A chemical banned more than 30 years ago but persisting in the environment is threatening killer whales and dolphins living in ocean waters around Europe, a study suggests.

Polychlorinated biphenyls, or PCBs, once used in the production of paints, flame retardants and electrical equipment, were banned in the U.S. in 1979 when they were found to be highly toxic. The UK followed with a ban in 1981.

However, PCBs are long-lasting and still linger in the environment and have an impact on the entire marine food chain, researchers at the Zoological Society of London say.

They may be harming Europe's orcas and dolphins, they warn.

ZSL scientists studying more than 1,000 orcas, also known as killer whales, and dolphins and porpoises that were either stranded or that were biopsied reported finding dangerously high levels of PCB.

Such elevated levels can damage the marine mammals' immune system and interfere with breeding, they explain.

"Few coastal orca populations remain in western European waters," says says Paul Jepson, a wildlife veterinarian with the zoological society and leader of a study appearing in Scientific Reports.

Those that are still there are very small and exhibiting low or zero rates of reproduction, he says.

Long-living top predators like killer whales would be exposed to PCBs in the marine food chain, they say, with the toxic chemicals settling in the animals' fatty tissues.

"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," Jepson says.

Around 300,000 tons of PCBs were produced in Europe between 1954 and 1984, he says, and is slowly leaching out of landfills and into rivers and estuaries, and from there, eventually into the marine environment habitats of whales and dolphins.

Levels found in cetaceans — orcas, dolphins and porpoises — around Europe may be higher than elsewhere in the world because PCBs were banned there later than in other parts of the globe, researchers say.

"Our findings show that, despite the ban and initial decline in environmental contamination, PCBs still persist at dangerously high levels in European cetaceans," Jepson says.

A large part of the problem is that PCBs were specifically designed to be resistant to heat and chemicals and to not degrade with time.

"They were designed to last a very long time, so it is incredibly hard to destroy them," Jepson says.