DNA is the primary target for the induction of biological effects from radiation in all living organisms. There are broad similarities in radiation responses from different organisms, but differences in radiation sensitivity. The range in leth­ality from acute exposure to radiation varies by three to four orders of mag­nitude amongst organisms, with mammals being among the most sensitive and viruses being among the most radioresistant.51

Damage from radiation is initiated by ionisation which occurs if the radia­tion has sufficient energy to eject one or more orbital electrons from the atom in which it interacts. Ionising radiation is characterized by a large release of energy which can break strong chemical bonds. The ionisation process and resulting charged particles can subsequently produce significant damage to biological cells termed ‘‘direct effects’’. However, much of the biological damage from radiation is due to ‘‘indirect effects’’ from free radicals, which are the fragments of atoms that remain after being ionised. Free radicals have an unpaired or odd number of orbital electrons, and are thus chemically unstable. Such free radicals can easily break chemical bonds, and are a main cause of damage from radiation exposure.52

Free radicals are not unique to radiation; they are produced in response to many stressors. Damage caused from the free radicals is so abundant that efficient repair mechanisms have evolved within all biological species to counter their effects.

Radiation and the free radicals produced can damage DNA by causing several different types of lesions for which there are efficient DNA repair processes.52 However, errors in repair can result in cell death (through apop­tosis), chromosome aberrations or mutations. Mutations can be deleterious, neutral with no apparent effect (which can persist over many generations) or, rarely, may offer a selective advantage. The fate of mutations and their impacts within a population are dependent on the type of cell in which they occur. Mutations in reproductive germ cells can decrease the number of gametes, increase embryo lethality, or be inherited by the offspring, resulting in their alteration. A mutation within a somatic cell can lead to cell death, or, if DNA damaged is mis-repaired the mutation in the somatic cell can lead to cancer. The risk of non-fatal cancer for humans has been estimated at 1 x 10~5 per mSv.52

The deleterious effects of ionising radiation to biological systems are primarily dose dependent. The effective dose depends not only on the gross energy deposited, but also on the type of the radiation and the radiation sensitivity of the affected tissue. In SI units, the effective dose to humans is the Sievert (Sv), which is the absorbed dose (the gray; Gy) adjusted by two dimensionless weighting factors: the radiation weighting factor to account for the biological effectiveness of the absorbed radiation, and the tissue weighting factor to account for differences in the radiation sensitivities of different organs of the body. These weighting factors have been developed for human radiation biology — no such factors exist for other organisms. Thus, dose to wildlife is expressed in Gy, rather than Sv (although dose rates may be presented on a weighted or unweighted basis).