Despite their being banned for decades, polychlorinated biphenyls (PCBs) threaten the long-term viability of more than 50% of the planet's killer whale population, reports a new model-based study. To reach this sobering conclusion, its authors used data about PCB levels in killer whales around the world, as well as whale population size estimates and data on whale reproductive responses to PCBs. The results provide a clearer picture of the fate of this marine mammal, the tissues of which collect a higher concentration of select contaminants than any other. Toxic, carcinogenic and persistent in the environment, PCBs impact both reproduction and immune function in mammals. While the production of PCBs was banned in the United States in 1978, their widespread industrial use led to ubiquitous global distribution. Killer whales, one of the largest marine predators, are particularly sensitive to PCB contamination due to bioaccumulation and to the chemicals' ability to be transferred from mother to calf. As a result, PCB concentrations are exceedingly high in the tissues of killer whales, which may be contributing to observations of population declines. Compared to marine mammals such as dolphins, however, little is known about the extinction risk from PCBs to killer whales, in part due to challenges in monitoring this highly mobile species. Here, to get a better handle on the potential impact of PCBs on global killer whale populations, Pierre Desforges and colleagues developed a risk assessment model capable of forecasting the effects of PCB exposure on these creatures in all the world's oceans, and over the next 100 years. The model combines data on blubber PCB concentrations from 351 killer whales around the world and the chemicals' known toxicological effects, and it simulates both accumulation of this chemical in whale tissue and its transfer to offspring. The results of the researchers' model simulations show that concentrations in killer whales are closely tied to proximity to PCB production and use, and that high levels of the chemical have a significant impact on both population size and sustainability. According to the model, whale populations in the Arctic and Antarctic, where PCB concentrations are low, will continue to grow, or only be modestly reduced, in years ahead. However, according to Desforges et al., in regions where concentrations are highest - Japan, Brazil, the northeast Pacific, the Strait of Gibraltar and the U.K. - killer whale populations could be headed for a complete collapse within the next century. The status-quo efforts to protect killer whales from conservation threats are likely to be impeded because PCBs have remained at problematic levels, the authors add.