Plasticizers are additives that increase the plasticity of a material. For polymer clay, phthalate esters are often used as plasticizers to keep the clay pliable until it is cured.
Possible exposure to the compounds used as plasticizers in polymer clay is the main safety concern of people working with this material. However, it has been shown in numerous studies that this exposure is extremely low and does not pose any health risks.
Despite the fact that polymer clay is used by adult artists (hobbyists as well as professional artists), polymer clay is categorized as children's toy, which means that this product is held to higher safety standards than those for the adult-intended products.
Please refer to my tutorial for polymer clay safety and polymer clay baking tutorial for more information about the safe handling of polymer clay.