How can I tell if my baby's rash is poison ivy?

If your baby has swollen, red patches of skin on his face, arms, or legs, there's a chance it's poison ivy, poison oak, or poison sumac — especially if you've taken him to an area where one of those plants may be growing.

© Scott Camazine / Science Source

The rash, known as contact dermatitis, sometimes first appears in streaks or lines where the skin touched the poisonous plant. It usually shows up 12 to 48 hours after contact, though it can take up to a week to appear. Blisters follow and become crusty within a few days.

In the United States, poison ivy is most common in the Midwest and the East but can be found throughout the country except in the Southwest, Alaska, and Hawaii. Poison oak is most often found in the West and Southeast. Poison sumac prefers standing water, especially along the Mississippi River and in the eastern parts of the country.

Is the rash dangerous?

No, but it's very itchy. And because the rash can become infected, it's important to keep an eye on it.

Why do these plants cause rashes?

They contain a potent, irritating oil called urushiol. About 85 percent of the population is allergic to urushiol, making it the most common allergy in the country.

While no one's born with the allergy, exposure to the plant produces sensitivity to it. In other words, your body responds only after repeated exposure. So your baby may brush against the plant several times without problem, only to break out in a rash the next time he meets up with it.

The oil's contained in the sap of the plant's leaves, stems, and roots, and it takes only one-billionth of a gram of urushiol to cause a reaction. Most cases show up in the spring and summer, when sap's plentiful and the plants bruise easily, but it's possible to get poison ivy, oak, or sumac any time of year.

How did my baby get poison ivy?

Your baby either touched the plant oil or something with the oil on it — for example, a ball, a gardening tool, or a pet that had come in contact with the plant.