Harmful changes that occur to inanimate materials and living organisms as a result of exposure to energetic electrons, nucleons, fission fragments, or high-energy electromagnetic radiation. In organisms this can cause changes to cells that alter their genetic structure (i.e. mutations), interfere with their division, or kill them. In humans, these changes can lead to radiation sickness, radiation burns (from large doses of radiation), or to long-term damage of several kinds, the most serious of which result in various forms of cancer (especially leukaemia). Ultraviolet radiation can cause distortion of DNA in the form of covalent linkage of adjacent thymine bases in one of the DNA strands. Unless corrected by the cell’s excision repair mechanism, these thymine dimers interfere with DNA replication and may result in cancer.