Some heavy metals are neurotoxic. For instance lead, mercury, nickel, zinc, cadmium, chromium and manganese. The commonest source of heavy metal pollution are industrial and mining activities, petroleum exploration, exploitation, processing and effluent management, atmospheric condensation and sewage disposal. Natural phenomena such as earthquake, landslides, tornadoes and cyclones have been implicated. Nuclear reactor accidents, and solid weapons sometimes discharge heavy metal pollutants, which constitute potential dangers to the environment.