Every year, poison ivy, oak and sumac send about 43,000 Americans to the emergency room. The plant isn’t the real problem, though. Urushiol oil is. Urushiol is the allergen in all three plants, and it is the most common cause of allergic contact dermatitis in North America, affecting an estimated 10 to 50 million Americans a year. If you understand how this oil works, you can stop most rashes before they start.

Why does urushiol matter so much at work?

Plants can irritate skin in several ways, from thorns and sap to reactions triggered by sunlight. But allergic contact dermatitis from the Toxicodendron family, which includes poison ivy, oak and sumac, is the most common by far. As a result, it costs employers real money in medical bills and lost work time. Farm, forestry and other outdoor crews see it the most.

What is urushiol, exactly?

Urushiol is an oil made in the sap of Toxicodendron plants. It is clear to slightly yellow, and it turns black as it reacts with air. You will find it in the stems, roots, leaves and fruit skins of the plant.

Chemically, urushiol is a blend of related compounds. Each one has a ring-shaped catechol group with a 15- or 17-carbon chain attached. The mix varies from plant to plant. In general, the more unsaturated that carbon chain is, the stronger the reaction.

The oil stays in the sap until the plant is damaged, or until it leaks out naturally in late fall. Once it gets out, it lasts. Urushiol can stay active on clothing, tools and pet fur for months, even years, if no one washes it off. So wash yourself, your clothes and your gear after any contact.

How does urushiol cause a rash?

Urushiol molecules are too small to set off the immune system on their own. Instead, they bind to proteins in your skin. Your body then sees that new compound as foreign, and in people who are sensitive, it triggers a delayed allergic reaction called a type IV hypersensitivity reaction.

Here is how it plays out:

1. Langerhans cells in the skin pick up the urushiol compound.
2. They carry it to immune cells called T lymphocytes.
3. The T cells travel back to the contact site and release chemicals that cause redness, swelling and itch.
4. Meanwhile, keratinocytes, the cells in your outer skin layer, release their own signals that make the reaction stronger.

Researchers have also found that a molecule called IL-33 plays a big role in the itch. It acts on nerve receptors that send the itch signal to the brain. That is one reason antihistamines often don’t do much for poison ivy itch. This is still an active area of research, though, so expect the science to keep moving.

Because urushiol is so similar across all three plants, if you react to one, you will probably react to the others.

Why do some people react worse than others?

Sensitivity depends on genetics, age and past exposure. Between 50 and 75 percent of U.S. adults react to poison ivy, oak and sumac. In big cities, where the plants are less common, the number drops closer to 20 percent. About 10 to 15 percent of people are tolerant.

Exposure is common for forest service workers and other outdoor crews, and hands, arms and legs take the most contact. In addition, how much oil touches the skin, and for how long, changes how bad the rash gets.

What does the rash look like, and how long does it last?

The rash usually appears 24 to 48 hours after contact, though it can show up anywhere from 5 hours to 15 days later. Look for red bumps and blisters in sharp lines or streaks where the plant touched the skin. Without treatment, it can last about 3 weeks, and up to 6 weeks in very sensitive people.

Crews fighting wildfires face another risk: burning plants can put urushiol into the smoke, which can cause a widespread rash and irritate the airways. Long-term problems are rare, but skin discoloration and secondary infections can happen.

How is a poison ivy rash treated?

Treatment focuses on easing the itch. Common options include baking soda or colloidal oatmeal baths, cold compresses and calamine lotion. For stronger reactions, doctors often prescribe topical corticosteroids, and severe cases may need oral or injected steroids. Some newer topical treatments are under study. Always follow a doctor’s advice for a bad reaction.

How do you prevent it in the first place?

Prevention beats treatment every time. Here is what works:

• Know the plants. Learn what poison ivy, oak and sumac look like and where they grow.
• Cover up. Wear gloves and protective clothing around known patches.
• Use a barrier before exposure. I.C. Ivy Block uses zinc acetate, an OTC skin protectant, plus dimethicone and bentonite to help form a barrier on the skin.
• Wash fast. Washing within about 10 minutes of contact can remove up to half of the urushiol. I.C. Ivy Cleanse is made for this job, with micro scrubbers and surfactants that help lift urushiol from skin.
• Clean your gear. Wash clothing, gloves and tools so the oil doesn’t get you twice.

For more practical tips, read our post on poison ivy myths, or book a lunch and learn for your crew.

The bottom line

Urushiol is small, stubborn and everywhere your crew works. But it follows simple rules. Block it before you go out, wash it off fast, and clean everything it touched. Do that, and most of your team will never need the ER visit.