How does poison ivy cause rashes?
Poison ivy is a plant containing a “poison” or irritating oil called “urushiol”, which causes an allergic reaction in the majority of people. Around 85% of people will experience urushiol-induced contact dermatitis if they're exposed to it, a reaction that generally manifests itself in the form of an itchy red rash, bumps on the skin, or even blisters and weeping sores in more severe cases.
Brushing against a poison ivy plant transfers the surface urushiol oil to whatever has rubbed against it, be that skin, clothing, or even animal fur. When the urushiol meets the skin, it will bind and be absorbed if it is not immediately washed off. It's good to be aware of this, as it's not the plant itself that causes the rash, but the oil – so you can catch a poison ivy rash without even coming into contact with the plant! If you pick up a gardening tool, for example, that has urushiol on it from having touched poison ivy, or if you pat a dog that has (unknown to you) brushed against the plant, you'll pick up the transferred urushiol and develop the distinctive rash.
The rash itself is not contagious – that is to say, you can't catch a poison ivy rash from touching someone else who has it. Nor will you spread your own rash by touching or scratching it, as long as you have washed since coming into contact with the plant. Urushiol will not be present on the surface of the skin of someone with a poison ivy rash, but will have been absorbed by the skin. Once this happens, it can't be transferred, and so won't transfer on to anything else you touch.
That's why it's important for you to wash as soon as you realize you've had contact with poison ivy, even if there are no visible symptoms yet. If you catch it in time, before the urushiol is absorbed by the skin, you can prevent the allergic reaction from taking place. The best way to do this is by applying rubbing alcohol to the area to kill the urushiol, rinsing it off with cold water, and then washing thoroughly with soap and water. It's imperitive that you don't touch or scratch the affected area before you do this – this will not only rub the oil into your skin (thus helping it to absorb before you can wash it off), but also transfer it to your hands and under your fingernails, increasing the risk of spreading it to other parts of your body through touch.
If you already have the rash, you need to make sure you wash anyway to ensure that no more urushiol remains on the surface of your skin. You should also wash the clothing that you were wearing, and even your pet dog or cat if you suspect that there's a chance they could have some of the oil on their fur! This will decrease the risk of further infection through contact with unseen urushiol on surfaces other than poison ivy itself. And of course, it's advisable to kill any poison ivy plants in areas where you or your pets are likely to be spending time again.
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