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Prevention and Treatment of Rhus Dermatitis and Other Weed Allergies

For gardeners, landscapers, and outdoor athletes such as golfers, spring is their time to shine. Unfortunately, the joy of these pursuits is often dimmed by encounters with plants such as poison ivy and wild parsnip. 

Every year, approximately 50 million Americans are treated for painful, weeping rashes caused by allergies to Toxicodendron species such as poison ivy, poison oak, and poison sumac. This so-called “Rhus dermatitis(Rhus, Greek for “sumac,”  was once the  botanical name for the 

Toxidodendron genus) is the most commonly-diagnosed allergy in the United States. Anywhere from 50-85% of Americans have a sensitivity to Toxicodendron. 

Toxicodendron species are not the only plants to cause rashes and irritation.Wild parsnip (and its invasive Apiaceae cousin, giant hogweed), ragweed, stinging nettle, and wood nettles are all capable of producing rashes and other painful symptoms when people unwittingly come into contact with their oils and prickly fibers. 

Anyone who spends time outdoors can experience reactions to these common weeds–even those with no prior history of sensitivity. Fortunately, there are things you can do to prevent exposure to the irritating compounds in these plants. When exposure does occur, there are things you can do to minimize the discomfort and speed up healing. 

Urushiol: What It Is, Why It Hurts, and Weed Allergy

Urushiol is a fat-soluble compound found in the leaves, stems, and vines of Toxicodendron species. It is produced when leaves or other plant matter are bruised–for example, when we brush against it or step on it during a hike. Pulling a vine from a tree or fence is the most common story in the dermatology clinic. 

Once contact is made, urushiol penetrates the outer layer of skin or stratum corneum. The full effects are not noticed immediately in most cases, but an immune response begins. Langerhans cells break down the urushiol and “present” the molecules to T cells in the lymph system. This in turn sounds the alarm for several bursts of cytokines and chemokines.

Over several hours or even days, the body mounts and reinforces immune responses against the stubborn urushiol molecules. The result is inflammation: redness, itching, and fluid-filled papules or vesicles form in lines over skin that might have only been itchy and red before.

This is what is known as a Type IV hypersensitivity, a reaction mediated by immune system cells–in this case, the T cells. It is a “delayed” reaction that escalates as more cells are recruited to expel the urushiol. Itching and rash forty-eight hours after exposure is the classic story. 


 depicting poison ivy treated by Rhus Dermatitis



Other Plant-Based Irritants

Other plants can also cause irritation or severe reactions, although the irritation is caused by different mechanisms. 

Stinging Nettle is such a well-known skin irritant that its Latin name, urtica, is the root word for the medical term for hives–urticaria. When someone unwittingly grasps or brushes against a nettle plant, the downy-looking trichomes or “hairs” on the leaves and stems inject irritating substances such as formic acid (also found in ant venom), serotonin, acetylcholine, and histamine. These substances all cause inflammation in the skin

Wild parsnip, a less “civilized” cousin of carrots and celery, is an invasive weed that was brought to North America by the first European settlers. It and its close kinsman, giant hogweed, are both umbelliferous plants from the Apiaceae family, and they both cause a skin reaction called phytophotodermatitis–a long word that indicates a rash caused by plant-induced light sensitivity. Although they are not native to North America, they’ve spread so successfully here that they have become nuisances. 

These plants are identifiable by lacy clusters of flowers called umbels. Wild parsnip has golden yellow umbels that usually do not exceed six inches in diameter. Giant hogweed is a tall, shrub-like plant with large, white umbels that are often larger than 24 inches in diameter.

The sap of these plants contains chemicals called furanocoumarins that make the skin more sensitive to damage from UV radiation. This is not an allergic reaction–but in its early stages, phytophotodermatitis symptoms can be mistaken for rhus dermatitis or other contact allergies. It is more like a localized sunburn or chemical burn; there is no immune response such as that seen with rhus dermatitis. 

