Wearing long sleeves, gloves, and slacks can help prevent a reaction to poison ivy, but clothes are not an impenetrable barrier: Urushiol-containing sap can seep through cloth.
Poison ivy can be dangerous even when you're disposing of it in a fire. During burning, urushiol is carried by smoke and dust particles. Airborne particles may coat surrounding surfaces with urushiol and cause diffuse dermatitis and potentially severe airway irritation.
Prevention is the best defense. You should always be alert for poison ivy when walking and especially look down before you sit down. It is frequently found along streams or lakeshores, its growth encouraged by moisture. Protective clothing may help, but contaminated clothing and tools must be disposed of or cleaned to avoid additional contact. Diligent washing of the skin with soap and water or swabbing with alcohol is only helpful if done soon after exposure. There are some protective agents available. They should be applied at least 15 minutes before possible exposure. In 1995, Ivy Block was reported in a controlled trial to completely protect 70 percent of exposed volunteers and partially protect the rest. It has been approved by the FDA. Several other barrier creams appear to be effective, including StokoGard Outdoor Cream, Hollister Moisture Barrier, and Hydropel. Much less active or ineffective were Ivy Shield, Shield Skin, Dermofilm, and Uniderm. Dermashield is not effective. Desensitization using orally ingested poison ivy extract was used in the past but has been withdrawn from the market because of side effects. Very sensitive hikers should consult their dermatologist. I have seen no recent reports of vaccine development.
There is no effective specific treatment. Severe cases may require prescription ACTH or potent steroids orally or by injection for effective control. There are prescription steroid ointments with high concentrations of drug for local treatment in these cases. For less severe cases, the best non-prescription skin treatment for most stages of the rash is 1% hydrocortisone ointment (Cortaid, Cortizone, etc.), which in my experience is moderately effective; lower concentrations are not. Ointment is more enduring than cream. It is not effective in the blister stage. Antihistamines are ineffective either topically or orally. Many popular and folk remedy skin treatments (lotions, compresses, Epsom salts, calamine lotion, bicarbonate of soda, etc.) are at best only mildly soothing and do not improve the rash. A few are harmful by further irritating the skin. Herbal remedies, including plantain, feverfew, and jewelweed, are ineffective.
