The danger in some toxic substances is that they don't biodegrade or recycle back into the ecosystem. While many modern pesticides are made to degrade into their original elements and molecules so that they aren't toxic as they move through the ecosystem, other substances such as plastics and other manmade chemicals don't biodegrade. These nonbiodegradable compounds are called persistent organic pollutants (POPs)because they stick around for long time, polluting the environment and causeing illnesses. Some common POPs include.