Made with Xara Talk to us and ask the expert  Welcome to Operational Radiation Safety “Πάν μέτρον άριστον” For external exposure, the intensity of radiation falls with distance from the radiation source, just as the brightness of light falls with distance from a light  bulb. The total amount of external exposure will also depend on the length of time the person remains close to the source. Radioactive material can be taken into the body by consuming foodstuffs and liquids with radioactivity in them, by inhaling radioactive gases or aerosol  particles, or by absorption through wounds in the skin. The material taken in will internally expose the organs and tissues for as long as it remains inside the  body. The time the radioactive material remains in the body will depend on the way it was taken in and the physical and chemical form of the material.  Radioactive gas will have a short residence time, while inhaled aerosols will take longer to clear the lungs. Ingested material will reside for some time within  the gastrointestinal tract but then radioactive elements will be taken into the body and metabolized in the same way as non-radioactive forms of the same  chemical element. What levels of radiation do people receive?  Radiation is a natural phenomenon. The earth itself is composed of minerals that contain the naturally occurring radioactive elements uranium and thorium.  This presence creates a field of radiation, which varies from place to place depending on the local geology. Cosmic radiation from the sun and from outer  space also continually penetrates the earth's atmosphere adding to this field that represents a source of external exposure. The inert radioactive gas, radon,  is created by the uranium and thorium in the soil; it percolates through the soil, and concentrates in the indoor air of buildings. Inhalation of radon gas leads  to internal exposure to radiation, which varies significantly from place to place depending on the nature of the buildings and local geology. In addition, small  amounts of other naturally occurring radioactive materials are present in foodstuffs and water and contribute to internal exposure. The external and internal  exposure together deliver a small dose of radiation to everyone on the planet, known as background radiation. In addition to radiation dose received from  natural sources of background radiation, a number of human activities enhance exposure, e.g. flying at altitude (greater levels of cosmic radiation), medical  uses of radiation, the generation of nuclear energy, and other industrial uses of radiation or radioactive material.  A measure of the total radiation dose received is expressed in the unit sievert (Sv) or fractions according to the metric system: a millisievert (mSv) is one-  thousandth of a sievert; a microsievert (µSv) is one-millionth of a sievert. The rate of accumulation is expressed as dose rate or dose accumulated per unit  time e.g. in units of microsieverts per hour (µSv/h). A direct measurement can be made of radiation dose rates from sources external to the body in  microsieverts per hour. The dose received by a person is the given by the dose rate multiplied by the time of exposure. The intake of radioactive material into the body is expressed in terms of becquerels. One becquerel (Bq) represents one disintegration of a radioactive atom  per second. The radiation dose arising from the intake is also expressed in sieverts, and is assessed using the concentration of radioactivity in food, water or  air (Bq/kg or Bq/L or Bq/m3), the intake rate, the metabolism of the particular radionuclide and its half-life.