This CICAD deals with the environmental aspects of manganese and its compounds. Manganese (Mn) is a naturally occurring element that is found in rock, soil, and water. It is ubiquitous in the environment and comprises about 0.1% of the Earth's crust. Crustal rock is a major source of manganese found in the atmosphere, with ocean spray, forest fires, vegetation, and volcanic activity being other major natural atmospheric sources. The major anthropogenic sources of environmental manganese include municipal wastewater discharges, sewage sludge, mining and mineral processing, emissions from alloy, steel, and iron production, combustion of fossil fuels, and to a much lesser extent, emissions from the combustion of fuel additives. Manganese is an essential nutrient for microorganisms, plants, and animals. There is also evidence that it, can protect organisms against the effects of more toxic metals. However, excess levels of manganese are toxic to plants and animals, though there is a wide range of variation in tolerance to it, both between and within plant species.