Once urushiol touches the skin it begins to penetrate. If you're among the estimated 85 percent of the population sensitive to the sappy substance, a reaction will usually appear within 12 to 48 hours as a line or a streak of rashes resembling insect bites.
Redness and swelling will begin in a couple of days, with blisters and severe itching lasting from a few days to several weeks. For those with darker-colored skin, small dark spots can remain even after the rash heals.
Potential sensitivity to the three plants is hard to define or categorize. Children are often infected for the first time between ages 8 and 16, and often with severe cases that can include swollen eyes and severe fever. Although any part of the body is susceptible, thicker-skinned areas like the soles of the feet and palms of the hand are often unaffected.
Although sensitivity to the rash can decline with age, adults should not assume they're immune even if they never suffered from the rash as children. Fair-skinned people are the most susceptible.
"Prevention is the best cure," Walsh said. "If you know what you're dealing with and take preventative measures, that's the best way to avoid the stuff."
However, if you think you may have had contact with poison oak, ivy or sumac, wash all potentially exposed areas with cold running water from a stream, lake or garden hose as soon as possible. If washing is done within 15 minutes of contact, the water can neutralize the sap and prevent it from spreading.
When you return home, wash all clothing outside to prevent resin from transferring to rugs or furniture. Since urushiol can remain active for months, make certain to wash all camping gear or equipment that also may be carry the resin.
If you develop a rash, avoid scratching the blisters. Although the fluid in blisters will not spread the rash, fingernails may carry the resin or germs that could cause an infection.
Successful treatment for the rash varies. While some firefighters have become immune by taking long-term prescription medication that contains the plants' extract, this procedure is not recommended for the general public. The tedious process can cause severe cases of the rash and also prompt other side reactions.
In some instances, hydrocortisone cream as well as calamine and Caladryl lotions are recommended to help dry oozing lesions. Other over-the-counter products such as rubbing alcohol and Tecnu lotion works well to neutralize the toxins if used within a few hours after contact.
In some extreme cases, the allergic reaction to urushiol has caused kidney damage and neurological problems. On the contrary, rest assured that several common beliefs about poison oak, ivy and sumac are myths. The rash, for instance, cannot be passed from person to person.
James Raia, co-author of Tour de France For Dummies, is a freelance writer in Sacramento, California. Visit his web site: www.byjamesraia.com.