Human effects on biogeochemical cycles

In discussing the chemistry of near-surface environments on Earth it is important to distinguish between different types of alteration to Earth systems caused by humans. Two main categories can be distinguished  

1 Addition to the environment of exotic chemicals as a result of new substances synthesized and manufactured by industry. 

2 Change to natural cycles by the addition or subtraction of existing chemicals by normal cyclical and/or human-induced effects. 

The list in Table 1.2 is by no means complete. It has been calculated that the chemical industry has synthesized several million different chemicals (mainly organic) never previously seen on Earth. Although only a small fraction of these chemicals are manufactured in commercial quantities, it is estimated that approximately a third of the total production escapes to the environment. 

The impact of these exotic substances on the environment is difficult to predict, since there are often no similar natural compounds whose behaviour is 

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