Why were PCBs historically used in oil transformers and why discontinued?

PCBs were historically used in transformer oils because of their excellent dielectric strength, thermal stability, and fire resistance. They reduced fire risk and performed well under high electrical stress. However, they were later found to be highly toxic, environmentally persistent, and harmful to human health. Due to these risks, PCBs were banned in most countries, and safer mineral and ester-based oils replaced them.