A person may not be immediately aware that they’ve come into contact with the sap. However, if the sap is not washed away before the skin is exposed to UV rays, then red blisters can form on the skin that it touches. These burns can last for days or weeks; hypopigmentation or hyperpigmentation of the affected skin can persist for months. On rare occasions, the skin can scar. Quick treatment reduces the likelihood of abnormal pigmentation or scarring.

Anyone who spends a lot of time outdoors is at risk of coming into contact with these plants. Hikers, gardeners, landscapers, farm workers, mushroom hunters, and roadside maintenance workers are especially at risk. 

Protecting Yourself From Plant-Induced Dermatitis

You should familiarize yourself with the above-mentioned plants and avoid them where possible. While exposure can occur without your awareness, being aware of the plants in your vicinity will help you avoid uncomfortable encounters with these weeds and their caustic oils. Wear long sleeves, long pants, socks, and shoes when you are hiking, golfing, or working outdoors. Gardeners should wear thick gloves when pulling weeds and wash up immediately with soap and water. .

Never burn Toxicodendron species; the smoke can be toxic to inhale and can even cause dermatological symptoms. Every exposure increases your likelihood of a reaction, even if you’ve never reacted to these compounds previously. 

Some advocate homeopathic treatment with rhus tox tablets, which contain small amounts of urushiol, as a preventative measure if you anticipate spending time in places you might be exposed to urushiol. The idea is that this low level of exposure desensitizes the immune system to urushiol. This claim is not without controversy; some studies suggest the treatment has merit while others say it increases the likelihood of reactions. This is a matter best taken up with your dermatologist and allergist. 

Soaps and detergents destroy the toxic properties of Toxicodendron oils.If you are exposed to the toxic compounds in these plants, wash the skin immediately with a strong detergent like dish soap, scrubbing vigorously and rinsing thoroughly. Immediately launder all clothing worn for hikes, gardening, farm work, or golfing. Be aware, also, that outdoor pets can carry Toxicodendron oils on their fur without reacting to it (remember that humans are the only animals sensitive to urushiol). If you hike with your dog, for example, they should get a vigorous scrub-down with appropriate detergents on a sponge, just as you should. Petting an animal with Toxicodendron sap on its fur puts you at risk. 

Treatment For Weed Allergies

These plants are too varied and too prolific to avoid completely if you spend time outside. Despite our best efforts, reactions can occur. Fortunately, there are treatments available to comfort inflamed skin–and while the reactions caused by these plants have varying mechanisms, the treatments are all about the same. When precautions don’t work 100%, there is SmartLotionⓇ.

  • For exposure to Toxicodendron species: Thorough washing as described above is your first order of business after known or suspected contact with these plants. Rubbing alcohol works well; washing in cool or lukewarm water with dishwashing or hypoallergenic laundry detergent can help get rid of oils on the surface of the skin. Any clothes, pets, or tools that have come into contact with Toxicodendron species should be washed with detergent, also; urushiol sometimes takes years to evaporate, and you could re-expose yourself by handling items that are not adequately cleaned. 

Itching and redness can be alleviated by taking oral antihistamines and applying cool compresses and oat washes. A topical corticosteroid eczema cream such as SmartLotionⓇ is also useful. Remember that symptoms can take hours or even days to appear, so monitor carefully. 

Some people erroneously believe that urushiol can be spread to other parts of the skin by fluids in blisters from poison oak or poison ivy. This is not true. However, urushiol is notoriously hard to clean off, and if any trace of the oil remains, it can easily be transferred to previously unaffected skin. This is one reason that baths are not advised after exposure; showers are more effective at getting the oils off of the skin quickly. 

On occasion, exposure to higher-than-average levels of urushiol can cause a phenomenon called “black spot poison ivy,” with linear scatterings of black dots interspersed among the lesions. This is caused by high levels of urushiol oxidizing with exposure to air. Do not try to remove these lesions; if you see black spots, or if your rash is particularly debilitating, visit with your doctor. Blisters are the most common indication for treatment with oral steroids.

