Polychlorinated biphenyls (PCBs) are organic compounds that were once used in industrial products such as paints and adhesives. However, open use of PCBs has been banned since 1970s, when it became clear that they have adverse environmental effects, particularly on wildlife populations. The compounds are toxic to many animal species, including birds, fish and mammals. PSBs also take an exceptional long time to degrade, so they are still a significant air and water pollutant. Fishing is still restricted in the Great Lakes district, for instance, because of the risk that contamination could be passed to humans. According to the paragraph, on top of their toxicity, why else are PCBs considered harmful to the environment?