Linda Birnbaum: If you have lots of them in a molecule, that is going to make the molecule forever in the environment because there is no natural way that has evolved to break that bond. So the carbon fluorine bond, for example, does not break down in the presence of sunlight. Other very persistent chemicals, things like dioxins, will actually break down in the presence of sunlight over time. It takes a long time, the estimate is about 10 years. No one has been able to demonstrate that these chemicals can break down with natural processes of light, heat, or even oxygen levels.