Suspect poison ivy or exposure to related plants if you notice redness, itching, and linear arrangements of vesicles over skin within 48 hours of outdoor activity (or contact with laundry or pets that have been exposed to the plants). See a doctor for treatment. Rashes typically resolve within two to three weeks. Working out and sweating prolongs weed dermatitis. Workouts must be curtailed until the symptoms are resolved.

  • Exposure to stinging nettles also demands immediate washing of the area. Cool compresses, topical antihistamine cream, and topical steroid creams such SmartLotionⓇ help calm the redness and itching. These irritations last for a few hours to a few days.

  • Exposure to wild parsnip, giant hogweed, or related plants demands immediate removal from sunlight or other sources of UV radiation. That should be followed by thorough washing with hypoallergenic detergents. Laying a cool, damp cloth over the affected skin is beneficial. Under a doctor’s guidance, SmartLotionⓇ can safely be applied for itching and redness.

  • Board-certified dermatologist Dr. Steve Harlan developed SmartLotionⓇ to safely treat his patients with chronic, recurring bouts of dermatitis–including those whose hobbies and professions put them at risk for repeated brushes with plant-based dermatitis. 

    SmartLotionⓇ contains prebiotics that help the skin maintain a healthy microbiome as it heals, which allows a small amount of hydrocortisone to work quickly and safely without the risk of skin atrophy or topical steroid withdrawal. Under a physician’s care, most patients can augment their prescribed treatments with SmartLotionⓇ.  

    For his patients with rhus dermatitis or other weed-related itching, Dr. Harlan recommends applying SmartLotionⓇ up to four times daily until the rash is gone–much as one would treat urticaria (hives). Stinging can occur with the first few applications. Spritzing the skin with cool water before applying SmartLotionⓇ  and applying ice packs for up to two minutes afterwards is helpful in these instances. If the weed dermatitis has not begun to clear within two weeks, see your dermatology provider for evaluation. 

    Even with the risk of weed-related dermatitis, time spent in nature is never time wasted. With a watchful eye, a proactive prevention plan, and safe options for treatment, these brushes with the plant kingdom’s feistier subjects need not prevent you from enjoying a day on the hiking trails or golf course. 

    Facts About Poison Ivy and Its Relatives:

    • European colonists were initially charmed by the abundant populations of poison ivy that twined through the Eastern hardwood forests of the Northeast American coast; Colonial Governor John Smith compared it fondly to the English ivy that grew over the manor houses in England.

    • Poison Ivy was a popular specimen plant in the gardens of European monarchs and the growing merchant class for a few hundred years because of its brilliant foliage; it was intentionally grown in the gardens of English aristocrats and even at Versailles in France. Marie Antoinette was said to be a fan of the plant.

    • Thomas Jefferson so admired the plant that he intentionally grew it at his Virginia estate, Monticello, as an ornamental plant.

    • Humans are the only animals who react to urushiol; poison ivy is beloved by deer for its leaves and by birds for its berries, and it is considered an important part of the ecology of Eastern hardwood forests. 

    • Japanese lacquerware, often seen in glossy black or red, is called urushi; it is made by treating the sap of the lacquer (urushi) tree at high heat until the toxic compounds break down. When it hardens, it is like a hard plastic, and it has no toxic qualities. Urushi boxes and decor are expensive and highly sought-after by collectors.

    • Rhus dermatitis gets its name from the old botanical name for the Toxicodendron genus, rhus, which came from the Greek word for sumac. The red sumac used in Mediterranean cooking is related to Northern American poison sumac, but it is not the same plant and it is safe for consumption. Poison sumac has white berries.


    We hope you enjoyed our article on weed allergy and Rhus Dermatitis. You can find more articles on our blog, or head over to our knowledge center to find out how SmartlotionⓇ  can help manage flare-ups.


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    - Zula Elwood 

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