Basic Data on Radioactivity

Basic Data on Radioactivity and its Effects on Health, the Population and the Environment

Radioactivity is a natural phenomenon and is commonly found in the environment in which we live. It is an umbrella term for several types of radiation and is also used professionally to refer to “ionising radiation“.

As a normal natural phenomenon, it has certain characteristics that we as humanity have learned to use to our advantage over the last 100 years. Its uses are truly diverse. For example, we use it in medicine for examination purposes, but also as a therapy, in industry to sterilize food, or to check the strength of welds. By understanding the principle of fission of radioactive elements, we have also come to their peaceful use for energy production.

However, radioactivity also has negative aspects and should be handled with care. Over the years, a number of principles have evolved, to mention just two:

  • ALARA (“As low as reasonably achievable”), which is the principle that every person, and especially every worker, who comes into contact with radiation should be exposed to as little of it as possible – as little as is reasonably achievable.
  • a cautious approach – especially present in nuclear facilities, where employees should always ask whether what they are going to do carries any negative consequences.

On the following pages you will find more detailed information on the nature of ionising radiation, the ways in which it can interact with the human body, the consequences of overexposure to radiation and how to protect yourself against the negative effects of radiation.

Ionizing Radiation, Sources of Ionizing Radiation

Ionizing radiation is radiation whose particles have enough energy in their interaction to release an electron from the electron shell of the nucleus.

Charged and uncharged particles of ionising radiation transfer their energy to the material environment as they pass through the mass, with the greatest transfer of energy occurring in the form of ionisation of atoms. When a neutral atom is ionised, an electron is knocked out of the electron shell of the nucleus, producing a positive and a negative ion.

Particles can acquire sufficient energy to ionize the environment directly through the radioactive conversion of a radioactive substance (radionuclide), through various charged particle acceleration processes occurring in space and technological devices, or through other physical processes leading to the creation of a particle with the potential to directly or indirectly ionize the environment. Sources of ionising radiation are divided into natural and artificial.

Natural sources of radiation are those that exist in nature independently of human activity, such as cosmic rays and ionising radiation produced by the conversion of natural radionuclides found in water, soil and air.

Artificial sources of ionising radiation are those sources which are man-made:

  • equipment that produces ionising radiation – for example: x-ray tubes, particle accelerators, nuclear reactors,
  • radioactive sources containing artificial radionuclides – for example: radionuclides produced by irradiation of inactive materials or by fission, but also including radioactive sources produced by the treatment or processing of natural radioactive materials.

Ionising radiation is divided into directly ionising and indirectly ionising radiation. Directly ionising radiation is made up of charged particles that have enough energy to ionize. For example, alpha radiation, beta radiation, proton radiation. Indirectly ionising radiation is made up of uncharged particles that have enough energy to release directly ionising (charged) particles or to cause a change in the nucleus that can then cause ionisation. These include, for example, gamma rays, X-rays and neutron rays.

Basic characteristics of common types of ionizing radiation:

Alpha (a) radiation (and the particles are composed of 2 protons and 2 neutrons)

  • has little penetration, it is shielded by a few cm of air or a sheet of paper;
  • in soft tissue penetrates only a fraction of a millimetre,
  • depending on the energy, an alpha particle can create more than a hundred thousand ion pairs as it passes through a mass before it is blocked,
  • causes serious damage to cells or organs if internal contamination with alpha radionuclides occurs;

Beta (b) radiation (made up of negative electrons or positive positrons)

  • is more penetrating than alpha radiation, mainly light materials (plastic, aluminium) are used for shielding,
  • it penetrates decimetres to metres in air, millimetres to a few centimetres in soft tissue,
  • external exposure to beta radiation can cause a high dose in the skin and in the lens of the eye,
  • internal beta contamination results in higher doses in organs where beta radionuclides accumulate.

Neutron radiation (n) (formed by particle neutrons)

  • neutrons are very penetrating;
  • they can cause serious biological damage;
  • neutrons can be shielded by materials containing hydrogen (e.g. water, polyethylene).

Gamma radiation (g) ( photon radiation, a photon has neither mass nor charge)

  • is emitted from the nucleus of the atom, it is very penetrating;
  • can be shielded especially by heavy materials (e.g. lead, steel, concrete);
  • poses a risk from both external and internal radiation;


  • arises in electrical devices when electrons are blocked or when the energy of the electrons in the shell of an atom is changed;
  • has the same properties as gamma rays, but often has lower energy and therefore less penetrating power.

