With a copper iud only partially expelled, how is it possible to become pregnant if the area is still considered a toxic environment for sperm?

IUD method of action: The Copper IUD's mechanism of action(effectiveness) has nothing to do with the sperm. It works by preventing an already fertilized egg from implanting in the uterine lining(endometrium) when it is placed correctly. If the IUD is not in the correct intrauterine location then conception & implantation will occur.(pregnancy).