Contact Dermatitis (CD) is a skin inflammation that occurs when substances touching your skin cause irritation or an allergic reaction.  Airborne substances (ragweed, insecticide sprays) can also cause CD.  The resulting red, itchy rash isn't contagious or life-threatening, but it can be very uncomfortable.   Successful treatment consists of identifying the culprit and avoiding it so that the rash can resolve in two to four weeks. 

Irritant CD is a substance that damages your skin's outer protective layer; often on hands, fingers, face (plants, soap, chemicals, cosmetics), resulting in non-allergic, inflammatory skin reactions or rash.  Not everyone will develop an allergy, but no one is safe from an irritant.  Allergic CD (ACD):   a substance to which you're sensitive (allergen) triggers an immune reaction in your skin (plants, natural rubber, metals, jewelry, perfume, cosmetics, hair dyes).  The first encounter sensitizes the allergic individual, resulting in the production of white blood cells that recognize the allergen.  Usually, the second and subsequent encounters with the allergen stimulate the allergic response.   

Photoallergic/Photosensitization CD is a substance that causes ACD only after you apply a substance (topical or ingested) and sunlight then strikes your skin (sunscreens, ointments with NSAIDs, oral drugs such as hydrochlorothiazide/water pill

Four days after pruning a Honeysuckle vine (Lonicera spp.) with no gloves, in a sleeveless shirt, I had a rash on elbows/inner arms that soon spread down the sides of torso, to lower back and butt.   Another four days later, my thighs and legs were rashy.  Elbows and thighs felt like alligator skin.   Thankfully, not on face or other sensitive areas. 

"Insane" itching with no relief.  I washed with Tecnu, Dr. Bonner, dish detergent (couldn't find oatmeal soap without perfume).  BUT soap irritates, so a simple, cool shower is better.  I applied cold compress, calamine, anti-itch gel, hydrocortisone cream, Triamcinolone cream (left over from a poison oak bout), aloe, tea tree oil, calendula gel, banana peel, cucumber.   Nothing helped for more than 30 minutes.   I took Antihistamines (diphenhydramine) and felt drowsy and tired.  A LOT of different antihistamines on the shelf, so I asked the pharmacist which one to use for itching.   I did not try onion, rubbing alcohol, white vinegar (with salt or baking soda), cooked oatmeal covered with gauze or saran wrap, blow drying or chlorine (swimming pool).   

The only inkling of relief was soaking in Epsom salts, 20-25 minutes, two to three times a day. (A woman at Bed, Bath & Beyond battling poison oak said it was the only thing that helped.  I concur).  Sometimes I included a handful of natural colloidal oatmeal powder or used it alone.   Sometimes I boiled water, dissolved colloidal oatmeal and sponged down.  Maybe the wash cloth helped spread the rash, but for a short period I was joyfully relieved during two weeks of hell.      

I finally contacted my doctor in Santa Rosa.  She indicated:  1) Triamcinolone was a good idea because Hydrocortisone (over-the-counter) is too "weak";   2) very unlikely to benefit from antibiotics since it's not painful and no pustules;  3) occasionally they might prescribe oral prednisone, especially if the rash is in a wide area, on genitals or face, but this has the disadvantage of causing anxiety/sleeplessness;  4) this is not something for the ER unless you develop shortness of breath or swelling of lips. 

She prescribed Triamcinolone.  By now, the rash was starting to abate.  The itching, though lighter, continued for 1-2 weeks (my skin was really dried out by now).  Soaking in the tub remained the most helpful of anything. 

This is a stark reminder to always wear gloves in the garden when handling plants or gardening materials.  Long sleeves and long pants is good practice, especially if you are allergy-prone or have a compromised immune system.   Remember, you can develop allergies later in life.  Remember, the rash can take 2-4 weeks to resolve.  If you see a rash developing and it alarms you, see your doctor.  This was the most convincing argument to save my money and get a Shingles shot--which my doctor has been recommending for four years.   Shingles means pustules and nerve pain.  This rash was agony enough. 

Sources:   (1)  California Master Gardener Handbook, for sale at http://anrcatalog.ucdavis.edu/GrowingVegetablesFruitsNuts/3382    (2)  Mayo Clinic  http://www.mayoclinic.com/health/contact-dermatitis/DS00985   (3) Info on"poisonous"  (including toxicity level) and "safe" plants:  http://ucanr.edu/sites/poisonous_safe_plants/files/154528.pdf  and http://aggie-horticulture.tamu.edu/earthkind/landscape/poisonous-plants-resources/common-poisonous-plants-and-plant-parts/