Ionizing radiation

Ionizing radiation is radiation with sufficient energy to ionize atoms or molecules, for example by removing electrons from them.

It may travel as electromagnetic waves, such as X-rays and gamma rays, or as particles, such as alpha particles, beta particles and neutrons.

Ionizing radiation should be distinguished from non-ionizing radiation, which does not have sufficient energy to ionize atoms or molecules. Examples of non-ionizing radiation include radio waves, microwaves, infrared radiation and visible light.

Common types of ionizing radiation include:

• alpha particles, which consist of two protons and two neutrons;
• beta particles, which are electrons or positrons emitted during certain radioactive transformations;
• gamma rays, which are high-energy electromagnetic radiation emitted from an atomic nucleus;
• X-rays, which are electromagnetic radiation produced by processes involving electrons or generated by devices such as X-ray machines; and
• neutrons, which are uncharged particles that may be produced in certain nuclear reactions and processes.

These types of radiation differ in their physical properties, energy, penetration and interactions with matter. These differences influence how radiation interacts with matter and how energy is deposited in tissues, and are therefore important when assessing radiation exposure and its potential health effects.

Atoms of the same chemical element have the same number of protons but may have different numbers of neutrons. These different forms of an element are called isotopes.

Some isotopes are stable, while others are unstable and spontaneously transform, or decay, releasing energy in the form of ionizing radiation. Such unstable isotopes are called radioisotopes or radionuclides. Radionuclides are characterized by properties including the type and energy of radiation they emit and their physical half-life.

Examples of radionuclides include iodine-131, caesium-137 and cobalt-60.

Particulate radiation is a form of ionizing radiation. This consists of atomic or subatomic particles (electrons, protons, etc.) that carry kinetic energy, or mass in motion. This comes in several forms, including alpha and beta particles.

Alpha particles and beta particles are considered directly ionizing because they carry a charge and can, therefore, interact directly with atomic electrons through coulombic forces (i.e. like charges repel each other; opposite charges attract each other).

The neutron is an indirectly ionizing particle because it does not carry an electrical charge. Ionization is caused by charged particles, which are produced during collisions with atomic nuclei.

The third type of ionizing radiation includes gamma and X rays, which are electromagnetic, indirectly ionizing radiation. These are indirectly ionizing because they are electrically neutral (as are all electromagnetic radiations) and do not interact with atomic electrons through coulombic forces.

The physical, or radioactive, half-life of a radionuclide is the time required for its activity to decrease through radioactive decay to half its initial value.

Physical half-lives vary greatly between radionuclides, ranging from fractions of a second to millions of years. For example, iodine-131 has a physical half-life of approximately 8 days, whereas carbon-14 has a physical half-life of approximately 5730 years.

Physical half-life should not be confused with biological half-life, which describes how quickly a substance is eliminated from the body through biological processes.

Radioactivity is the process by which unstable atomic nuclei spontaneously transform and emit ionizing radiation. The activity of radioactive material describes the rate at which these nuclear transformations occur.

Activity is measured in becquerels (Bq). One becquerel corresponds to one nuclear transformation, or decay, per second.

The becquerel describes the activity of radioactive material. It does not by itself indicate the radiation dose that a person receives.

Radiation dose is used to describe the energy deposited by ionizing radiation in matter and, through radiation protection quantities, to characterize exposure. Different dose quantities serve different purposes.

Absorbed dose is the amount of energy deposited by ionizing radiation per unit mass of matter or tissue. It is measured in gray (Gy), where 1 Gy corresponds to 1 joule per kilogram.

Equivalent dose is a radiation protection quantity that takes account of the type of radiation by applying radiation weighting factors to the absorbed dose in an organ or tissue. Equivalent dose is expressed in sievert (Sv).

Effective dose is a radiation protection quantity that additionally takes account of the relative sensitivity of different organs and tissues to radiation-induced stochastic effects by using tissue weighting factors. It is also expressed in sievert (Sv).

Effective dose is used principally for radiation protection purposes. It should not be interpreted as an individual's measured dose or as a precise estimate of an individual's health risk.

