What Do Cashews, Mangos, and Poison Ivy Have in Common?

It itches, it oozes, it drives you crazy! Here are all the surprising ways you can end up getting a poison-ivy-like reaction and how to avoid them.

Sabrina Stierwalt, PhD
5-minute read
Episode #340

Avid listeners of this podcast may recall my past revelations that I have a super power. When I encounter poison ivy or poison oak, even in the tiniest amount, my body’s immune system explodes in a systemic reaction, creating a situation that requires fast treatment. It’s itchy, uncomfortable, and I’ve spent many sleepless weeks on steroids trying to keep the rash at bay. 

Well, stop rolling around in poison ivy, you might be thinking. I wish! Unfortunately, for me, it’s usually my dog that frolicks in the plant. Dogs do not react to the plant’s oils, so she carries on none the wiser but brings the oils to me. 

In one instance, however, I broke out in a poison ivy-type rash after visiting a friend in downtown Los Angeles and my dog was nowhere in sight. This was a bit much, even for me. I think I saw one tree the entire time, and it certainly wasn’t hosting a nefarious vine. So, how was that possible? It was then that I learned—the hard way—that poison ivy is just one member of a family of trees, the anacardiaceae family, that can carry an oily irritant that causes a rash.

You see, my friend had cooked for me a popular meal from her homeland of Trinidad, which was full of delicious, fresh mango. Mango, it turns out, is also a member of the anacardiaceae family.

What is the difference between poison ivy, poison sumac, and poison oak?

The anacardiaceae family of trees, sometimes called the cashew family, includes mango, poison ivy, poison oak, sumac, Peruvian pepper, pistachio, and you guessed it, cashews. They all contain urushiol, the sneaky oil that causes the skin to erupt in rashes for some people and is the bane of my existence, but to varying degrees. The oil is found in all parts of the plant, including the leaves, stems, and even the roots. 

The anacardiaceae family of trees, sometimes called the cashew family, includes mango, poison ivy, poison oak, sumac, Peruvian pepper, pistachio, and you guessed it, cashews.

Urushiol is the reason cashews are never sold in the shell and are typically roasted. The oil is found on the outer shell (similar to mangos, where it’s found on the skin) and roasting the cashews at high temperatures can kill any remaining oil that makes its way through to the nut. The Center for Disease Control published a report in 1982 about a batch of  >7500 bags of shell-contaminated cashews being sold in Pennsylvania and Maryland, mostly as part of a Little League fundraiser. Around 20 percent of the unlucky cashew-eaters developed a rash. 

Also in the family is Toxicodendron vernicifluum, or the Japanese lacquer tree. This tree produces the sap used in painting those beautiful lacquered boxes. There have been cases of rashes developing in reaction to contact with lacquerware because the oil remains in the paint. 

How does poison ivy work?

According to the American Academy of Dermatology, around 85 percent of people develop a rash when they get urushiol on their skin either through direct contact, indirect contact, or via the air. The oil is hardy so it can travel by way of, say, a soccer ball, a gardening spade, or even your pet.

Your skin starts to absorb the oil immediately, but typically the rash doesn’t appear until 12 to 72 hours after coming in contact with the oil. In the best cases, the rash is typically very itchy and uncomfortable but will go away on its own in 1 to 3 weeks. For those with my super power, steroids may be required to quiet the body’s immune system reaction. 

Your skin starts to absorb the oil immediately, but typically the rash doesn’t appear until 12 to 72 hours after coming in contact with the oil.

The National Institute of Occupational Safety and Health in the US (NIOSH) has a great video illustrating how contact with poison ivy inspires a response from our immune system to produce a rash. The video uses cartoons, presumably in an attempt to make the whole ordeal cuter instead of showing the awful, itchy, oozy mess that it is. 

When the urushiol gets on the skin, chemicals from the oil, called haptens, encounter skin proteins, which then latch onto them. Special protective cells in our skin, called Langerhans cells, then grab the antigen pair and send a message to the immune system that an invader is present. The immune system responds to the call for help by sending helper T-cells to investigate the problem. For those who are not allergic to the oil, those T-cells declare a false alarm and don’t elicit a further response. 

If you are allergic, however, the body responds by producing a barrage of cells to fight the invasion, including cytokine and chemokine proteins (these are the ones that make you itchy and swollen), macrophages (a type of white blood cell meant to digest foreign substances), t-lymphocytes (infection fighters), and more helper T-cells. All of these cells work to kill off the cells that have latched onto the chemicals from the oil, but they also kill off some healthy cells. In the process, they also make the skin blistery and itchy. 

Yes, that’s right—the more times you get an urushiol rash, the worse your reaction gets.

The extent of your reaction depends on whether you are allergic, the extent of your allergy, how much of the oil you’ve come into contact with, and how many times you’ve had a reaction to the oil in the past. Yes, that’s right—the more times you get an urushiol rash, the worse your reaction gets. I told you—a super power. 

How to avoid poison ivy

  1. Avoid the plant altogether as best you can. The American Academy of Dermatology provides a map of where you can find members of the anacardiaceae family (and other rash-inducing plants) throughout the United States. The Center for Disease Control also provides maps of the geographic distribution of poison ivy, poison oak, and poison sumac throughout the United States and Canada. With climate change and rising temperatures, however, the areas where poison ivy thrives are expected to expand.
  2. Poison ivy can be particularly challenging to avoid since it comes in varying shapes and colors. A handy memory trick that I was taught as a child, and that describes most poison ivy plants in the eastern US, is to look for a woody vine and to remember that “with leaves of three, let it be.” Learn how to identify the plant with this helpful guide from the Center for Disease Control.
  3. If you know you’ve made contact with the plant, wash your skin immediately using soaps made for degreasing like dishwashing detergent. This may not be able to stop the rash from popping up but may contain the rash by not spreading the oil to other parts of your skin.
  4. Also wash your clothing or any surface that may have also encountered the plant, like your shoes or your pet.
  5. To prevent infection, try not to scratch the rash—easier said than done, I know.
  6. If the rash is minor, you can try soothing the itch with calamine lotion, oatmeal baths, or hydrocortisone cream. Although no scientific studies have proven its use, strong anecdotal evidence suggests breast milk is an effective cure for even the toughest rashes. If you experience swelling or trouble breathing or swallowing, see a doctor right away.
Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.