About 75% of Bay Area adults are allergic to poison oak. Mysteriously, children rarely develop the allergy until about age 10, even though their exposure may be great. Allergy may be triggered by the first exposure or by the two-hundredth. Poison oak allergy cross-reacts with poison ivy, poison sumac, and mango rind.

Poison oak rash may develop differently when it occurs for the first time, or if it has been years since the previous attack, as opposed to when it occurs repeatedly. In repeated attacks the rash and itch usually start 12 hours to 2 days after exposure; the rash reaches a peak in 4 to 5 days; and it fades in about 10 days. With the first attack the itch and rash may not start until 3 to 5 days after exposure; new spots may appear for 10 days, and the attack may last 3 to 4 weeks.

The gradual appearance of new spots over this 10-day period has given rise to myths about the "spreading" of poison oak. These new spots appear where the poison oak touched the skin the first day, but the sites contacting a low dose react later than sites, which received a large dose. The rash itself cannot be "spread" by scratching, and the blister fluid does not spread the rash.

It is the sappy resin in poison oak stems, roots, and leaves to which people become allergic. Breaking through underbrush, pulling up "weeds" or reclining on the plants increases exposure. If the skin is immediately washed with soap (any kind) and water, a reaction will not occur. However, after about 15 minutes, the resin chemically binds to skin and then only the excess can be washed off. Before washing, this excess can be spread to other skin sites by rubbing. A common pattern occurs when a person gets poison oak resin on the palms and fingers by inadvertently handling the plant, and then wipes sweat from the face, or touches other areas (especially the penis, held while urinating). Later the palms do not break out, probably because the thick skin there prevents absorption, but the thin-skin areas of the face and genitals react violently.

The risk of being exposed to poison oak resin on pets and clothes is probably slight. On cloth, fur, and leather, the resin binds strongly and degenerates fairly quickly. On hard, smooth surfaces, such as tools, the resin can remain "active" for weeks, so wash equipment after use in poison oak areas.

Mild poison oak rash can be treated with cool soaks, calamine lotion or over-the-counter anti-itch creams. Cortisone creams available without prescription (hydrocortisone 1%) may not be strong enough to be of benefit. Severe poison oak rash requires prescription steroid creams, or, often, prednisone tablets. None of these treatments shortens the duration of the attack so they must be used for as long as it occurs, which depends on whether it is a repeated attack or a first-time one (or the first in many years).