A dangerous transformation
Most of the anthropogenic mercury emissions come from industry. However, natural sources such as erosion and volcanic eruptions also contribute to atmospheric mercury. All the air around us contains gaseous mercury that is not that reactive and thus not harmful, either to animals or to humans, at normal concentrations.
Concentrations worldwide are fairly similar. But it appears that a reaction between sea salt, sunlight and atmospheric mercury transforms the less hazardous gaseous mercury in the air into more reactive mercury. When this more reactive type of mercury is deposited on the ground, it can be converted into toxic methylmercury -- which then can poison the entire food chain.
Accumulates in the food chain
When mercury enters the food chain, it is taken up by microorganisms, and then by ever larger organisms. Marine mammals, polar bears and humans are the top of the food chain in the Arctic, and thus are subject to the most contamination, because the farther up the food chain you go, the higher the concentration of mercury becomes.
Mercury is stored in the body and there is much evidence that the contaminant damages the nervous system. Mercury can also have a serious effect animal health, but also threatens people who largely live off marine mammals. Some studies of children in the Faeroe Islands have shown learning disabilities which are suspected to be linked to high mercury concentrations in the food that they eat.