Contact with the poison ivy, oak, or sumac plants causes a rash (allergic contact dermatitis). The rash is caused by contact with the oil (urushiol) in these plants. Urushiol is an allergen, so the rash is actually an allergic reaction to the oil in these plants.

You may be more or less sensitive to the plants than other people. A more sensitive person will have a more severe reaction and will react after minimal contact with the oil.

The degree of sensitivity to poison ivy, oak, or sumac is affected by:¹

• Physical activity. The urushiol may spread during activity.
• How much contact you have had with the plant.
• Age. Sensitivity may change with age. Young children are less likely to have sensitivity to urushiol; sensitivity peaks between ages 8 to 15. Some people, however, do not show a sensitivity to urushiol until middle age.
• How well your immune system functions.