Life on Earth has evolved and is sustained by the light and heat of the Sun. In addition to this essential and almost entirely benign flux of electromagnetic energy, living organisms have always been subject to the hazards of natural ionising radiation. In the twentieth century man's activities added somewhat to these hazards. On the other hand, ionising radiation is used to great advantage in industry and for diagnostic and therapeutic purposes in medicine, and nuclear power is not without its benefits. The interaction between ionising radiation and living tissue is therefore a matter of great interest and importance.

Ionising radiation and biological damage

The basic unit of living tissue is the cell. Cells are complex structures enclosed by a surface membrane. A cell has a central nucleus. This contains DNA (deoxyribonucleic acid) molecules, which code the structure, function, and replication of the cell. The famous ‘double helix’ of the DNA molecule has a diameter of about 2 nm. About 80% of a cell consists of water.

The induction of cancer or of hereditary disease by low levels of ionising radiation is believed to be related to damage to the DNA molecules. This can happen by direct ionisation of the molecule, or indirectly through ionisation of the water molecules in the cell. The break-up of a water molecule may produce a hydroxyl (OH)- ion that is highly reactive chemically and may attack the DNA molecule.