Oct 11, 2018
Science Thursday: Poison Oak, Poison Smoke?
Type: Research Synthesis
I came out of last week looking like someone who’d just been released from a hard labor camp. My hands were covered in blisters from digging soil pits for a research project, and my arms were covered in bruises from hauling t-posts and hog panels to those same research sites. (So many bruises, in fact, that I’ve been too embarrassed to wear short sleeves for the last five days.) To add insult to injury, I had “White bites” from my fire boots on the fronts of both ankles, and I was dehydrated and sore from two full days of burning and hiking across hundreds of acres in Humboldt County’s steep coastal rangelands. In short, I felt amazing. There’s no better feeling than being truly tired and sated — body and soul — after a week of inspiring work.
Saturday morning brought an added (and less welcome) complication. I brushed a hand across my arm and felt a familiar pattern before I saw it: an invisible line of small bumps that eagerly jumped at my touch, excited to be itched. Poison oak.
I had been musing over poison oak only a few weeks prior, as my co-worker and I were putting the final touches on our burn units. Both units are in coastal rangelands encroached by coyote brush, poison oak and Himalayan blackberry, and after many decades without fire, those three shrubs had formed an impenetrable thicket, dense and unpleasant enough that ranchers use it like a cattle fence. The poison oak was roughly 15 feet tall — thick, ropy vines whose oily leaves were just showing the first tints of fall. Poison oak, with its electric pinks and oranges, happens to be one of my favorite fall plants, but I appreciate it like I would a mountain lion or a rattlesnake: with great respect for its power and beauty, but no desire to be more intimate.
Recall that poison oak, Toxicodendron diversilobum, is in the same genus as poison ivy (T. radicans) and poison sumac (T. vernix), and all of them are in the larger plant family Anacardiaceae, along with mangoes, cashews, ginkgo and many other species. Plants in this family contain varying amounts of urushiol, a highly allergenic compound to which only humans and a few other primates are sensitive (Gladman 2006). Contact with urushiol can cause contact dermatitis, which at its best looks something like what I have (small itchy bumps) and at its worst can result in painful, weeping sores and even infection (Lee and Arriola, 1999, PDF, 698KB). A majority of people in the United States are clinically sensitive to urushiol, but many people, especially in urban areas, never have the opportunity to realize their sensitivity.
In the fire field, the effects of Toxicodendron dermatitis are very real. Research shows that urushiol-caused dermatitis is responsible for 10 percent of USDA Forest Service lost-time injuries, and treatment has, in some years, absorbed 1 percent of California’s workers’ compensation budget (Gladman 2006).
When I was younger, I thought that I was in the 10–15 percent of people who are genetically tolerant of urushiol. I grew up in an area where it abounded, and the longer I went without getting the rash, the more brazen I became. I’d walk through it, move it out of the way for people, and do other things that, in hindsight, seem a bit unwise. At 22, I clumsily moved a cut vine, and it swung around and broke the skin on my back. Turns out, I’m not in the top 10 percent when it comes to poison oak! And now I forewarn other emboldened people: literature shows that for some people (like me), sensitivity only shows itself after many repeated exposures — sometimes after years or decades. Also, children are more tolerant than adults, so not getting it as a kid doesn’t mean you’re in the clear for adulthood.
Like many parts of rural life, poison oak has an almost mythical quality. People have strong beliefs about how it spreads, how to prevent it, and how it affects them personally. Some people swear that by itching their rash, it can spread it to other body parts, or to other people. Others brag that they are not susceptible at all, presumably because they are genetically superior, or perhaps because they have built up resistance by eating poison oak buds in the early spring or otherwise inoculating themselves. And other people avoid it at all costs, insisting that even looking at poison oak will cause them to break out. (Note: these claims have mostly been proven wrong, with the exception of poison oak inoculation, which remains unproven and contentious.)
Last week before we burned, I received a call from a woman who’d read my prescribed fire press release. She was inquiring about the likelihood of our burns producing urushiol-laden smoke that would affect her where she lives, nearly 15 miles away. I have to admit that made me chuckle, because my co-worker and I had just been saying that stories about poison oak smoke are like urban (or rural) myths: everyone has heard of someone else who’s been affected by poison oak smoke, but it’s hard to find firsthand accounts of smoke-induced dermatitis, especially ones that aren’t confounded by physical contact with the plant.
