The Remarkale Chemistry of Poison Ivy

I’ve always thought that one of the characteristics of an effective blog is the author’s ability to take an everyday experience that happens to him or her and use it to further the overall goal of the blog. The overall goal of this blog is to make me really observe, understand and dial in to the annual process of how the living world wakes up, a good part of which is understanding what goes on in plants all around us, and how those processes work. The good news today is that one of those great, to-the-point experiences has happened to me; the bad news is that it really, really itches.

Tangent: Actually, a pretty reasonable characteristic of an effective blog would be if anyone ever read it, but we’ll just leave that one aside for now…

One of the goals of the Twin Corral Box Canyon trip was to explore all of the side canyons we missed in 2001, and in this we were largely successful. The last one we explored on Monday was one we’d bypassed on the first trip: a narrow canyon blocked by a thick mini-jungle of vegetation. This time we bush-whacked our way up it, getting scratched and whacked by willows, Gambel Oak and brush of various types. The going was slow, and Steve and I split up, each trying different routes up and out of the jungle. From about 50 feet ahead and to the left of me, Steve called out, “Hey, there’s Poison Ivy in here.”

Now one of my weaknesses as an outdoorsman is that I have never learned to accurately identify Poison Ivy. So I called back, “What’s it look like?” After a moment Steve called back, “Uh green… and leafy…” Thanks, Steve.

Steve found a good path up and out of the jungle, and onto some slickrock where we found decent hand and footholds that brought us up to a slickrock saddle separating the 2 main forks of the side canyon. We poked around for several minutes, agreeing that we should wash our legs when we got back down at the first decent pool of water we found, in case we’d brushed up against the Poison Ivy. We returned the way we came, then continued hiking down-canyon another 10 minutes or so before finding a pool and washing our legs, probably 30-40 minutes after first encountering the Poison Ivy.

Thursday morning I woke up with a violently itchy rash across the back of my left upper leg, and a spot on my left knee, and since then, I’ve committed myself to understanding how Poison Ivy does its thing.

Poison Ivy, Toxicodendron radicans, grows in every US state except Alaska, Hawaii and California. California of course has plenty of the closely-related, and similarly functioning Toxicodendron diversilobum, or Poison Oak as it is commonly known. Both plants are misnamed. They are neither Ivy nor Oak, but actually woody vines that grow either as climbing vines or low ground-shrubs.

In the East, Poison Ivy is common in all sorts of wooded or partially-wooded areas, and does really well in “disturbed” or cleared locations. In fact, one of the interesting things about Poison Ivy in the East is that its frequency is apparently much greater today than it was prior to European settlement, due to all the subsequent “disturbance” of forest and woodland.

But in the West, the only time I seem to come across Poison Ivy is in canyon bottoms. In fact certain canyons, like the Black Canyon of the Gunnison in Colorado or Young’s Canyon off the Dark Canyon in Southern Utah, are notorious Poison Ivy “hot spots”.

The sap of Poison Ivy, Poison Oak and other Toxicodendron species contains stuff called urushiol. Urushiol isn’t a specific chemical, but rather a class of chemicals, more specifically known as catechols, each of which is characterized by a Benzene ring with a long “tail” of additional Carbon and Hydrogen atoms.

Quick Chemistry Review: Benzene is a common organic chemical compound used in lots of plastics. (It used to be an additive in gasoline until it’s carcinogenic properties became clear.) A single Benzene molecule has 6 Carbon and 6 Hydrogen atoms and forms a hexagonal ring. A “tail” is a series of additional atoms in a linear structure attached to the main hexagonal ring.

The primary catechol molecule in Poison Ivy is Pentadecyclcatechol. The primary catechol molecule in Poison Oak is Heptadecyclcatechol. The main difference between the two: Penta-blah-blah has 15 Carbon atoms in its tail; Hepta-whatitz has 17.

Side note: It’s actually a lot more complicated than this; there are 4 different variants in each of these catechols, and the mixture of each type in a given Toxicodendron species or subspecies affects the effect of that specific plant’s urushiol…

When the sap/urushiol of Poison Ivy is exposed to the air, either through crushed leaves/stems, insect bites/damage or whatever, the Pentadecyclcatechol molecules oxidize (which means that they’re chemical structure changes in a way that involves the loss of one more electrons) and in this oxidized state they’re highly reactive.

When oxidized Pentadecyclcatechol molecules contact skin, they rapidly penetrate the epidermis and work their way into the dermis. (The dermis is the living, pinkish under-layer of skin you see when you skin your knee, and which oozes blood when exposed until it scabs over.) Before they do this, they can be washed off the skin, but only with the help of soap; urushiols are hydrophobic, meaning they repel water.

Once in the dermis, the oxidized Pentadecyclcatechol molecules chemically bond with proteins in the cell membranes, after which they are impossible to wash off. The entire process takes around 15 minutes.

The altered proteins are seen as foreign bodies by the body’s immune system, which is what triggers the itching, blistering, etc.- it’s an autoimmune reaction. The reaction really gets going when the Pentadecyclcatechol binds to membranes of Dendritic T-Cells, which are basically white blood cells with a whole lot of “arms” sticking out. These T-Cells play an important role in triggering autoimmune reactions, and the catechol-affected T-Cells set off the full alarm as it were, kicking the full-blown autoimmune response into high gear.

Side note: AIDS patients, with their impaired immune systems, often suffer from low T-Cell counts. An odd-but-logical side affect of a low T-Cell count is that AIDS patients usually suffer a milder or no reaction to contact with Poison Ivy or Poison Oak.

By the time Steve and I reached the pool and washed our legs, we were probably 15-30 minutes too late; by that time the Pentadecyclcatechol molecules had penetrated into the dermis of our legs and begun binding to cell membranes. Even if we’d reached a pool sooner, it’s unlikely our “washing” (glorified rinsing, really) would have had much effect; without soap we were probably just pushing the urushiols around a bit. Lesson learned? On my next canyon trip, day-hike with a watch and a bit of camp soap in my daypack.

It’s hard to see the Beauty of the World when it itches so damn bad. I’ve swiped some of my younger son’s steroid-based anti-eczema cream, and that seems to be helping a bit, but I’ll be glad when this little home-chemistry class has run its course.

It’s been rainy and cold in Northern Utah ever since my return. But it’s finally starting to dry out and warm up, and the Gambel Oak is exploding into life.