Polychlorinated biphenyls (PCBs) have been banned by a worldwide agreement since 1986, and were banned in some countries several years before that. But 30 years later, the toxic compound is still showing up in the wild. According to a study published Thursday in Scientific Reports, European striped and bottlenose dolphins and killer whales have some of the highest concentrations of PCBs in their blubber ever recorded worldwide. The researchers behind the study say that these concentrations are high enough to cause declines in population and make recovery from other ecological challenges more difficult.

First, a bit about PCBs: The man-made chemicals were first brought into production lines in 1929, and for half a century they were used in cooling and insulating fluids for transformers and capacitors, as well as in many different kinds of plastics. But then came evidence that PCBs can cause a whole host of health problems in humans and other animals, and they were banned. However, since they don't break down easily in the environment, they've persisted even after their ban.

Earlier, industrial-use PCBs used to leak into waterways, and some still do from waste-management plants and contaminated sites. They stick to sediments and are carried through waterways, building up in concentration (and toxicity) the higher up you go in the food chain. They can even be passed from mother to child through breastfeeding.

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Because PCBs are fat-soluble, they build up to especially high levels in blubbery marine animals. In the new study, 1,000 whales, dolphins and harbor porpoises were tested for PCB in their blubber. The porpoises were the only species without record-breaking levels. The scientists were also surprised to find that levels were the same in male and female animals, which is unusual because females usually offload most of their PCBs to calves when they nurse.

“The long life expectancy and position as apex or top marine predators make species like killer whales and bottlenose dolphins particularly vulnerable to the accumulation of PCBs through marine food webs," lead author Paul Jepson of the Zoological Society of London said in a statement. "Our findings show that, despite the ban and initial decline in environmental contamination, PCBs still persist at dangerously high levels in European cetaceans."

In a teleconference held on Tuesday, Jepson added that the situation was "really looking bleak." He and his co-authors urged policymakers to find ways to prevent more PCBs from leaking into the environment, and to find ways to mitigate marine mammal exposure to the chemicals already sitting in ocean sediment.

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"We think there is a very high extinction risk for killer whales as a species in industrialized regions of Europe," he said.

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