Plastics are formed when small molecules are linked together into long chains ("polymerization"). Sometimes, the materials that result aren't quite what a manufacturer has in mind. So, for example, when a plastic needs to be opaque, a pigment can be added. If it needs to be lightened, air bubbles can be pumped in. Similarly, if a plastic is too stiff or inflexible, a plasticizer can be added to fix these problems. It's the chemicals that are added as plasticizers which are the focus of our concern.