text: In the biosphere, PCBs can be degraded by the sun, bacteria or eukaryotes, but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, meta- or para-substituted PCBs undergo biodegradation faster than more substituted congeners.
  In bacteria, PCBs may be dechlorinated through reductive dechlorination, or oxidized by dioxygenase enzyme. In eukaryotes, PCBs may be oxidized by the cytochrome P450 enzyme.
  Like many lipophilic toxins, PCBs undergo biomagnification and bioaccumulation primarily due to the fact that they are easily retained within organisms.
  Plastic pollution, specifically microplastics, are a major contributor of PCBs into the biosphere and especially into marine environments. PCBs concentrate in marine environments because freshwater systems, like rivers, act as a bridge for plastic pollution to be transported from terrestrial environments into marine environments. It has been estimated that 88-95% of marine plastic is exported into the ocean by just 10 major rivers.
  An organism can accumulate PCBs by consuming other organisms that have previously ingested PCBs from terrestrial, freshwater, or marine environments. The concentration of PCBs within an organism will increase over their lifetime; this process is called bioaccumulation. PCB concentrations within an organism also change depending upon which trophic level they occupy. When an organism occupies a high trophic level, like orcas or humans, they will accumulate more PCBs than an organism that occupies a low trophic level, like phytoplankton. If enough organisms with a trophic level are killed due to the accumulation of toxins, like PCB, a trophic cascade can occur.
  PCBs can cause harm to human health or even death when eaten. PCBs can be transported by birds from aquatic sources onto land via feces and carcasses.