The rash that develops upon exposure to any of these plants is an allergic reaction to the oil found in the plant sap. The oil is called urushiol. It is found not only in the plant's leaves, but also in its stems and vines. The first time you come in contact with poison ivy, poison oak, or poison sumac, you may not have a reaction to it. At that point, your body will develop a sensitivity to it. The next time you come in contact with poison ivy, your skin will react with an itchy rash 12 to 72 hours afterward. Some people are highly allergic to the oil. About 3 in 20 people never develop an allergic reaction to it. The allergic reaction also varies over time. Children who are sensitive may not be sensitive as adults.

Animals usually don't react to the urushiol oil. But it can stick to pet fur, as well as to any clothing or tools that brush against the plants. If you burn poison ivy, the oil can become airborne. It can cause a rash when it comes in contact with your skin. If you breathe in (inhale) the smoke from burning poison ivy, it can cause a serious allergic reaction. This can include a rash, conjunctivitis, and trouble breathing.

Poison ivy grows as a vine or a shrub in well-drained forests and woods. Poison sumac grows in swamps and peat bogs. Poison oak, like poison ivy, grows as a shrub or vine. Poison ivy is most common in the East and the Midwest. But it can be found all over the U.S. Poison oak is found on the East and West coasts.

One saying goes: "Leaves of 3, let them be." The plants may look slightly different by region and time of year. But some general traits can help you identify them. These include the 3 leaflets of poison ivy and poison oak. In the spring, poison ivy has reddish leaves. In the summer, they are green. And in the fall, they turn yellow, orange, or red. It may have white berries. The markings on the berries make them look like a peeled orange. Its leaves can be smooth, shiny, and hairless or velvety, rough, and hairy. The poison ivy plant grows as a vine, but also as a low-growing shrub. Poison oak also has 3 leaflets, but its leaves look like oak leaves. The undersides of the leaves are much lighter than the tops and are covered with hair. Poison oak grows as a shrub, tall clumps, or long vines. If the poison oak plant has berries, they will hang in clusters and be yellow-white. Poison sumac grows as a tall shrub or small tree and has 7 to 13 oval leaflets attached to a central stem. Leaves will be orange in spring, green in summer, and orange, red, or yellow in the fall. The leaves may have spots that look like paint on them. Its glossy, cream-colored or pale yellow berries grow in drooping clusters. (In comparison, the nonpoisonous sumac tree has red berries that grow in upright clusters and jagged leaf margins.)

The rash can appear on any skin surface that has been exposed to the urushiol oil. The rash appears 12 to 72 hours after exposure. Areas of the body where the skin is thinner, such as the face, are more likely to develop the rash. Areas such as the palms of the hands where the skin is thicker are less likely to develop a rash. The rash does not spread. But it may appear to spread, because some skin may take longer to develop a rash. Scratching the rash will not spread it. For a severe poison ivy rash, see your healthcare provider. Also visit your provider if your poison ivy rash is on your face or genitals, or if it covers more than 20% of your body. Also see your provider if you inhaled the smoke from burning poison ivy and had an allergic reaction and trouble breathing.

Most exposure to poison ivy occurs during the spring and summer, when the plants have leaves. But it can still be a problem in autumn and winter. If you trim out poison ivy vines without realizing what they are, even if the vines are dead, you can get the oil from the vines on your hands, clothing, or tools. If you burn yard waste that contains poison ivy, the smoke can cause an allergic reaction.

Use a degreaser with water or rubbing alcohol and a lot of water to clean the skin. If you can wash off the oil within 5 minutes, you likely won't develop a rash. If the oil has been on the skin for more than five minutes, you should still wash your skin. You will likely develop a rash. But washing will help prevent the oil from spreading. Wash any clothing with detergent. And be careful not to let the clothing touch furniture or flooring, because the oil can spread there. Take care to clean contaminated tools or outdoor gear. The oil will remain on them until the next time you use them.

Add oatmeal to your bath water and soak in it to ease the itching. Commercial oatmeal baths are also available. Calamine lotion, hydrocortisone cream, or an over-the-counter antihistamine can relieve minor itching. If the itching is severe, talk to your healthcare provider about a prescription cortisone treatment.