Mercury is a toxic, naturally occurring heavy metal. It occurs in three forms, elemental mercury (as a liquid or vapour), inorganic mercury compounds and organic mercury compounds. It is readily transformed from elemental and inorganic forms into its most toxic form, the organic compound methylmercury, by soil bacteria in oxygen-poor conditions.
Elemental mercury and inorganic and organic mercury compounds can be transformed in the environment from one form to another in a series of complex processes. In general, however, elemental mercury exists mostly in the atmosphere, while inorganic and organic mercury compounds exist mostly in land and water environments.
Sources of mercury in New Zealand fall broadly into two categories:
Globally, most concern has been expressed over the accumulation of anthropogenic mercury from diffuse sources in aquatic ecosystems. Bioaccumulation and biomagnification of mercury may result in adverse effects on the aquatic animals and associated wildlife, but can also cause increased dietary intakes of mercury in the human population due to higher concentrations of methylmercury in fish.
There is limited evidence that mercury exposure is of concern in New Zealand. Apart from occupational exposure in some specific occupations (dental workers), eating long-lived predatory fish, or fresh water fish caught in geothermal regions, is the most likely route for mercury exposure in New Zealand.