Copper occurs naturally in an environment that shows high density relative to water and toxicity at low concentrations. Due to these characteristics, copper is classed as heavy metal. Copper is fairly innocuous at very low levels, high quantities such as that found in some industrial wastewater streams can pose a hazard to human health and to the environment. In humans, long-term exposure to slightly elevated copper levels can result in mild symptoms such as headaches and gastrointestinal upset. Chronic exposure to high levels of copper is linked to a number of severe health impacts, including cognitive impairment in adolescents, as well as diseases of the liver, kidneys, brain, and nervous system.