DDT or dichloro diphenyl trichloroethane is the best pesticide and also an established carcinogen. This is still found in the waters of Los Angles county at a very high level even after many years of its ban. U.S. Environment Protection Agency released a report on this.
The high level of DDT in the sea is due to the dumping of this substance by DDT manufacturing chemical plants into local sewers which eventually lead into the sea. This has affected the marine life to a larger extent.
DDT is said to cause damage to liver, nervous and reproductive system. Though it is banned in many of the countries, scientists opine that it is an effective tool to fight malaria. In 2006, WHO has suggested the use of DDT indoors to battle against malaria.