Leaves of three, let ‘em be.” If only it were that simple to prevent a run-in with poison ivy or one of its two partners in crime, poison oak and poison sumac.

Staying away from poisonous plants is of course good advice. But for the 75 percent of people who are allergic to them — meaning they develop an itchy red rash, swelling and blisters when their skin comes in contact with their oil, urushiol — more vigilance is vital.

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First, some (mostly) good news

It’s relatively easy to avoid a bout with poison ivy if you know you’re going to be around it — while gardening, say, or hiking in the woods — by wearing gloves, long sleeves and long pants.

According to the Mayo Clinic, the amount of urushiol that gets on skin determines the severity of the rash it causes. So although the oil is present on all parts of a poisonous plant, lightly brushing against one probably won’t lead to a rash.

If you have more intense contact with the plant — you grab it with your bare hands, for example, or you fall into patch of it while wearing shorts — you can expect the rash to appear 12 to 48 hours after exposure and to last two to three weeks. Fortunately, you can’t catch poison ivy from someone else’s rash, nor will it spread to other parts of your body via blisters. 

Sensitivity to urushiol can change over time. “Someone who’s had contact repeatedly and never had a reaction may become susceptible later in life,” explains Bruce Robinson, MD, assistant clinical professor of dermatology at Mt. Sinai Medical Center in New York City. On the other hand, a person can also become less sensitive as they age.

How well your immune system functions is also a factor: Ironically, a person with a strong, healthy immune system is more likely to experience a severe reaction than someone whose immune system is weakened or compromised.

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Hands off these carriers

Aside from touching the plant directly, you can also develop a reaction from these sources.

Pet fur. If your pooch rolls on the ground or bounds through hedges where poison ivy is growing, his fur can get coated with urushiol. A pat on the head or belly scratch can transfer the oily residue to your skin. Be aware of where your dog (and cat) hang out. If your pet manages to find a patch of poison ivy, put on gloves and give him a good scrub, advises Robinson. Because it's formulated to dissolve grease, adding Dawn dishwashing liquid to the bath can help cut through the oil, adds David Adams, MD, a dermatologist at Penn State Hershey Medical Center.

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Outdoor toys, tools and gear. “The oil from poison ivy can linger for days or weeks on inanimate objects,” says Joseph Fowler, MD, clinical professor of dermatology at the University of Louisville in Kentucky. 

“We’ve seen cases of exposure from garden tools that were used the previous year, so the oil can definitely be active that long or even longer,” notes Adams.

Wear gloves and add rubbing alcohol to soapy water when cleaning anything that might have come in contact with poison ivy, oak or sumac — for example, camping equipment, kids’ toys, weed eaters and the underside of lawnmowers.

Fingernails. “Urushiol can persist in hard to reach places on the body, such as under fingernails,” says Fowler. When washing your hands after being in an area where you know (or even think) there’s poison ivy, scrub under your nails with a brush.

Shoes. One of the sneakiest conduits for poison ivy is footwear. “If you walk through poison ivy, the resin can get on your shoes and laces. When you next touch your shoes or tie (or untie) them, the oil can get all over your hands, and then you can spread it to other parts of your body,” explains Robinson. Take off your shoes before you wash your hands.

Smoke. Be extra cautious when clearing brush and clippings and then torching it in your yard. “Some of the most severe allergic reactions occur when smoke from burning poison ivy contacts skin,” says Adams. A better bet? Have a professional garden service haul away old vines and scraps that may contain poison ivy.

Holiday greenery. Oh Christmas tree, what’s that hanging from your branches? It just might be poison ivy that’s still viable, according to Adams. What look like dead vines wrapped around an evergreen can be poison ivy that’s still got enough active urushiol to cause a rash. Even poisonous plants that have been pulled out of the ground and appear dead can harbor viable urushiol, adds Robinson. Don’t touch. 

Jennifer Kelly Geddes is a New York City-based writer and editor who specializes in parenting, health and child development. She’s also the mom of two teen girls.