B. Exposure and Exposure Pathways

Humans are permanently exposed to ionising radiation because they live in an environment where there are both natural and artificial sources of ionising radiation. The ways in which ionising radiation affects humans are called exposure pathways.

If the source of ionising radiation is outside the human body we call it external exposure. If the ionising radiation is emitted by radionuclides that are inside the organism they irradiate, we call this internal exposure.

a) Exposure to natural ionising radiation

External exposure

The terrestrial component and cosmic rays contribute to the external exposure to natural ionising radiation. Terrestrial radiation is caused by radionuclides found in almost all geological formations. However, the nuclide composition and activity of individual rocks and minerals varies widely, so the level of radiation depends on local conditions. The level of cosmic ray exposure depends on altitude, latitude and the phase of the solar cycle. The dose increases with altitude and latitude.

Internal exposure

Natural radionuclides are present not only in soil but also in air and water. Radionuclides that are in a soluble form are also present in surface water but especially in groundwater. Gaseous natural radionuclides are also present in the atmosphere. They enter the atmosphere by diffusion from the Earth’s crust or are produced in the atmosphere by the interaction of cosmic rays. Of the gaseous natural radionuclides, the radioactive isotopes of radon, water vapour containing the radioactive isotope of hydrogen – tritium, carbon dioxide containing the natural radioactive isotope of carbon are mainly found in the environment. Radioactive aerosols are small dust particles that are composed of or trapped by naturally occurring radioactive nuclides.

Radionuclides normally found in air, water or soil are absorbed by plants mainly from soil and irrigation water. Some radionuclides may be absorbed by plants through respiration or by above-ground parts.

Radionuclides present in elevated forms in the atmosphere, in water and in feedstuffs may be a source of internal contamination of livestock.

Radioactive nuclides normally enter the human body through breathing (inhalation) and ingestion of food and water. Consumption of agricultural products, whether plant or animal, fruits, teas, game, mushrooms contain natural radionuclides. Similarly, drinking water in which minerals containing radionuclides are dissolved contributes to the total intake of radionuclides.

b) Exposure to artificial sources of ionising radiation

In addition to natural radionuclides, gaseous substances and aerosols are released into the atmosphere and the ecosystem as a whole, for example from nuclear installations or from sites where radioactive sources are produced or used. The most common are radioactive isotopes of noble gases (argon, krypton, xenon), the radioactive isotope carbon (C-14) in the form of CO2 or as an organic gas, gaseous forms of iodine and other volatile substances, and radioactive aerosols containing a radionuclide or a mixture of radionuclides that are used in the workplace.

The full spectrum of artificial radionuclides also enters the environment through the discharge of effluents from nuclear facilities, radiochemical laboratories or nuclear medicine sites into surface water (rivers, seas) and the release of solid radioactively contaminated materials for reuse or landfill.

It is only permitted to discharge (gaseous and liquid) or release (solid materials) into the environment if they have a mass or volume activity below the permissible limit, and the total activity released over a period of time (per day, month, year) is also limited. The permissible activities are set by legislation and decisions of public health authorities.

Artificial radionuclide activity in the atmosphere, in soil and in water was also contributed to by the atmospheric nuclear weapons experiments in 1945-1980 and the Chernobyl and Fukushima accidents.

Artificial radionuclides contribute to radiation exposure through similar exposure pathways as natural radionuclides.

However, the largest contributor to the average exposure from artificial sources of ionising radiation is the exposure to which patients are exposed during diagnostic radiological methods using X-rays and the use of radionuclides for diagnostic purposes.

Natural radioactive nuclides (40K, 14C, 226Ra) are present in the human body in approximately constant concentrations. A balance has been established between their intake by food and their excretion. Short-term or single intakes of a radionuclide may occur during the handling of radioactive substances or contaminated equipment or in the event of an accident at a nuclear installation. In this case, the dose depends on how quickly the activity of the radionuclide in the body decreases. The rate of activity decline depends on the physical half-life of the radionuclide and the rate of metabolic excretion of the radionuclide from the body. The rate of activity depletion is characterised by the effective half-life. It is the time after which the activity of a radionuclide in the body is halved. In internal exposure, there is usually uneven exposure to individual organs because some radionuclides are not dispersed evenly in the body but accumulate in a particular organ or tissue. The organ in which the radioactive nuclide produces the greatest exposure is called the critical organ for that particular nuclide. The effective half-lives and critical organs of several important radioactive nuclides are given in the following table.

