Complex sentence: From the initial synthesis of PCB until 1914, large amounts of PCB were released into the environment, to the extent that there are still measurable amounts of PCB in feathers of birds currently held in museums.
Split into simpler sentences: From the initial synthesis of PCB until 1914, large amounts of PCB were released into the environment. So much so that there are still measurable amounts of PCB in feathers of birds currently held in museums.