So when Sunday morning came and my thighs, back and arms were covered in a light but irritating poison oak rash, I had to wonder. I had been wearing boots, gloves, long pants and a long-sleeve shirt under my Nomex, so how did this rash come to be?
Sometimes in science, you find a point that is well referenced in the literature, but its citation trail seems to lead nowhere. That’s how my review of the effects of poison oak smoke has been. I can find any number of papers claiming that inhalation of poison oak (or ivy or sumac) smoke can cause systemic dermatitis, but the citations are sketchy. For example, a paper in the International Journal of Vaccines and Vaccination (PDF, 626KB) claims “a severe reaction may happen if a sensitive person inhales this smoke, or even death.” Wow! That’s certainly cause for alarm, until you check their reference and see that they’ve simply referenced the scientific name of the plant — no scientific literature whatsoever.
One of the most detailed descriptions of the effects of poison oak smoke is in a Pacific Northwest (PNW) Extension paper (PDF, 293KB), which states:
“Smoke from burning poison ivy and poison oak has poisoned people who were otherwise immune. Inhalation of such smoke causes lung poisoning that can require hospitalization and intensive care. The oil is not volatile at bonfire temperatures. Any transmission in smoke is by droplets on particles of dust and ash in the smoke, rather than from vapors.” (Burrill et al., 1994)
This publication provides no references. Another dead-end street.
Oddly enough, some of the only original research I found came out of India, where researchers looked at urushiol-caused dermatitis resulting from ceremonial traditions that involve burning the seeds of Semecarpus anacardium, another plant in the poison oak family (Bhatia et al., 2014, PDF, 626KB). In some parts of India, these seeds are used to purge the curse of the “evil eye,” a malevolent expression that can cause injury or misfortune to others. Those who were treated with smoke from the burning seeds consistently developed dermatitis on their hands, arms and faces — areas in close proximity to the smoke and to the burning seeds.
In writing this blog, I’m not claiming that smoke can’t cause a poison oak rash; in fact, I’m sure many of you have stories to share. However, I am noting that even the peer-reviewed literature on the topic is in some ways furthering the mythical status of the plant and its family. And for good reason! Urushiol — this ever-powerful and internationally feared oil — is mostly invisible. You never quite know if you were exposed, partly because you can’t see the oil, but also because the plant tissue has to be injured for the oils to be released, so lightly brushing vines or leaves can be OK (Gladman 2006). Likewise, if you are exposed, you can’t really tell if you were able to wash it off in time, or if it’s still on your boots and clothes (urushiol can persist indefinitely in a dry state!). And its smoke is even more elusive. In my case, did I breathe poison oak smoke and have a systemic reaction, or was it on my clothes when I undressed that evening? Or maybe it was all over Millie, the cute cattle dog that I cuddled on the fireline? There’s no sure way to know, and in some ways, I like that. Poison oak keeps us in check, humbled. Even the cockiest person may be just one exposure away from cleansing their curse.
Burrill, L. C., Callihan, R. H. and Parker, R. (1994). Poison Oak and Poison Ivy. Pacific Northwest Extension.
Bhatia, K., Kataria, R., Singh, A., Safderi, Z. H. and Kumar, R. (2014). Allergic Contact Dermatitis by Semecarpus anacardium for Evil Eye: A Prospective Study from Central India. Indian Journal of Basic and Applied Medical Research, 3, 122-127.
Derraik, J. G. (2007). Heracleum mantegazzianum and Toxicodendron succedaneum: Plants of Human Health Significance in New Zealand and the National Pest Plant Accord. The New Zealand Medical Journal (Online), 120(1259).
Gladman, A. C. (2006). Toxicodendron Dermatitis: Poison Ivy, Oak and Sumac. Wilderness and Environmental Medicine, 17(2), 120-128.
Lee, N. P. and Arriola, E. R. (1999). Poison Ivy, Oak and Sumac Dermatitis. Western Journal of Medicine, 171(5-6), 354.
Pekovic, D. D. (2016). Vaccine Against Poison Ivy Induced Contact Dermatitis, A Lingering Scientific Challenge. International Journal of Vaccines and Vaccination, 2(1), 00023.
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