“Leaflets of three, let it be.” This old-time saying contains a warning the wise traveler should heed. Of the three most common poisonous plants — poison ivy, poison oak and poison sumac — two can be identified by their “leaflets of three.” Found throughout the United States and Canada (except in Hawaii, Alaska and the desert Southwest), these wicked weeds cause allergic reactions ranging from a mild rash to lifethreatening anaphylactic shock.

Obviously, the best way to avoid the severe itching, redness, swelling and blisters — typical results of contact with these botanic beasts — is to be able to identify and avoid them altogether. Both poison ivy and poison oak can grow as vines or bushes with the aforementioned leaves formed of three leaflets. Poison sumac is generally found in boggy, swampy areas and carries leaves of seven to 13 leaflets. All three are green to yellow-green during the growing season, turning reddishorange in the fall.

The actual culprit here is urushiol, a colorless or clear yellow oil found in the sticky sap of these plants. Any cut, crushed or bruised part of the plant, either dry and dead or growing, can transmit this substance to your skin. Contact may be direct (skin to sap), indirect (skin to any object with urushiol on it: family pet, tools, athletic equipment) or airborne (burned plants release sap-coated particles into the air which are then inhaled and irritate eyes, nose, throat and lungs). In a sensitive individual, the latter exposure could lead to an anaphylactic reaction.

If you do come in contact with these weeds, immediately wash your skin with cool water and mild soap and remove and wash your clothing. If water isn’t available, rubbing alcohol may dissolve and remove the oils from your skin. If you can remove the oil within 10 minutes, you are unlikely to develop a rash. If the oil penetrated the skin, a reaction appears within 12 to 48 hours. Treat a mild rash with calamine lotion, Aveeno oatmeal soap or soothing bath treatment, or cool compresses of water or milk to reduce itching. An oral antihistamine such as Benadryl may also provide relief.

Scratching may cause the blisters that follow the initial itchiness to weep and then crust over. Wounds often take 10 or more days to heal, and your doctor may prescribe an antibiotic if an infection has resulted from the scratching. The good news is that the rash does not spread by touch, although it may look that way based on the pattern of the breakout, which occurs at a slower rate on areas of the forearms, legs and trunk where skin is thicker, slowing the absorption of urushiol.

Do not attempt to treat a severe reaction on your own or wait it out in your hotel room or campsite. Call the local emergency number or contact your hotel’s concierge to get immediate treatment. Try to stay calm and prevent further exposure to the offending agent. As always, have some means of communicating to medical personnel your health history, allergies and prescribed medications. Treatment may include Epinephrine and steroids to decrease the immune response and relieve inflammation from the toxin.

Two to three weeks after your initial exposure, the rash and itching should be resolved. Maintain your treatment regimen until the symptoms have completely disappeared to ensure they will not return. Keep in mind that future exposure to these weeds will likely result in a more severe reaction than your first — reason enough to let them be!