Some plants have the ability to cause contact dermatitis (an unpleasant skin reaction).
Sometimes just simply brushing against the plant will do it, but in other cases, you actually have to bruise the plant or snap off a leaf or stem before a reaction will occur, because it’s contact with the sap that is harmful.
Note that in some cases, dermatitis will only occur if the skin is exposed first to the sap and then to the sun. This is called photodermatitis or phototoxicity. With these plants, there is therefore less risk of a reaction on a cloudy day.
Note too that sensitivity varies greatly from one individual to another: what can send one person to the hospital (and yes, that can happen!) may have no effect on another. But since you never know whether you’re sensitive or not, please wear gloves and long sleeves if you have to handle the following plants.
*Plants causing photodermatitis.