PCBs (Poly-Chlorinated biphenyls) are harmful to beluga whales due to their fat-soluble nature and potential carcinogenic effects. Belugas, which are cetaceans, accumulate high levels of PCBs primarily because they are at the top of the food chain and possess a thick layer of fat for insulation. While PCBs are known to be carcinogenic in laboratory mice, the primary concern for belugas is not cancer but reproductive interference. High PCB concentrations in orcas, particularly in regions like Puget Sound, have led to significant health issues, including reduced reproductive success and shorter lifespans. The impact of PCBs on belugas highlights the broader environmental and ecological challenges faced by marine mammals in polluted habitats.