The objective of this article is to reflect on the history of PCB alerts in France since the 1980s in order to identify physical, social and political reasons which explain why these chemical compounds are still the object of lively controversies, despite having been banned. After recalling the circumstances of their scattering into the environment at large and examining knowledge that was produced at various times on their toxicity, we describe the events which lead to the crisis of 2005. We are interested more particularly in the emergence of citizen concerns and claims about environmental and health issues which, in the case of PCBs of the Rhône, succeeded in weighing in the orientations of research and public authorities’ choices. At the same time, we discuss what the standards of risk assessment have taught us about PCBs and what they still cannot tell us about them. We also investigate what knowledge is still lacking as it corresponds to issues left aside or ignored while they would interest the public directly.