Poison oak is a deciduous (loses leaves in winter), woody plant that can have a shrub or vine form. In open areas under full sunlight, poison oak forms a dense leafy shrub usually 1 to 6 feet in height. In shaded areas, such as in coastal redwoods and oak woodlands, it becomes a much taller climbing vine, supporting itself on other vegetation or upright objects by means of aerial roots.
Leaves normally consist of three leaflets with the stalk of the central leaflet being longer than those of the other two; however, occasionally leaves are composted of five, sever or nine leaflets. Leaves of true oaks, which are superficially similar, grow single, not in groups. Poison oak leaves are alternate on the stem. Each leaflet is 1 to 4 inches long and smooth with toothed or somewhat lobed edges. The diversity in leaf size and shape accounts for the Latin term diversilobum in the species name. The surface of the leaves can be glossy or dull, sometimes even somewhat hairy, especially on the lower surface. In spring, poison oak produces small, white-green flowers at the point where leaves attach to the stem. Whitish-green, round fruit form in late summer. In early spring the young leaves are green or sometimes light red. In late spring and summer the foliage is glossy green, and later turns attractive shades of orange and red.
IMPACTAlthough a native, poison oak can be found in great abundance where established vegetation is disturbed, particularly along roadsides, in uncultivated fields, and on abandoned land. It is also a problem in wood lots, Christmas tree plantations, rangeland, and recreation areas. While it can reduce optimal grazing area in rangeland or pastures, the primary concern associated with poison oak is the allergic reaction it causes in many people.
All members of the genus Toxicodendron, including poison oak, poison ivy, and poison sumac, cause allergic contact dermatitis. About two million cases of skin poisoning are reported in the United States each year, primarily caused by these three species. In California, the number of working hours lost as a result of dermatitis caused by poison oak make it the most hazardous plant in the state.
Contact with poison oak leaves or stems at any time of the year can cause an allergic response. When the allergen contacts the skin surface in sensitive individuals, it is rapidly absorbed into the surrounding cells. Within 1 to 6 days, skin irritation and itching will be followed by water blisters, which can exude serum. Contrary to popular belief, the exuded serum does not contain the allergen and cannot be transmitted to other regions of the body or other individuals. The dermatitis rarely lasts more than 10 days.
Although 50% of the population is clinically sensitive to poison oak and poison ivy, about 75 to 85% can potentially develop allergy if exposed to a sufficiently high concentration of the toxin. Once a reaction to the toxic has occurred, the body responds with a cell-mediated immunity, which is a delayed hypersensitivity. Those individuals who have developed delayed hypersensitivity are sensitive to the toxin and repeated exposures further increase sensitivity. Conversely, long period with no exposure will reduce an individual’s susceptibility to the allergen. There is no known difference in sensitivity to poison oak among races or between sexes. Animals do not usually suffer skin irritation from contact with poison oak because they are protected by fur; dogs, however, can contact poison oak on their nose or underbelly. On the other hand, livestock may graze on the tender foliage.
In addition to direct contact with the plant, transmission of the allergen can occur from a number of other sources including smoke particles, contact with objects such as clothing, gloves, and tools, or contact with animals, particularly pets. When poison oak is burned, the oils can be transported on the smoke particles. Breathing this smoke can cause severe respiratory irritation.
After coming in contact with the allergen, the best way to prevent skin irritation is to pour a mild solvent, such as isopropyl alcohol (rubbing alcohol), over the exposed area and then follow this with plenty of cold water (warm water enhances penetration of the oil) within a few minutes of exposure. If isopropyl alcohol is not available, just wash with lots of cold water. But you need to wash within 5 minutes of exposure to prevent a rash. Even if it is too late to prevent the rash, washing the skin to remove excess plant oil will keep the rash from spreading. The poison oak toxin is an oil, so it does not dissolve in water. Sufficient quantities of water, however, will dilute the oil to the point where it is no longer harmful.
Using only a small amount of water or disposable hand wipes is more likely to spread the toxin than remove it. Soaps can be used to wash, but only if with copious amounts of water; otherwise, they too will spread the toxin. An important point to remember when washing with isopropyl alcohol or soaps following exposure to poison oak is that they will remove the skin’s protective oils. Therefore, wash with them only if you are done working outside for the day. If there is a possibility of re-exposure to poison oak within 6 hours, just wash with lots of water. Be sure to thoroughly wash the hands as they serve as the major route for transfer of the allergen to other parts of the body, especially the face.
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