Because of some of its unique properties, mercury has become a pollutant of global concern that poses risks to the health of humans and the environment. When mercury from natural and anthropogenic sources is released to the atmosphere, it may be transported from its point of origin and enter the global mercury cycle to be ultimately wet or dry deposited in aquatic and terrestrial ecosystems. Converted by bacterial action in lakes and waterways to the more toxic form known as methylmercury, it then bioaccumulates in fish and shellfish. The study of these processes allows estimates to be made for the global mercury budget. Bioaccumulation is the process by which a substance builds up in a living organism from the surrounding air or water, or through the consumption of contaminated food. Methylmercury gets concentrated as it is transferred up the food chain to birds, animals, marine mammals and humans in a process known as biomagnification. Through this cycle, mercury can contaminate entire food webs, posing a serious threat to ecosystem health and particularly to the higher order species in the food chain.

High levels of any mercury exposure for humans can cause severe health problems immediately, but it is the accumulation of low quantities of mercury that is the greater risk to future mothers and their babies. In most chemical forms, mercury is a neurotoxin - this means it can cause damage to the brain and central nervous system. It also affects the kidneys and lungs. Methylmercury, one of the most toxic forms of mercury, is known to affect learning ability and neuro-development in children.