Source of radiation Annual effective dose
Average Typical scope
Directly ionising and photon component 0,28
Neutron component 0,10
Cosmogenic radionuclides 0,01
SUM 0,39 0,3 – 1,0
When staying outdoors 0,07
When staying indoors 0,41
SUM 0,48 0,3 – 1,0
Inhalácia Uranium thorium decay series 0,006
Radon (Rn-222) 1,15
Thoron (Rn-220) 0,10
SUM 1,26 0,2 – 10
Ingestion K-40 0,17


Internal exposure depends on the physical properties of the nuclide (type and energy of the emitted radiation, half-life), its chemical form (soluble or insoluble form) and on its biochemical and physiological properties (rate and extent of absorption in the gastrointestinal tract, subsequent deposition and excretion).

C. Quantities Expressing the Degree of Severity of Exposure

In radiation protection, different quantities are used to express the severity of exposure. In this material, for simplicity, only the effective dose is used, the unit of which is Sv – Sievert. This quantity gives a sense of the probability of health damage from low-dose whole-body exposure (stochastic effects), taking into account not only the magnitude of the dose, but also which organs were exposed and the type of radiation that caused the dose. In addition, radiation protection also uses a dose-equivalent quantity that applies only to a specific organ. Its unit is also the Sv.

In the case of high dose exposures, where deterministic effects may occur, the quantity absorbed dose is used. The unit of this quantity is the Gray (Gy),

D. Effects of Ionising Radiation on Humans

In most materials, recombination occurs within a very short time after ionization, whereby free electrons and ions are paired to form a neutral atom. In some substances, ionisation may result in chemical changes (e.g. in photographic emulsions) or physical changes (e.g. colour change, change in conductivity).

The effects of radiation on living organisms depend on the amount of energy the radiation has imparted in matter, the type of radiation (how much energy the radiation has imparted per unit length of its pathway), and which organ or organs have been irradiated. Not all tissues and organs are equally sensitive to radiation. The most sensitive organs and tissues are those in which new cells are formed most intensively, including the haematopoietic organs and epithelial cells in the digestive tract. The most resistant are nerve cells, muscle cells and bone cells.

Effects on humans are divided into deterministic and stochastic.

Deterministic effects are those that we can identify as a consequence of exposure. They occur only at high doses, above the threshold dose. For exposures below the threshold dose, clinical symptoms do not appear and a deterministic effect does not occur. The higher the dose above the threshold level, the more severe the course of the disease will be. Deterministic effects usually appear after a short period of time, within hours to weeks, the higher the dose, the sooner. Deterministic effects include, for example, acute radiation sickness, opacity of the lens of the eye and various degrees of erythema. Examples of threshold doses: skin erythema 2-5 Gy, irreversible skin damage 20-40 Gy, permanent sterility 3-6 Gy, ocular cataract 0.5 Gy, acute radiation sickness more than 3 Gy.

Stochastic effects are non-specific, meaning that the same disease can be caused by causes other than radiation exposure. The higher the dose, the more likely the disease is to occur, but the severity of the disease is not related to the dose. A linear dose dependence of the probability of a stochastic effect is assumed. However, the latency period (time from exposure to the onset of clinical symptoms) can be very long, up to decades, for stochastic effects. Stochastic effects include tumours (carcinomas) and genetic damage.

A linear dose dependence of cancer risk is evident from epidemiological studies. Three major epidemiological studies are presented in the above graph as a gold standard. A study of persons who were exposed to the atomic bomb explosion in Hiroshima and Nagasaki but survived (more than 80,000 follow-ups), a study following nuclear power plant workers (more than 400,000 follow-ups), and a study of patients who were exposed to medical radiation (more than 800,000 follow-ups). The hormesis model is a low-dose beneficial effect phenomenon, whereas the Supra-linear model is based on the fact that low dose levels have a more detrimental effect than higher dose levels.

E. Exposure Level

a) Exposure to natural ionising radiation

The level of exposure to natural sources of ionising radiation is not the same everywhere. The 2008 report of the United Nations Scientific Committee on the Effects of Ionising Radiation (UNSCEAR) gives the global average annual effective dose per capita from natural and artificial sources of ionising radiation.

