Poison ivy (Toxicodendron radicans -- eastern poison ivy/Toxicodendron rydbergii -- western poison ivy) normally grows as a vine or shrub. It can be found throughout North America (except in Alaska, Hawaii and the desert). It grows in open fields, wooded areas, on the roadside, and along riverbanks. It can also be found in urban areas, such as parks or backyards. Poison ivy plants typically have leaf arrangements that are clustered in groups of three leaflets, though this can vary. The color and shape of the leaves may also vary depending upon the exact species, the local environment, and the time of year. The plant may have yellow or green flowers, and white to green-yellow berries, depending on the season.
The oily resin called urushiol is what causes the rash once a person is exposed by skin contact to poison ivy, poison oak and poison sumac. Urushiol is not only found on the leaves of these plants but is also found on the stems, flowers and roots. Exposure to a less than a grain size of table salt of urushiol will cause a rash in up to 90% of individuals. The oil on these plants is active after the plant has died.
Keep in mind that a person does not have to have personal contact with the plant to contract the oil on their skin. If your dog, pet, clothing, tools, vehicle etc. comes in contact and the oil urushiol from these plants and you touch those surfaces, the oil can affect your skin in the painful rash as well. If these plants are burned, the urushiol particles can land on the skin causing a rash along with inhaling the smoke from the urushiol oil can effect the lungs.
Sensitivity to urushiol occurs when individuals come into contact with it. The first time a person is exposed, they may not develop a rash. However, with repeated exposure, sensitivity develops that ultimately leads to the development of the characteristic rash. Most people (about 85%) will develop sensitivity, while a small percentage of individuals (about 15%) never develop an allergic reaction to urushiol.
On a personal note; I walked through a meadow full of poison ivy at Ft. Leavenworth Kansas when I was six years old. I did not have a shirt on and was wearing shorts. When I walked out of the woods the sting of the urushiol oil took effect and I could hardly walk. I made it home and my mother put me in a bathtub with lukewarm water and oat meal. The oat meal stopped the sting. Ever since I was so heavily exposed to the urushiol oil, I have not had a reaction to urushiol since.