Regional and global aspects of the mercury cycle

Source-exposure-effect paradigms for risk assessment of pollutants has worked well in most cases. How well does it work for mercury, an atmophilic element with sizable natural as well as anthropogenic sources and complex chemistry, found throughout the globe at extremely low concentrations (picomolar) in air and water and somewhat higher concentrations (nanomolar) in soil and biota? The paradigm seems to operate quite well with mercury, but the complexity of mercury cycling can overwhelm simple interpretations, requiring development of additional theory and substantial field studies and intercomparisons.