Symptoms
The rash of poison ivy, poison oak, and poison sumac is characterized by red bumps that itch and blister Figure 01. The red bumps usually come on within 12 to 48 hours of exposure, and can affect any skin surface, although the soles of the feet and palms are usually spared. You may experience some swelling and tenderness around the affected area. Only the parts of your body that have touched the sap will break out. If you have brushed by one of these plants, the rash might appear streaky. The bumps may blister, crust, and scale after a few days. The rash usually clears up after a few weeks.
Figure 01. Poison ivy rash
Risk Factors
Poisonous plants can be found all over the country, but certain species predominate in particular geographic regions Figure 02. Poison ivy (Toxicodendron radicans) is almost ubiquitous in the US. The variety Rydberg’s poison ivy (Toxicodendron rydbergii) is most common in New England and certain parts of the mid-Atlantic and Rocky Mountain regions. As the name implies, eastern poison oak (Toxicodendron toxicarium) abounds in eastern states, but also dwells in parts of the midwest. Western poison oak (Toxicodendron diversilobum) is prevalent in western states, and poison sumac grows in the east and midwest.
Figure 02. Geographical distribution of poisonous plants in the US
Most rashes develop during warmer months. Spring and summer are prime times for poison ivy, poison oak, and poison sumac reactions. This is when the plants are loaded with sap and bruise easily. However, you can get a rash in the winter — particularly if you are burning wood that has urushiol on it.
Most people who come into contact with poison ivy, poison oak, or poison sumac will get the rash. However, sensitivity decreases over time. Not everyone who is exposed to a poisonous plant will have an allergic reaction. About 15% of the population has a resistance to them. And while sensitivity varies from person to person (some people are extremely sensitive and develop a terrible rash with extreme swelling), it generally diminishes with age. If you had poison ivy, poison oak, or poison sumac as a child, you may not get it as an adult.
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I'm Beth Isaac, PharmD. Welcome to PDR Health!
