Waste products from man's activities, released into the environment, distribute themselves between the atmosphere, the land, the sea and, through these, to the biota. There is a constant interchange of material between reservoirs leading, it is thought, to an ultimate sinking of trace materials into the depths of the ocean where they slowly accumulate on the ocean bed and are removed from the circulation system. Substances released into the air can rise into the upper atmosphere and spread all over the world before they finally sediment into the lower atmosphere and suffer washout. The overall lifetime of a particular pollutant is determined by its concentration in each reservoir and by its rate of transfer between different pairs of reservoirs. A major goal for scientists is to achieve a quantitative description of this process for the major pollutants so that in the future it may be possible to predict the changing concentrations with time, in each reservoir, of a potentially harmful pollutant of known input rate into the environment.