IT is the toxic legacy of Europe's industrial past, lingering unseen in the environment fifteen years after countries started cleaning up their act.
But the man-made pollutants still have the power to cause harm, and now scientists are warning that they are being passed down through generations of wildlife more than 15 years after their use was banned.
A fresh study of seals has found that seal pups are being slowly poisoned by chemicals found in their mothers' milk, which entered the food chain after leaking into the sea.
It comes after warning were sounded that Scotland’s unique resident pod of killer whales is threatened with extinction because of the high concentrations of the illegal elements found in the waters along the west coast.
Seals pups need to feed to build up blubber.
The latest probe into the issue has found that levels of the persistent organic pollutants (known as POPs) are falling very slowly in Scottish waters, even though the UK and many other countries nearby not using or producing them for decades.
The pollutants include PCBs - or polychlorinated biphenyls - which were widely used as stabilisers in paints, sealants and in electrical transformers and also traces of the pesticide DDT, used as a pesticide in Europe before before its harmful effect on the environment was fully understood.
Blamed for killing off wildlife as well as insects, especially birds, it was widely banned across Europe in the 1970s and 80s.
READ MORE: On the breeding beach with Scotland's grey seals
Now known as legacy pollutants, POPs are no longer manufactured or used but don’t break down easily in the environment and are toxic to humans and wildlife.
They reach the sea through incineration, water run-off or careless disposal in landfill, and are able to travel a long way from where they were released because they are so stable.
The substances enter the food chain through fish eaten by the mother seals, and are then passed onto their offspring when they suckle.
They can then be found in the young seal's blubber, which they need to survive out at sea. The latest study compared POP levels present in Scottish grey seal pups today to concentrations measured in 2002.
The research, led by Abertay University in partnership with the University of St Andrews Sea Mammal Research Unit and the University of Liege in Belgium, found that while some POPs, including many of the PCBs, had fallen by around a quarter during that time, other pollutants, like DDT and its breakdown products, had not changed.
Dr Kimberley Bennett, who led the project and is a lecturer in Biomedical and Environmental Science at Abertay, said: “Environmental POPs fell dramatically in the environment immediately following the 2001 Stockholm Convention ban, but our research shows that this fall has slowed dramatically in recent years.
“In previous studies we’ve already shown that even low POP concentrations can alter fat tissue function in seal pups, and it now seems likely that these negative biological effects will continue well into the future."
Last year it emerged that the community of eight Orcas found off the Western Isles, which have been recorded since the early 1980s, may not be able to reproduce due to huge levels of deadly PCB’s in the sea.
READ MORE: Record year for birth of grey seal pups around east coast
An autopsy carried out on Lulu, a killer whale which was found on Tiree in 2012, revealed the mammal had one of the highest levels of the man-made chemical ever recorded - more than 100 times above the level that scientists say will have biological consequences for a species.
Fears were raised for the very future of the unique west coast pod by the Hebridean Whale and Dolphin Trust, whose Hebridean Marine Mammal Atlas confirmed the Scotland's outer islands as a global hotspot for whales, sharks and dolphins, with 30,000 animals recorded there since 2002.
But it also documented how the UK’s only resident Orca pod will likely become extinct in the coming decades, as a calf has never been recorded despite it consisting of four males and the same number of females.
One of Scotland's Orca
The Trust’s evidence was used to identify the boundary of Scotland’s first protected area for harbour porpoise, approved by the Scottish Government in 2016.
Researchers were also the first to suggest that bottlenose dolphins live year round off Scotland’s west coast.
Recent estimates suggest that Europe produced between 299,000 and 585,000 tonnes of PCBs.
But while industry has stopped using the chemicals in the manufacture, millions of tonnes of continue to be in circulation.
The contaminants were found to harmful to wildlife since the 1970s, with marine mammals facing the biggest threat because they feed at the top of the food chain.
Killer whales have an 11-month lactation period during which they produce very high-fat milk for their calves and the higher the fat, the easier it is for PCBs to dissolve in it. While PCBs are no longer produced, they are extremely hardy as they were designed to resist extreme heat.
READ MORE: Seal pups poisoned by chemicals in mothers' milk
And although the so-called "dirty dozen" chemicals are banned from being produced and released into UK waters under the Stockholm Convention, they are still finding their way into the sea through sewage and landfill.
The Stockholm Convention is a UN treaty which was brought forward with the aim of “eliminating or restricting” the production and use of POPs.
Lulu had PCBs measuring 957mg/kg and at these levels, species stop reproducing.
Abertay's Dr Kimberley Bennett said that the time had now come to look into ways of removing the deadly toxins from the sea, as natural processes were not enough.
She said: "We need additional measures to clean up POPs that go beyond the ban, but because they are effectively locked into the food chain, that’s going to be very difficult to achieve.”
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