The average annual effective dose from all natural sources is approximately 2.4 mSv/y.
According to the data presented in the UNSCEAR Report of 2000, the average effective dose in Slovakia is about 3.2-3.5 mSv per year. It should be noted that annual effective doses in the range of 1 to 13 mSv per year are common, and there are also locations in the world where exposure from natural sources is in the tens of mSv per year range (e.g. Guarapari area in Brazil, Ramsar in Iran, Yangijang in China, Kerala in India, etc.).

The following table provides an overview of the doses from each exposure pathway from natural ionising radiation. The largest contribution to the effective dose is due to inhalation of the natural radioactive noble gas radon, and this is greatest for indoor exposure. Radon is found in various rocks and therefore in soil, water and the atmosphere. Radon released into living spaces from building materials or seeping from the soil and geological bedrock into the interior of a building contributes to human exposure.

Source of radiation Annual effective dose
Average Typical scope
Directly ionising and photon component 0,28
Neutron component 0,1
Cosmogenic radionuclides 0,01
SUM 0,39 0,3 – 1,0
When staying outdoors 0,07
When staying indoors 0,41
SUM 0,48 0,3 – 1,0
Inhaling Uranium thorium decay series 0,006
Radon (Rn-222) 1,15
Thoron (Rn-220) 0,1
SUM 1,26 0,2 – 10
Ingestion K-40 0,17

b) Exposure to artificial sources of ionising radiation

The above-mentioned UNSCEAR Report also provides data on the average contribution of artificial sources of ionising radiation to human exposure. Medical exposure is the largest contributor. According to the UNSCEAR report, the global average effective dose from medical exposure was 0.65 mSv per year in 1997-2007.

The nuclear-fuel cycle accounts for only a tiny fraction of the global average human dose. The main global contributors are long-lived radionuclides, especially tritium and the radioactive isotope carbon (C-14), and to a lesser extent Cs-137 and Sr-90 radioisotopes and trans-urans. According to the UNSCEAR report cited above, the global average dose per individual due to the nuclear fuel cycle is less than 0,0002 mSv per year.

Contributing to the global exposure of the population to man-made radioactive substances is radiation from fallout, which is radioactive substances that were released into the atmosphere during atmospheric nuclear weapons experiments in the last century. The 2008 UNSCEAR Report estimated that the global average effective dose to an individual due to fallout was less than 0.005 mSv per year. In 1963, the average effective dose per person due to radionuclides released in atmospheric experiments reached a maximum of 0.11 mSv.

For the first year after the Chernobyl accident (1986), the average effective dose to an adult in Slovakia from radionuclides released into the environment during and after the accident was 0.22 mSv (Report on the radiation situation in the SSR after the accident at the Chernobyl nuclear power plant, VÚPL Bratislava 1987).

However, the average exposure of the Slovak population to medical radiation is significantly higher than the global average. The Slovak Republic is one of the countries with a high standard of medical care and thus we are subjected to medical exposure more often than the global average. In countries with advanced medical care, among which Slovakia is one, the average per capita dose from diagnostic medical exposure is up to 1.9 mSv.

According to the cited UNSCEAR Report from 2008, the average effective dose to the patient for a CT scan of the chest is 7.8 mSv, for a CT scan of the abdomen 12.4 mSv, for a CT scan of the pelvis 9.4 mSv, and for a CT scan of the head 2.4 mSv. For radiological interventional methods, the average doses, depending on the type of scan, range from 5 to 12 mSv. However, there are also interventional examinations where the effective dose can reach several tens of mSv. The average effective dose to the patient is 0.3 mSv for a mammographic X-ray, 3.4 mSv for an upper gastrointestinal X-ray, 7.4 mSv for a lower gastrointestinal X-ray, and 0.1 – 0.2 mSv for a routine chest X-ray.

In addition to the global exposures mentioned above, short lived radionuclides contribute to radiation exposure at the site where the nuclear installation is in operation. In the area of both Jaslovské Bohunice and Mochovce, doses to the population in the vicinity are calculated by modelling calculations. The doses to the population due to the operation of nuclear installations are so low that it is not possible to measure them directly. According to the calculations performed, the exposure of representative persons (the most exposed) in the vicinity of both Bohunice and Mochovce in recent years is below 0.001 mSv per year.
For the sake of interest, it is worth mentioning that the average effective dose to workers who entered the controlled area at nuclear facilities in Slovakia at least once in 2013 was 0.24 mSv per year (the data is from the Annual Report of the Public Health Authority of the SR for 2014). These doses are so low because very sophisticated and complex approaches are used to regulate the doses, but this is also due to the fact that many of the monitored workers are only in the controlled area for a very short period of time and are only in areas where radiation levels are not elevated. The maximum effective doses to a worker in nuclear installations in 2013 did not exceed 15 mSv.

