The amount of damage radiation can cause to living matter varies considerably. The ionisation provoked in cells all along the radiation pathway can lead to modifications in the chemical structure of simple molecules such as water as well as the more complex, indispensable molecules of DNA. Our cells are therefore highly vulnerable, but have the capacity to conduct simple repairs in the case of light damage. Living matter proves surprisingly resistant.
Living organisms are affected first and foremost on the level of their cells, whose most sensitive element is the nucleus where the DNA is stored. Radiation can weaken a body genetic material by either ripping holes in the DNA structure or changing the chemical bases it uses. The cell can conduct basic repair work on the radioactively-induced damages, which are similar to those caused by chemical or thermal processes. This repair capacity, however, is limited. It can be rendered useless if too much energy is deposited in a cell in too short a time frame. The badly-repaired wounds can then lead to instant or delayed cell death (an inability to undergo cell division) in which case the damage is limited to the cell. It may also lead, in some cases, to mutation which is then transmitted to daughter cells.
At higher intensities, radiation can affect ordinary (somatic) cells or those which play a role in reproduction (also known as seminal cells). In the former case the consequences can lead to the appearance of cancers, while damage inflicted to seminal cells can cause a reduction in fertility or even mutation (though nothing has yet been observed in humans). It is important to remember that not all organs are equally sensitive. Reproductive organs such as the male gonads are more than 20 times as sensitive as skin. Finally, the type of radiation also determines how dangerous the radioactivity can be – as alpha particles are 20 times as toxic as gamma photons for identical energy deposition.
Radiotoxicity also depends on an oft-overlooked factor known as radioactive dose rate, which involves dividing the radioactive dose (in grays) by the exposure time. A dose absorbed in a given length of time is more toxic than the same dose absorbed over longer periods. It is almost as though the damage done is so great that the cells ability to repair themselves is swamped and therefore becomes less effective. This factor can be used to our advantage when it comes to targeting malignant tumours. In radiotherapy, very high doses are applied over very localized areas and for very short lengths of time.