Ionising radiation such as u.v., X-rays, gamma rays and alpha and beta particles, damages living cells by damaging membranes, damaging DNA, creating free radicals which react with vital molecules. If DNA is damaged, cells may divide uncontrollably causing potentially cancerous tumours. If the mutation is in a sex cell, the mutation will be passed on to many generations. High doses of radiation kill living cells, high and low doses increase the risk of cancer and there is no safe dose.
People working with ionising radiation must wear a film badge. This is a strip of photographic film in a light-proof wrapper, with different areas covered by absorbers of different materials and different thicknesses, that blackens upon exposure to radiation. The dose equivalent is measured in sieverts (Sv).