Pesticides are chemicals used for crop protection and pest control, and are probably the most widely distributed contaminants in the environment over the last century. Although it is extremely difficult to obtain precise figures concerning their production and use per country [1], millions of tons of pesticides are produced and spread annually all over the world. Thousands of different synthetic molecules are used as pesticides: carbamates, thiocarbamates, dipyridyls, triazines, phenoxyacetates, coumarins, nitrophenols, pyrazoles, pyrethroids, etc. Most of these chemicals contain chlorine, phosphorus, tin, mercury, arsenic or copper atoms. Pesticides are divided into different groups according to their target, eg herbicides, against weeds and toxic vegetation; insecticides, against harmful insects; fungicides, against blights, mildews, mould and rusts; algaecides, for the sanitary control of lakes, channels, water pools, reservoirs; bactericides, against some pathogenic microbes; etc.

To be efficient, a pesticide must remain in the appropriate environmental compartment and be stable enough to act against the target pest for a certain period of time. However, less than 5% of these products are estimated to reach the target organism, the remainder being deposited on the soil and nontarget organisms, as well as moving into the atmosphere and water [2].