sentence: Natural biodegradation of PCBs in contaminated soil and sediment occurs at low rates ,17 and various attempts to accelerate biodegradation have been made.18 , 19 , 20 , 21Despite active research in developed nations spanning three decades , extensive regulatory action , and an effective ban on their production since the late 1970s , PCBs remain a focus of research and environmental attention.22 China is a developing country in which pollution prevention and control is most likely less effective compared to developed countries .