How do radioactive elements form naturally?

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Christian Küppers

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Born in 1958, is a graduate physicist and has been employed as a scientific employee in the field of nuclear technology and plant safety at the Öko-Institut e.V., Darmstadt office, since 1987. His work focuses on radiation protection, disposal of radioactive waste and environmental impact assessments.

Radiation occurs naturally on our planet. It is never completely harmless, the dose plays a major role. When is radiation dangerous for humans?

The two reactor domes of the Neckarwestheim nuclear power plant protrude against the backdrop of the municipality of Neckarwestheim, around 40 kilometers north of Stuttgart. (& copy AP)

Radioactive substances, called radionuclides, emit various types of radiation when they decay, which due to their high energy can generate charged particles (ions), e.g. in human tissue. This is why one speaks of ionizing radiation. The harmful effects of this radiation have long been known. The risk resulting from a certain radiation dose can now also be assessed well. The corresponding data are based primarily on long-term studies of the survivors of the two atomic bombs in Japan (Hiroshima and Nagasaki). The results of these investigations are supplemented and confirmed by further analyzes of a large number of people who were exposed to relatively high levels of radiation (e.g. patients on whom the radioactive contrast agent Thorotrast was used, or employees in nuclear weapons production).

The higher the dose, the greater the risk

Ionizing radiation can damage the human body in a number of ways. A distinction must be made between low radiation intensity and very high radiation intensity. Even a low dose of radiation can cause cancer or genetic damage in offspring. The severity of the damage does not depend on the level of the radiation dose, but the probability of damage occurring (so-called stochastic damage). The higher the radiation, the more likely a disease or a change in the genetic make-up will occur. A lower threshold for this effect is not known. It is therefore common in radiation protection to assume that any radiation exposure, no matter how low, can cause cancer, but with a lower probability. In addition, there are now indications of other possible damage that could be caused by ionizing radiation, for example diseases of the cardiovascular system.


With very high doses of radiation, deterministic (acute) damage occurs, the type and severity of which in turn depends on the level of the dose. These include, for example, clouding of the lens of the eye through to acute damage as a result of very high doses of radiation, especially in the event of accidents (e.g. firefighters at the Chernobyl accident reactor or workers who died from uncontrolled chain reactions while handling uranium and plutonium). Very high doses of radiation can lead to death within a few days, as they damage the blood-forming tissue.

The two basic principles of radiation protection are derived from this possible damage, depending on the dose level, which are the basis of international recommendations and the German Radiation Protection Ordinance: First, every activity that can lead to radiation exposure must be justified. It must therefore serve a special purpose that cannot be achieved in any other way or only with unreasonable effort. Second, dose limits must be adhered to, which exclude acute radiation damage and limit the risk of so-called stochastic damage.

Sources of radiation exposure

The majority of the average radiation exposure comes from natural sources. These are radionuclides that were formed when our universe was formed, for example uranium-238 with a half-life of 4.5 billion years. The half-life is the time after which half of the existing amount has been converted by radioactive decay. Until the end of another half-life, half of the remaining half decays, i.e. a quarter and so on. For example, after ten half-lives, about 0.1 percent of the original amount is still present. From the origin of our universe there are mainly uranium and thorium. There are also natural radionuclides that are constantly produced by radiation from the cosmos, for example tritium and carbon-14.

The natural radiation exposure is mainly due to radon-222, which arises from radium-226 in the decay chain of uranium-238. Uranium and radium can be found in the ground, in ores or in building materials. As long as they stay there, they contribute little to radiation exposure. Only when radium-226 breaks down into the noble gas radon-222 can this gas be released from the soil and rock and spread into the environment. Radon and its decay products can then be inhaled and lodge in the lungs.

The intense radiation of the decay series leads to radiation exposure of the lung tissue, which is held responsible for around 5 percent of deaths from lung cancer in Germany. The level of radiation exposure varies from region to region. In areas with a higher concentration of radium in the soil, the entry of radon into living spaces can be technically reduced, in particular through gas-tight foundations or targeted ventilation of the soil under the house. When you are outdoors, the exposure is usually significantly lower than inside buildings.