Advances in science and technology have made it possible to detect very small traces of substances. Whereas a few decades ago such small quantities of substances could not be detected and hence were considered not to be present, today it is possible not only to identify them but also to measure their concentration. This has had an influence on our attitude towards certain substances even if they are only present in trace quantities. Today we frequently talk of environmental problems in connection with substances which only a few years ago were present in the same concentrations but were undetectable.

To illustrate the units used to represent such low concentrations, a sugar lump with a mass of 6g is dissolved in the following quantities of water. It is then present in the concentrations given below: