In a large variety of situations, human beings are exposed to the radiation of radioactive material - sometimes more, sometimes less. There are many different varieties of radiation - most radioactive material in the environment come from natural sources.

Natural background radiation can be distinguished into cosmic radiation and terrestrial sources. Terrestrial radiation originates from the soil, rocks, water, air. Cosmic radiation relates to sources found in outer space. Cosmic radiation for example leads to a higher radiation exposure for people on an airplane compared to those staying on the ground.

Additionally, there is man-made radiation, which originates from man-made devices, and which is commonly found in modern medicine, e.g. x-rays, nuclear medicine or other types of radiation used during cancer therapies. Man-made radiation is also caused e.g. during the technical use of radionuclides, during the transportation of radioactive substances or radioactive waste, and during the operation of nuclear power plants.

Both natural and man-made radiation have the same effects because they are both based on the same physical and biological processes. The effects are only dependent on the type of radiation (alpha- beta-, gamma- (α-, ß-, γ-) or neutron radiation) and the energy or rather the doses absorbed by the body. The differing effects of those radiation types can partly be explained by differing levels of penetration into the human body and the varying spread of the absorbed energy inside the body as a result of this penetration.

Atoms are the main building blocks of matter. They are comprised of an atomic nucleus and an electron shell. The atomic nucleus is made up of protons and neutrons. This combination is called nuclide. The neutrons and protons are always arranged in specific proportions. These proportions determine whether a nuclide is stable or whether it may transform itself into a different nuclide by emitting radiation. This characteristic is called radioactivity or radioactive decay. Depending on the type of decay, the nuclide will either emit α- or ß-particles, γ-radiation or neutrons of certain energy.

This process of nuclide transformation- also called radioactive decay - is measured in Becquerel (Bq). 1 Bq equates to one transformation or decay per second. Over time, the activity levels of radioactive material decrease. It is a law of nature that a given amount of a radioactive nuclide will decrease by 50% over a given period of time. This is the so-called radioactive half life, which varies with each nuclide and which is a distinctive feature of each individual nuclide. It can range from a split second to several billion years.

The dominant man-made radiation source is x-ray radiation. X-ray radiation is not a result of radioactive decay and does not originate in an atomic nucleus, but is produced in x-ray equipment by the deceleration of electrons inside atoms. When radiation energy - regardless of whether it is caused by the transformation of atomic nuclei or produced in x-ray equipment - impacts on humans, it is absorbed by the human’s tissue and may cause damage to it. The absorbed energy is called dose. It is measured, depending on the radiation type (for α, ß, γ, neutrons and x-rays) in Sievert (Sv) or Millisievert (1 mSv = 1/1000 Sv). In Germany, people are on average exposed to a dose of 2.1 Millisievert (mSv) of natural radiation annually; the mean dose emitted by medical treatments is also approximately 2mSv, while other man-made contribution to the exposure is considerably lower.