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