less urushiol was deposited. Areas of the body where skin is thicker, such as the palms of the hand and the soles of the feet, tend to be less sensitive. Secondary infections (impetigo, for example) can develop from scratching the affected skin. The rash disappears in 14 to 20 days, but healed areas are highly susceptible to further contact.

Some people are immune to the effects of poison ivy, but research has shown that repeated exposure leads to greater sensitization. Poison ivy and poison oak dermatitis is an anaphylactic reaction, meaning frequent contact with urushiol increases the likelihood of an allergic rash. Usually, people will not develop a reaction or experience any symptoms after their first contact with poison ivy or oak. If a rash does develop, the allergic response depends on the amount and frequency of the urushiol exposure and the sensitivity of the individual.

Smoke from burning poison ivy is extremely toxic because droplets of the sap adhere to particles of ash that are later inhaled. This type of poisoning can cause a systemic reaction, resulting in lung infections and possibly even hospitalization. Nearly one-third of the firefighters who battle forest blazes in California, Oregon, and Washington develop rashes or lung irritations from the airborne urushiol found in poison oak. Experts warn to be especially cautious of dead poison ivy or oak vines wrapped around firewood.

Other plants and plant parts also contain urushiol oil, including cashew shells, mango rinds, and ginkgo (Ginkgo biloba). People with allergies to these items will likely suffer a more severe reaction after contacting poison ivy or poison oak.