for those congeners with one to four chlorine atoms. In the atmosphere, PCBs may be degraded by hydroxyl radicals, or directly by photolysis of carbon–chlorine bonds (even if this is a less important process). Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the picogram per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1 ng/m3 or more. In Milwaukee, an atmospheric concentration of 1.9 ng/m3 has been measured, and this source alone was estimated to account for 120 kg/year of PCBs entering Lake Michigan. In 2008, concentrations as high as 35 ng/m3, 10 times higher than the EPA guideline limit of 3.4 ng/m3, have been documented inside some houses in the U.S. Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere. Biosphere 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,