A、They have great influence on people’s health. B、They exist permanently in the environment. C、Their production is too massive t

PCBs, were manufactured from the 1920s, but were banned in the US in 1979, in the UK in 1981 and in the rest of the EU in 1987.
   They were found to have a wide-ranging impact on human and animal health, from links to cancer, to restraining the immune system and causing reproductive problems. However, these chemicals are extremely durable and despite the ban they are still found in the environment.
   "Europe produced about 300,000 tonnes of PCBs from 1954 to 1984. That was about 15% of the world’s total," said Dr Jepson. "A lot of this PCB, we don’t know how much, has not been dealt with and is slowly leaking into rivers, from landfills, and eventually into the marine environment." The chemicals then gradually work their way up the food chain and into the top marine animals.
   The researchers analysed samples taken from more than 1,000 killer whales and dolphins in Europe’s waters. "Our findings show that, despite the ban and initial decline in environmental pollution, PCBs still persist at dangerously high levels in European whales," explained Dr Jepson.
5. When were the PCBs banned in the UK?
6. Why should PCBs be banned according to the news report?
7. What do we learn about PCBs from the research?