A Staff Report from the Straight Dope Science Advisory Board

How come some people are more vulnerable to poison ivy than others?

December 8, 1999

Dear Straight Dope:

How come I can get poison ivy all over me merely by walking by it (or so it seems)? And some of my friends can literally roll around in it and nothing comes of it?   Is it true that every time you suffer through a case of poison ivy, it gets easier for you to get it again?

P.S. Will soaking in a bathtub with a diluted amount of bleach in it get rid of poison ivy quicker?

The culprit in poison ivy (Toxicodendron radicans) is its sap, called "urushiol" (you-ROO-shee-ol) oil which is found in the leaves, stems, roots and fruit of the plant. Poison oak and poison sumac are two others in the plant genus Toxicodendron, also called Rhus plants. The name "urushiol" comes from "urushi," the Japanese name for lacquer. The Japanese lacquer tree has a small amount of urushiol in its sap, and some very sensitive people even react to contact with lacquered furniture from Japan. Cashew and brazilnut trees are related and contain some urushiol oil, too, as do the rinds of mangos. It is such a poisonous antigen that one billionth of a gram can cause allergic contact dermatitis in a very sensitive person. One quarter cup could theoretically make everybody on earth itch.

About 85 percent of people will acquire an allergy to the sap in poison ivy. The other 15 percent are presumably resistant, but there's no guarantee one of your friends won't become sensitive at some point. I say "acquire" because--like other allergies--people aren't born with it but develop the allergy after exposure. Repeated exposures can make one more sensitive. It's uncommon for anyone under the age of six to get the itchy rash and blisters familiar to anyone who's suffered through a poison ivy/oak/sumac reaction. (The doctor says, "just don't scratch it" and you want to punch him in the mouth. And hope there's some urushiol on your fist.) Sensitivity can fade with age, and some adults who suffered as children are no longer allergic or have a milder reaction to poison ivy in later years.

Growing up in the woods near the coast of California, I had much intimate experience with poison oak. I had a donkey with a fondness for the stuff, and she'd selectively munch through an area, leaving all other foliage. People used to borrow her for this all the time. Once I made the mistake of riding her wearing shorts (I was in the shorts) after she'd been wallowing in poison oak, and yikes! You can imagine.

You have to have direct contact with urushiol oil to get the allergic reaction. "Walking by it" would not be good enough. Theoretically you could even brush against the leaves and not come in contact with the oil that's inside the plant, but it's a very fragile plant and bruising--which releases the oil--happens easily. Another indirect way you can be exposed is through contact with contaminated objects (clothing, shoes, tools, animals). Urushiol oil can be active on any surface for five years and possibly longer.

I have heard that urushiol can become airborne from a lawn mower, but I've never heard of anyone actually catching it that way. You can be dangerously exposed to the toxic oil of poison ivy through the smoke when the plants are being burned. This can affect the throat and lungs, and can even be fatal.

So don't  burn the stuff. I would also recommend that you not touch your silly friends who are rolling around in it. Avoid the plants if possible (for plant identification, see http://www.aad.org/aadpamphrewo rk/PoisonIvy.html ), or wear long sleeve shirts and long pants if you can't stay out of it. Be careful touching any exposed clothing later, and wash them as soon as possible in detergent. Don't let your pets run around in poison ivy and if they do, hose them down, too. Some Forest Service employees spray antiperspirant deodorant on exposed skin (not the face) ahead of time if they know they'll come in contact with poison ivy. The aluminum chlorohydrate may help prevent penetration of the oil through the skin. There are other over-the-counter products available specifically for this purpose, too.

If you come in contact with poison ivy, the key is to wash yourself down with a lot of cold water immediately after exposure. Within fifteen minutes, the urushiol binds with the skin proteins and you can't get it off. A garden hose or a creek will work. A lot of water will dilute the poison and wash it off your skin--some dermatologists say a little water is worse than none, because you'll just spread the oil around on your skin. Avoid using a washcloth which will do the same thing. Applying rubbing alcohol immediately after exposure works to remove the urushiol, too, but don't do that if you're likely to be exposed again that day, because it removes the protective oils in the skin which could allow a worse exposure to poison ivy later.

On the web I saw many folk remedies to supposedly prevent or relieve poison ivy rash, including your diluted bleach in a bathtub example (and one guy recommended white shoe polish). None of the medical/dermatology sites or dermatologists I consulted recommended soaking in diluted bleach, though. Anything stronger than the chlorine you'd be exposed to in a swimming pool could be pretty bad for your skin. And, of course, if you have a severe case of rash and discomfort, see a dermatologist who can prescribe drugs to lessen the severity of your symptoms.

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