POPs are a group of man-made substances, most of which share characteristics like low water solubility (they do not easily dissolve in water), the ability to accumulate in fat (high lipophilicity), and resistance to biodegradation (they take a very long time to break down and stop being harmful). The name POPs refers to many pollutants such as pesticides like DDT and pollutants like PCBs. These chemicals come from pesticides, industrial chemicals, and are the unwanted by-products of industrial processes or combustion.