What is ionising radiation?
Ionising radiation is a form of energy released by atoms in the form of electromagnetic waves (gamma or X-rays) or particles (neutrons, beta or alpha). The spontaneous decay of atoms is called radioactivity, and the excess energy released is a form of ionising radiation. Unstable elements produced by decay and emitting ionising radiation are called radionuclides.
All radionuclides are uniquely identified by the type of radiation they emit, their radiant energy and their half-life.
The activity, used as an indicator of the amount of radionuclide present, is expressed in units called becquerels (Bq): one becquerel is one act of decay per second. The half-life is the time it takes for the activity of a radionuclide to decay to half its original value. The half-life of a radioactive element is the time it takes for half of its atoms to decay. It can range from fractions of a second to millions of years (e.g. the half-life of iodine-131 is 8 days, and that of carbon-14 is 5,730 years).
Sources of radiation
Humans are exposed to natural and artificial radiation every day. Natural radiation comes from numerous sources, including more than 60 naturally occurring radioactive substances in soil, water and air. Radon, a naturally occurring gas, is formed from rocks, soil and is the main source of natural radiation. People breathe in and absorb radionuclides from the air, food and water every day.
People are also exposed to natural radiation from cosmic rays, especially at high altitudes. On average, 80% of the annual dose that humans receive from background radiation comes from naturally occurring terrestrial and cosmic ray sources. Levels of such radiation vary in different regimes and in some areas the level can be up to 200 times higher than the global average.
Humans are also affected by radiation from man-made sources, ranging from nuclear power production to medical uses of radiation diagnosis or treatment. The most common artificial sources of ionising radiation today are medical devices such as X-ray machines and other medical devices.
Exposure to ionising radiation
Exposure to radiation may be internal or external and may occur in different ways.
Internal exposure to ionizing radiation occurs when radionuclides are inhaled, absorbed or otherwise enter the bloodstream (e.g. by injection, wounding). Internal exposure ceases when the radionuclide is eliminated from the body either spontaneously (with feces) or as a result of treatment.
External radioactive contamination can occur when radioactive material in the air (dust, liquid, aerosols) is deposited on skin or clothing. Such radioactive material can often be removed from the body simply by washing.
Exposure to ionising radiation can also occur as a result of external radiation from an appropriate external source (such as exposure to radiation emitted by medical X-ray equipment). External exposure ceases when the radiation source is closed or when a person moves out of the radiation field.
People can be exposed to ionising radiation in different circumstances: at home or in public places (public exposure), in their workplaces (workplace exposure) or in health care facilities (patients, carers and volunteers).
Exposure to ionising radiation can be classified into three cases of exposure.
The first case is planned exposure, which is due to the deliberate use and operation of radiation sources for specific purposes, such as in the case of medical use of radiation for diagnosis or treatment of patients, or the use of radiation in industry or for scientific research purposes.
The second case is existing sources of exposure where exposures already exist and where appropriate control measures need to be taken, e.g. exposure to radon in homes or workplaces or exposure to background natural radiation in environmental conditions.
The latter case is exposure in emergency situations due to unexpected events involving prompt action, e.g. in the case of nuclear accidents or malicious acts
Medical uses of radiation account for 98% of all radiation dose from all man-made sources; they account for 20% of total population exposure. Each year there are 3 600 million radiological examinations for diagnostic purposes, 37 million procedures using nuclear materials and 7.5 million radiotherapy procedures for therapeutic purposes worldwide.
Health consequences of ionizing radiation
Radiation damage to tissues and/or organs depends on the radiation dose received or absorbed, which is expressed in grays (Gy).
The effective dose is used to measure ionising radiation in terms of its potential to cause harm. Sievert (Sv) is the unit of effective dose that takes into account the type of radiation and the sensitivity of the tissue and organs. It makes it possible to measure ionising radiation in terms of its potential to cause harm. Sv takes into account the type of radiation and the sensitivity of organs and tissues.
Sv is a very large unit, so it is more practical to use smaller units such as millisievert (mSv) or microsievert (µSv). One mSv contains one thousand µSv and one thousand mSv is one Sv. In addition to the amount of radiation (dose), it is often useful to show the rate of release of this dose, e.g. µSv/hour or mSv/year.
Above certain thresholds, radiation exposure can impair tissue and/or organ function and can cause acute reactions such as reddening of the skin, hair loss, radiation burns or acute radiation syndrome. These reactions are more severe at higher doses and higher dose rates. For example, the threshold dose for acute radiation syndrome is approximately 1 Sv (1000 mSv).
If the dose is low and/or the exposure is prolonged (low dose rate), the resulting risk is significantly reduced, as the likelihood of recovery of damaged tissue is then increased. Nevertheless, there is a risk of long-term effects, such as cancer, which may appear years or even decades later. Effects of this type do not always manifest themselves, but their probability is proportional to the radiation dose. This risk is higher in the case of children and adolescents, as they are much more sensitive to radiation than adults.
Epidemiological studies in exposed populations, such as atomic bomb survivors or radiotherapy patients, have shown a significant increase in cancer incidence at doses above 100 mSv. In some cases, more recent epidemiological studies in people exposed as children for medical purposes (childhood CT) have concluded that cancer probability may increase even at lower doses (in the 50-100 mSv range).
Prenatal exposure to ionising radiation can cause fetal brain damage at high doses exceeding 100 mSv between 8 and 15 weeks’ gestation and 200 mSv between 16 and 25 weeks’ gestation. Studies in humans have shown that before 8 weeks or after 25 weeks of gestation there is no radiation-related risk to fetal brain development. Epidemiological studies suggest that the risk of fetal cancer after exposure is similar to that after early childhood exposure.
WHO has established a radiation programme to protect patients, workers and the public from the health risks of radiation exposure in planned, existing and emergency exposure situations. This programme, which focuses on aspects of public health, covers activities related to the assessment, management and communication of radiation risks.
In line with its core function of ‘setting norms and standards, facilitating compliance and appropriate monitoring’, WHO is working with seven other international organizations to revise and update international standards for basic radiation safety (BSS). WHO adopted new international BSS in 2012 and is currently working to support the implementation of BSS in its Member States.