F. Ionising Radiation Protection System

The primary objective of radiation protection is to ensure that deterministic effects do not arise and that the risk of stochastic effects is as low as reasonably achievable. Radiation protection is guided by three principles:

  • (1) Principle of justification states that sources of ionising radiation may only be used if the benefit of their use outweighs the harm to health that the use of ionising radiation sources may cause.
  • (2) Principle of optimalization says that in all circumstances the level of exposure should be kept as low as reasonably achievable.
  • (3) Principle of limitation states that, in activities leading to exposure, the exposure of workers and members of the public must not exceed the established exposure limits. The limit for exposure of workers is 20 mSv per year, ) the limit for exposure of the public to sources of ionising radiation used in activities leading to exposure is 1 mSv. Exposure of the public to natural sources of radiation and medical exposure of the patient are not limited and are not included in the exposure.

Basic rules for limiting exposure:

  • Time – the shorter the exposure, the lower the dose,
  • Distance – the greater the distance a person is from the source of ionising radiation, the lower the dose,
  • Shielding – placing shielding (absorbing) material between the radiation source and the person reduces the dose.

G. Radiation Protection in the Event of an Accident at a Nuclear Installation

The Emergency Plan plays an important role in the emergency protection system. The plan shall contain, inter alia, the actions and response levels at which actions will need to be taken and the reference levels for the management of exposure of response workers. The measures are intended to ensure that the doses to the public and to workers who may be exposed as a result of the accident are as low as achievable, so that deterministic effects do not occur.

An accident at a nuclear installation (Section 27(3)(c) of Act No 541/2004 Coll.) is considered to be a situation in which urgent measures may be necessary to protect workers and the public. In such a situation, the nuclear installation (e.g. nuclear reactor) may not be fully under control and radioactive substances may be released into the environment.

Leaking radioactive substances can spread primarily through the atmosphere in the form of a radioactive plume. A radioactive plume contains:

  • gaseous radioactive substances – in particular radioactive isotopes of the noble gases krypton and xenon, iodine, volatile substances such as tellurium, caesium, ruthenium
  • radioactive aerosols containing a mixture of radionuclides, including radionuclides which are less or slightly volatile (e.g. strontium, barium, transurans)

Particulate matter may be released from the plume and settle on accessible surfaces. Surface contamination of soil, objects, plants and uncovered people and animals occurs. If it rains during the passage of a radioactive plume, radionuclides are washed out of the atmosphere and thus become more sedimented (deposited) on the Earth’s surface. In open terrain, external exposure of persons to the radioactive plume and to the radionuclides deposited on the surface. In addition, inhalation of radioactively contaminated air contributes to radiation exposure. It is assumed that at this stage people in the affected area will not consume water and food that could be contaminated. In the late phase and beyond, it is envisaged that contamination of food will be monitored and only food meeting the criteria will be allowed to be consumed.

Protective measures – interventions – are implemented to limit exposure in an emergency situation. The aim of protective measures is to reduce exposure by limiting or interrupting exposure pathways.

As the situation is not, or may not be, fully under control in an accident, there are no limits on the exposure to persons involved in the response or to the population that may be exposed as a result of the accident.

However, the exposure of persons involved in emergency response activities (nuclear operators, firefighters, rescue workers, police officers, evacuation bus drivers, members of monitoring groups, etc.) is planned and managed so that reference levels are not exceeded.

Reference levels for exposure for activities to prevent the development of serious effects of ionising radiation on human health or to prevent the development of a radiological emergency with potentially serious societal and economic consequences may be established for an effective dose to intervening staff from external exposure for occupational exposure in an emergency situation greater than 100 mSv but not greater than 500 mSv. If it is not possible to comply with the exposure limits for the worker, then other rescue work shall be planned and carried out in such a way that the effective dose to the persons carrying out the work does not exceed 100 mSv and the equivalent dose to the skin is 500 mSv throughout the performance of the work. (§ 145 of Act No 87/2018 Coll.).

Precautions for the protection of the public shall be taken so that deterministic effects are excluded in any case and exposures are as low as reasonably achievable. When deciding on the implementation of the measures, the general criteria for the adoption of protective measures pursuant to Annex 12 of Act No 87/2018 Coll. shall be taken into account.

Updated: 06.09.2022