From ionisation to biological effects:
The most significant biological consequences of radiation result from the damage done to DNA molecules. The dissociation of simple molecules such as water – highly abundant in the human body – liberates highly active (and aggressive) ‘free radicals’ into the body. The severity of the effects depends on the density of ionisation in the medium. The more intense and concentrated the energy deposited in the medium, the more damaging the radiation can be. No matter what the intensity, however, no hereditary effects (such as the transmission of cancerous genes, for instance) have been observed in humans.
IN2P3/NEPAL