Radiation is ever present, not only in atom bombs and nuclear power stations, which have the sign below in areas containing radioactive material.
Radioactive elements in rocks decay producing radon, which is a heavy gas and may accumulate in people's home. Radon has radioactive isotopes, and decays to produce alpha particles – the most dangerous sort of nuclear radiation, because all the energy from alpha rays is absorbed easily. Because radon is a gas, it can be breathed directly into people's lungs.
Cosmic rays constantly hit the earth, producing carbon – 14 which becomes absorbed into living tissue.
As the nuclear radiation passes through biological tissue, it loses energy, breaking molecular bonds and creating ions. Many biological bonds are very weak – eg hydrogen bonds, of which there are many in DNA. Hydrogen bonds must break easily because the form the 'staircase' which breaks to allow DNA to replicate. X rays and Gamma rays may be absorbed by the electrons in an atom, ionizing the atom.
The interaction of radiation with biological matter is extremely complex. It is well known that excessive exposure to radiation, including sunlight, X rays, alpha and beta particles can destroy tissues. Mild cases can result in sunburn. Greater exposure can result in very severe illness or death by a variety of mechanisms, including massive destruction of tissue cells, alteration of genetic material and destruction of the components in bone marrow that produce red blood cells.
In all cases of radiation exposure cancer may result. Greater exposure to radiation will mean a higher chance of getting cancer, but in fact a single instance of exposure to radiation may cause cancer. A single instance of sunburn may cause a genetic radiation that leads to cancer decades later.