The National Institutes of Health and the National Park Service outline ways to identity the three most common types of plants that causes itching in humans: poison ivy, poison oak and sumac.
The perennial plant Toxicodendron radicans, or poison ivy, grows through the western hemisphere, from Canada and south to the West Indies and Mexico. Humans encounter poison ivy when travelling through wooded areas, roadsides, clearings, meadows, fence rows and thickets. Poison ivy has three leaflets, the middle one being the longest, and has smooth, round fruit that is green or grey. Flowers on the poison ivy plant are yellow and bloom from May through July. During all seasons, the plant emits an irritant that causes a skin rash 8 to 48 hours after humans touch it or an object that has touched it.
Like poison ivy, poison oak plants have three leaflets. To distinguish it from the ivy, the leaflets are shiny, smooth and rounded or toothed. In the early stages of its development and in the dry season, the plant displays a red colour. Throughout its growth, the vine changes from red to orange, pink and even purple. The poison oak grows throughout the western hemisphere. In the east, it thrives in sandy soil, sand hills, dry barrens and pine woods. In the west, humans can find the plant in lowlands, thickets and wooded slopes. The plant is found directly off the beaten path of man-made hiking trails, which is where humans may also come into contact with disease-spreading ticks.
Poison sumac grows in water, and humans encounter it along the Great Lakes region and the eastern coastal plain. It thrives in habitats such as bogs, river bottoms and swamps. Because it grows in water, poison sumac poses a greater risk of irritation through indirect contact. The plant is distinguished by small shrubs or trees with alternating leaves containing 7 to 11 leaflets. The leaflets are arranged in pairs with a single one at the end of the midrib. Green and white coloured fruits hang in clusters, and its yellow and green flowers are randomly dispersed.
The roots, stems, leaves and fruit of these poisonous plants contain the sap, which causes human skin to blister and itch. The severity of the rash depends on the amount of sap you are exposed to and your sensitivity to it. The National Park Service advises washing your skin and clothing immediately after coming in contact with the sap. You should also seek over-the-counter medicine and doctor-prescribed treatments to combat unbearable itching or pain. It is unsafe to burn these plants. Airborne sap can irritate the nose, eyes, throat and respiratory system.