(excerpted from http://www.ucihs.uci.edu/emergmed/_ems/Oct%2099T.html )
Anything that causes an allergy-related rash after contact with the skin is called contact dermatitis. We’ll focus on the most common cause in wildland firefighting, which is poison oak (a member of the Rhus genus of plants). Members of this group of plants include poison ivy, poison sumac and poison oak. Poison oak dominates west of the Rocky Mountains and below 4000 feet. A large percentage of the population is sensitive to poison oak, up to 50% according to some authors. And about 15-25% have serious reactions, enough to temporarily prevent them from working. The component of the plant that is responsible for the skin reaction is a light oily resin called urushiol present throughout the entire plant. This resin penetrates the skin and sets up a reaction in the skin that later forms the characteristic vesicular or bullous rash (small fluid filled blisters).
The rash does not occur immediately after contact, however. Rhus dermatitis is a cell-mediated allergy which causes T-lymphocytes to react over a period of 2-10 days. The rash appears most often 3 or 4 days after exposure. Since it is not mediated by histamine, antihistamines are not particularly helpful in treatment. It is characterized by red, raised, itchy patches or linear or clustered vesicles (small fluid-filled blisters). The fluid within the vesicle is an inflammatory fluid created by the body from the allergic reaction. It will not further spread the rash if the vesicle is ruptured and the fluid leaks out. What can happen though is that gloves, clothing, shoes or tools contaminated with the resin can initiate a new skin reaction in proximity to an existing rash or at a new site. Scratching and opening the rash can set up a secondary skin infection unrelated to the allergic reaction.
The resin boils at about 200C (about 390F) so under fire conditions it may splatter onto soot particles which when airborne could be inhaled and result in respiratory swelling. This has rarely been reported but is theoretically possible. Contaminated soot particles could cause conjunctivitis or tracheobronchitis. In the absence of asthma or other underlying lung disease, inhalation injury or facial burns, this is one possible cause of respiratory compromise developing in an otherwise healthy firefighter several days into firefighting operations when there is still open burning of fuel with soot production.
More likely, skin exposure to poison oak occurs during the cutting of fire line and the contamination of nomex, gloves, tools and equipment with the resin. That resin can then be transferred to the skin through contact with the contaminated material and lead to the characteristic rash. The face, neck, forearms and hands are the most likely areas to be affected, but the resin can be spread to any part of the body from contamination on the hands. Hand washing and avoiding subsequent contact with potentially contaminated gloves, boots, tools & nomex until the next operational period will help to reduce exposure. Protective gear itself provides barrier protection to the resin, but the resin can persist on this equipment and lead to secondary exposure. It’s interesting to note that the resin is soluble in rubber gloves & will penetrate through to the skin. Once there the occlusive nature of the rubber glove and the sweat beneath the glove propagates the spread of the resin more widely over the hand. Vinyl gloves appear to be protective against resin penetration. Awareness of equipment contamination and hand washing are the most fundamental means to minimize the risk of contact with the resin when avoidance of the plant is impractical or impossible.
The resin is soluble in organic solvents like acetone or isopropyl alcohol (rubbing alcohol) used to prepare IV sites, but if a solvent is used be sure to use enough to flush the hands or tools well. Too small a volume of solvent may just spread the resin around rather than remove it. Tec Laboratories markets Technu which is a mineral spirit/polyalcohol combination used to decontaminate skin and tools after exposure. Clothing and protective gear act as a barrier to the resin. Barrier preparations for application to the skin are also available. Barrier preparations make delayed washing with soap and water or even water alone more effective in resin removal. One commercial product is a linoleic acid dimer called Stokogard Outdoor Cream. Another barrier product from Tec Laboratories is Armor. Barrier products should be applied to exposed or potentially exposed skin areas before exposure and periodically while on the fire line until water washing is possible. They can also be applied to clothing and protective gear likely to come in contact with the poison oak plant and then the skin at some later time, like shirt and pants cuffs. So protection from exposure to poison oak is a combination of protective gear, barrier preparation application before exposure (like Armor) followed by solvent (Technu, isopropanol) and water wash after returning to fire camp or whenever practical.
Once the rash develops it pretty much has to run its course....