As the weather warms up and we engage in more outdoor activities, we face a common enemy: poison ivy, poison oak, and poison sumac. Here are some basic facts about these annoying plants, as well as some tips on protecting yourself from their noxious effect: a blistery, itchy rash that may last for several weeks.

The rash is due to an allergic reaction to a chemical called urushiol, which is found throughout the entire plant (leaves, branches, berries, bark, and roots). More than half of all people are sensitive to this chemical but, fortunately, the rash tends to become less severe as we grow older.

Urushiol soaks into the skin within 10 to 60 minutes after contact but the actual rash usually doesn't start to appear for 12 to 48 hours after the plant was touched. Areas exposed to smaller amounts of urushiol, as well as those body parts covered with thicker skin, will take longer to develop a reaction. This means you shouldn't be surprised if new areas of redness, itching, and blistering still crop up many days after the rash's initial blossoming.

Here are some strategies to help you prevent or control the itchy rash:

Learn to recognize and avoid the plants. The general rule is "Leaves of three, let them be." Unfortunately, leaf colors and shapes vary a lot from plant to plant and from species to species. These plants can also grow as woody vines, shrubs, or even small trees. Furthermore, leafless branches (in wintertime, or even for a few years after the plant has died) still contain the poison, so never touch or pull on a seemingly dead poison ivy vine. Click here for some photographic examples of the different forms of poison ivy.

Create barriers between the leaves and your skin. If you can't seem to consistently avoid poison ivy, the next best protection against urushiol is to throw up barriers between you and it. Wearing long sleeves, long pants, and gloves can be helpful when walking or working in the woods or garden, although not always practical in warmer weather. Keep in mind that urushiol can soak through many materials, so heavy or prolonged exposures may defeat your cloth barriers. Creams containing bentoquatam (Ivy Block®) can protect the skin from urushiol, but do not always prevent a rash. Other creams may be helpful for some people.

Wash everything you may touch that could have come into contact with the plant. You can also get poison ivy from sources other than the plant itself. Once urushiol is deposited on something that item can spread the rash until the chemical is washed away. The most common examples are fingernails, garden tools, and clothing. In addition, outdoor pets can carry the oil on their fur and transfer large amounts to you when they return home.

Know that popped blisters cannot spread the rash. Contrary to popular belief, the fluid from within the blisters will not cause the rash to spread. New lesions appearing in straight lines a few days after the initial rash are usually due to smaller quantities of urushiol that got deposited on the skin as it brushed lightly against a twig or leaf. These lesser exposures take longer to show up as a rash on the skin.

Avoid scratching. You also can't spread the rash simply by scratching the affected area. But do try to resist the temptation, because scratching may injure the skin and increase the risk of infection. Another good reason not to scratch: Urushiol can linger under fingernails for several days, so (if you simply must scratch that fiery itch) be sure to wash well under your fingernails to avoid new exposures.

Even with your best effort, sometimes you just can't completely avoid these nuisance plants. Then it's time to seek out treatment.