ATLANTA (CBS Atlanta) – You can look, but you’d better not touch.

Poison ivy, poison oak and poison sumac plants are growing larger leaves and the oil that gives people rashes is becoming more potent, say researchers.

Warmer temperatures and an increase in the amount of carbon dioxide in the atmosphere is helping these poisonous plants to grow.

A substance in the plants called urushiol oil is to blame for the notorious itchy rash that develops in people who touch it.

Many varieties of poison ivy and poison oak have branches with three leaves, hence the popular phrase, “leaves of three, let them be.” But the rule isn’t foolproof: the leaves sometimes grow in larger clusters. Poison sumac may have clusters of 7 to 13 leaves. The leaves often have black spots which look like paint splatters, caused by blobs of oil that leak out become oxidized in the air and turn black.

If the oil from a poison ivy, poison oak, poison sumac or related plants touches the skin, remove the oil as quickly as possible to minimize harm.

The U.S. Forest Service recommends cleaning the skin with isopropyl (rubbing) alcohol within 10 minutes.

One complicating factor is that, while the oil starts penetrating the skin within minutes, the allergic reaction it causes often takes between 12 hours and two days to develop. That means people who are exposed and don’t realize it don’t necessarily get away from the plant as soon as they might have otherwise.

The National Institute for Occupational Safety and Health (NIOSH) created a video explaining how poison ivy causes itching and swelling of the skin.

According to the Centers for Disease Control and Prevention, the best protection is to avoid contact with the plants. And don’t try to remove or burn them because the oil can cause lung irritation if inhaled.

(TM and © Copyright 2015 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2015 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)

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