In the old days, hydraulic fluid contained PCBs. PCBs worked great; they never broke down. Unfortunately, they are also a hazardous material, and as such much $$$ is needed to dispose of it. If any of old PCB-containing oil was mixed into used non-PCB hydraulic fluid, then all the contents of the container become hazardous. PCB stands for Poly-chlorinated Biphenyls. That is why they check for chlorine content.