Each year, tens of millions of Americans who spend time in the great outdoors find themselves with an unpleasant, itchy souvenir of their day out. Exposure to poison ivy, poison oak and poison sumac often triggers an allergic reaction that causes skin to turn red, blister or swell. To make matters worse, the irritated skin can be intensely itchy.

About 85 percent of people are vulnerable to this type of skin reaction, according to the American Academy of Dermatology. A colorless, odorless resin from plant sap known as urushiol is responsible for the symptoms, which usually last from a week to 10 days.

How can you avoid this maddening summer itch? Try these tips:

Identifying the Enemy

The key to avoiding contact with urushiol is to know which troublesome plants are common to your region. Plants containing urushiol are common throughout all states accept Alaska and Hawaii. Different plants occur more frequently in specific parts of the country.

It is important to be able to identify the characteristics common to each plant. Urushiol is found in several plants, including the skin of mangoes and the nut shell of cashew fruits. However, the three plants most often responsible for that maddening itch are:

• Poison ivy. Leaves are usually arranged in groups of three, but also appear in fives or sevens. In each cluster the middle leaf grows on a long stalk and the leaves are usually fairly equal in size. Leaf edges are slightly notched or smooth. The leaves are shiny and green when young and bright red in the fall. The flowers are small with a yellow-greenish white color. In the United States, poison ivy tends to grow in disturbed areas and wet areas in the Midwest and the East Coast. It is not normally found on the West Coast.
  
  
• Poison oak. The leaves are larger than poison ivy and grow in groups of three, five or seven. It has fruit and flowers that appear similar to poison ivy, but the leaves have short, smooth hairs on the underside. Poison oak grows as a low shrub in the eastern United States in areas of sandy soil and pine forests. In the West, it grows in tall clumps or as long vines in low areas and wooded thickets.
  
  
• Poison sumac. Features clusters of seven to 13 leaves arranged in pairs with one leaf at the end. The small yellowish flowers mature into whitish-green berries that hang in clusters up to 12 inches (30 centimeters) long. Poison sumac tends to grow around wet areas such as bogs in the Southeast, but it is most common in the Great Lakes area and the East Coast. It may also grow as a tall but spindly shrub.