Poison oak

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Poison oak is a plant species commonly found in the southwest United States. It frequently causes skin rashes on people exposed to its oil urushiol.

Occurrence

Poison oak is especially common in the San Gabriel Mountains north of Los Angeles and presents a serious challenge for navigating some canyons in that area. Off-trail bushwhacking is especially likely to cause symptomatic exposure to poison oak because this increases the number of plants one brushes against and breaks the plants, exposing the circulatory fluids of the plants where the urushiol is located. Even dead plants still contain urushiol oil.

Appearance

The most common mnemonic for poison oak is "leaves of three, let them be" because a set of three leaves is the most reliable way to identify poison oak. This is not always helpful, however, as the plants are often without leaves around the winter season, but can still cause a rash during this time. When flowering, poison oak berries are small and white. The leaves vary substantially in size, color, shape, and sheen.

A few other plants share the general appearance and "leaves of three" characteristics, but the most common ones in the San Gabriels also have small spines. If the plant in question is covered with spines or small thorns, it is not poison oak.

Reaction

While some people do not exhibit any reaction after coming into contact with some amount of urushiol (these people are "immune"), most people will develop a very itchy rash (Urushiol-induced contact dermatitis) one to fourteen days after exposure that persists for one to three weeks. The delay between exposure and symptoms varies with the amount and type of exposure, the area of the body affected, and the individual himself. More detailed information may be found here.

Prevention

The easiest way to avoid the rash due to urushiol oil is to avoid exposure to the oil. Lightly brushing an unbroken poison oak leaf does not guarantee exposure to the oil, so it is often easy to avoid exposure entirely on established routes and trails by watching for poison oak along the edges of the trail and then carefully avoiding it when encountered.

Another alternative is to apply a product that blocks the oil from coming into contact with your skin, such as IvyBlock.

Transmission

The only thing that causes the rash is the urushiol oil. Any mechanism that spreads the urushiol oil will also spread the rash. This often includes hands, gear, and pets -- any of these things may carry the oil until it can be transferred onto skin at a later date and pets do not generally exhibit the rash that humans get. The active "shelf life" of the oil is essentially indefinite, so gear exposed a long time in the past may still be able to cause a rash if the oil has not been washed or rubbed off.

Once a rash forms, the plasma and/or pus does not carry the oil and will not cause additional rash.

Treatment

Before the rash develops

The urushiol binds with skin 10-30 minutes after exposure. Before that time, it may be possible to wash it off with soap and water, Tecnu, water and sand, or other similar means, but normal soap may not be effective because the oil is roughly the consistency of pine sap. After this time, it can still be advantageous to clean off any excess oil without allowing it to bind with skin. To attempt this, a cold shower is recommended to close pores to reduce the likelihood of oil entering them, and then wash with dish soap, Tecnu, or a similar product.

Typical treatments

Once the rash develops, the urushiol oil has already bound to the skin and cannot be washed off. The most typical treatment recommendation for mild to moderate cases is to treat the itching symptoms using calamine lotion while severe cases may warrant a doctor's visit where they may prescribe pregnisone, a steroid to reduce the allergic reaction. Scratching is discouraged as it may cause scarring or infection.

The American Academy of Dermatology tips are representative of typical treatment suggestions.

Exfoliants

Zanfel is a special soap containing exfoliating beads that can treat the rash even after it has started. It can be purchased in most drug stores, but is extremely expensive ($25-$50 for a 1 ounce tube). Mean Green Power Hand Scrub is a degreasing soap intended for use in machine shops, but it appears to be very similar to Zanfel while costing substantially less ($40 for 60 ounces). Tecnu Extreme is somewhat similar to these products, but is less abrasive and appears to be substantially less effective.

In this author's experience, these products are the only ones to successfully treat the rash after it has started. A sixty-second application tends to eliminate itching for six to eight hours. When used before a rash has developed, Mean Green seems to reduce the time between exposure and rash from 6 days to 1-2 days, and reduce the duration of the rash from 2-3 weeks to about 5 days.

Other treatments

Many proposed treatments may be found here, but they are unlikely to all be equally effective. Additional treatments viewed with a more critical eye may be found here.

Cleaning

Washing gear in a normal washing machine will typically wash off the oil from a light exposure. For heavy exposure, using Tecnu to clean gear is recommended.

Myths

There are many conflicting claims about urushiol. This section attempts to list some of the more common ones and provide links to sites on both sides of the claim when both kinds of sites exist. If you feel that a myth is addressed incorrectly here, please add an alternative answer along with a reference for that answer.

Urushiol exposure can become systemic

That is, the urushiol can get into your blood stream leading to a rash on parts of your body not directly exposed to the urushiol oil.

True

False

  • Penn State poison ivy myths
  • Asthma and Allergy Foundation of America describes a "systemic" as simply one where there is sufficient amount of exposure to affect other systems in the body apart from just the skin
  • Ben Pelletier: This seems unlikely. If the urushiol oil were being carried by the blood stream, one would expect rashes to be more common along veins and arteries, and the rash would be fairly evenly spread across the entire body according to vein/artery density. Instead, it generally shows up in concentrated patches in the places you think you haven't been exposed. A guess at how outbreaks on "unexposed" areas are caused is by the fact that different types and thicknesses of skin show the rash after different amounts of time, and gear can carry the oil causing additional exposure later.

Scratching can spread the rash

False

Ingesting small amounts of poison oak can build an immunity

False

True

Some people are permanently immune

False; even if you have had no prior symptoms despite exposure, you may still develop a rash at some point.

External links

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