Halogenated hydrocarbons such as chlorinated ethylenes are commonly found in the environment. For many years it was believed that most of these compounds originated from anthropogenic (human-made) sources. However, it was recently discovered that over 3500 organohalides are naturally occurring. For instance, some researchers have found that organohalides are sometimes used as a part of microbial defense mechanisms against predators. It is now an accepted fact that these compounds are generated from both anthropogenic and biogenic sources.