Ionizing radiation, health effects and protective measures (2023)

29. April 2016

All radionuclides are uniquely identified by the type of radiation they emit, the energy of the radiation, and their half-life.

Activity - used as a measure of the amount of radionuclide present - is expressed in a unit called becquerels (Bq): a becquerel is one decay per second. The half-life is the time it takes for the activity of a radionuclide to decay to half its initial value. The half-life of a radioactive element is the time it takes for half of its atoms to decay. This can range from a fraction of a second to millions of years (e.g. iodine-131 has a half-life of 8 days while carbon-14 has a half-life of 5730 years).

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radiation sources

People are exposed to both natural and man-made sources of radiation on a daily basis. Natural radiation comes from many sources, including more than 60 naturally occurring radioactive materials found in soil, water and air. Radon, a naturally occurring gas, escapes from rock and soil and is the main source of natural radiation. Every day people inhale and absorb radionuclides from the air, food and water.

Humans are also exposed to natural radiation from cosmic rays, especially at high altitude. On average, 80% of the annual dose of background radiation received by a person comes from naturally occurring terrestrial and cosmic radiation sources. Background radiation levels vary geographically due to geological differences. Exposure in certain areas can be more than 200 times the global average.

Human exposure to radiation also comes from man-made sources ranging from nuclear power generation to the medical use of radiation for diagnosis or treatment. Today, medical equipment, including X-ray machines, are the most common man-made sources of ionizing radiation.

Exposure to ionizing radiation

Radiation exposure can be internal or external and can be acquired through various exposure routes.

Internal ExposureIonizing radiation occurs when a radionuclide is inhaled, ingested, or otherwise enters the bloodstream (e.g., through injection or through wounds). Internal exposure ends when the radionuclide is eliminated from the body either spontaneously (e.g. through excretions) or as a result of treatment.

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Outside exposurecan occur when airborne radioactive material (such as dust, liquids, or aerosols) deposits on skin or clothing. This type of radioactive material can often be removed from the body simply by washing.

Exposure to ionizing radiation can also result from irradiation from an external source, such as a B. medical radiation exposure to X-rays. External radiation stops when the radiation source is shielded or when the person moves outside the radiation field.

People can be exposed to ionizing radiation in different circumstances, at home or in public places (public exposure), at their place of work (occupational exposure) or in a medical setting (as well as patients, carers and volunteers).

Exposure to ionizing radiation can be divided into 3 exposure situations. The first, planned exposure situations result from the deliberate introduction and operation of radiation sources with specific purposes, such as the medical use of radiation for the diagnosis or treatment of patients or the use of radiation in industry or research. The second type of situation, existing exposures, is when an exposure already exists and a decision needs to be made to control it – for example exposure to radon in homes or workplaces, or exposure to natural background radiation from the environment. The last type, emergency exposure situations, results from unexpected events that require an immediate response, such as: B. nuclear accidents or malicious acts.

Medical use of radiation accounts for 98% of the population dose contribution from all man-made sources and accounts for 20% of total population exposure. Annually, more than 3600 million diagnostic radiological exams, 37 million nuclear medicine procedures and 7.5 million radiation therapy treatments are performed worldwide.

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Health effects of ionizing radiation

Radiation damage to tissues and/or organs depends on the radiation dose received or the absorbed dose, which is expressed in a unit called Gray (Gy). The potential harm from an absorbed dose depends on the type of radiation and the sensitivity of different tissues and organs.

Thateffective doseused to measure ionizing radiation in terms of damage potential. The sievert (Sv) is the unit of effective dose that takes into account the type of radiation and sensitivity of tissues and organs. It's a way to measure ionizing radiation in terms of its potential for harm. The Sv takes into account the type of radiation and the sensitivity of tissues and organs.

The Sv is a very large unit, so it's more practical to use smaller units like millisievert (mSv) or microsievert (μSv). There are thousand μSv in one mSv and thousand mSv in one Sv. In addition to the amount of radiation (dose), it is often useful to express the rate at which that dose is delivered (dose rate), e.g. B. microsievert per hour (μSv/hour) or millisievert per year (mSv/year).

Above certain threshold values, radiation can impair the function of tissues and/or organs and cause acute effects such as reddening of the skin, hair loss, radiation burns or acute radiation syndrome. These effects are more severe at higher doses and higher dose rates. For example, the dose threshold for acute radiation syndrome is around 1 Sv (1000 mSv).


With a low radiation dose and/or longer release (low dose rate), the risk is significantly lower because the probability of damage repair is higher. However, there is still a risk of long-term consequences such as cancer, which can appear years or even decades later. Effects of this type do not always occur, but their probability is proportional to the radiation dose. This risk is higher in children and adolescents because they are significantly more sensitive to radiation exposure than adults.

Epidemiological studies of radiation-exposed populations, such as atomic bomb survivors or radiation therapy patients, have shown a significant increase in cancer risk at doses above 100 mSv. More recently, some epidemiological studies on subjects exposed to medical exposures during childhood (child CT) suggested that cancer risk may increase even at lower doses (between 50 and 100 mSv).

Prenatal exposure to ionizing radiation can cause brain damage in fetuses after acute doses greater than 100 mSv between 8 and 15 weeks of gestation and 200 mSv between 16 and 25 weeks of gestation. Before the 8th or after the 25th week of pregnancy, human studies have shown no risk of radiation to fetal brain development. Epidemiological studies indicate that the risk of cancer after fetal radiation exposure is similar to that after exposure in early childhood.

WHO response

WHO has established a radiation program to protect patients, workers and the public from the health risks of radiation exposure in planned, existing and emergency exposure situations. This program focuses on aspects of radiation protection in the field of public health and includes activities related to the assessment, management and communication of radiation risks.

In line with its core function of setting norms and standards and promoting and monitoring their implementation, WHO has worked with 7 other international organizations to revise and update the International Basic Safety Standards on Radiation (BSS). WHO adopted the new international BSS in 2012 and is currently working to support the implementation of the BSS in its Member States.

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