Absorbed dose, equivalent dose and effective dose are different quantities with different purposes and should not be used interchangeably.

People are exposed to ionizing radiation from both natural and human-made sources.

Natural sources include cosmic radiation and naturally occurring radioactive materials found in soil, water, air, food and the human body. Radon, a naturally occurring radioactive gas released from rocks and soil, is an important contributor to exposure from natural radiation sources. Exposure to cosmic radiation also occurs naturally and increases with altitude.

Human-made sources of ionizing radiation are used in medicine, industry, research and other applications. In health care, ionizing radiation is used in diagnostic radiology, image-guided procedures, nuclear medicine and radiotherapy.

Levels of exposure from natural and human-made sources can vary considerably depending on where people live, their occupation, medical care and other circumstances.

Radiation exposure and radioactive contamination are different concepts.

A person can be exposed to ionizing radiation without becoming contaminated. External exposure occurs when radiation from a source outside the body irradiates a person. Exposure decreases or ceases when the source is removed or shielded, or when the person moves sufficiently far from the radiation field.

Radioactive contamination occurs when radioactive material is present where it is not intended to be, for example on a person's skin, hair or clothing, or on objects and surfaces in the environment. Radioactive material on external surfaces may sometimes be removed through appropriate decontamination measures.

Radioactive material may also enter the body through inhalation, ingestion, a wound or other pathways. This is referred to as an intake of radionuclides and may result in internal exposure.

Once radionuclides are inside the body, they may irradiate organs and tissues until they undergo radioactive decay or are eliminated from the body.

Exposure to ionizing radiation does not usually make a person or object radioactive. Radioactive contamination, by contrast, means that radioactive material is present on or inside a person, object or in the environment. Under some specific circumstances, certain nuclear reactions can also induce radioactivity in materials.

For example, a patient undergoing a nuclear medicine procedure for diagnostic or therapeutic purposes may be subjected to precautionary measures after the procedure for a short period of time as radioactive substance inside their body will take time to leave the body or to stop being radioactive. This is done to protect the patient’s family/caregiver from unnecessary exposure to radiation.

The health effects of ionizing radiation depend on several factors, including:

• the absorbed dose, dose rate and duration of exposure
• the type and energy of radiation
• the organs and tissues exposed
• whether radioactive material is outside or inside the body
• Individual characteristics of the exposed person, including age at exposure.

Health effects from ionizing radiation are commonly considered in two broad categories:

• Tissue reactions may occur when the radiation dose exceeds a threshold. Above that threshold, the severity of the effect generally increases with dose. Examples following sufficiently high exposures include skin injury, cataracts and, after substantial whole-body exposure, acute radiation syndrome.
• Stochastic effects, principally some types of cancer, involve an increased probability of an effect occurring with increasing radiation dose rather than an increase in its severity.

The likelihood and severity of radiation-related health effects therefore will depend strongly on the dose and circumstances of exposure.

Age at exposure is an important factor influencing radiation-related health risk. Children are generally more susceptible than adults to certain radiation-induced effects, in part because some developing tissues are more radiosensitive and because children have a longer remaining lifespan during which long-term effects may develop.

Exposure during pregnancy requires particular consideration because the potential effects on the embryo or fetus depend on the radiation dose and the stage of pregnancy.

The system of radiological protection aims to manage exposure to ionizing radiation so that its beneficial uses can be realized while protecting people from harmful effects.

Radiation protection measures depend on the source, the exposure situation and the pathway of exposure. For external radiation, protective measures may include reducing exposure time, increasing distance from a source and using appropriate shielding.

Where radioactive material is involved, protection may also include controlling contamination, preventing the spread of radioactive material and preventing or reducing the intake of radionuclides.

The specific measures required depend on the circumstances and should be based on an assessment of the source, exposure pathway, radiation dose and people who may be exposed.

Ionizing radiation has important applications in health care. It is used for diagnosis, including radiography and computed tomography; for image-guided procedures; in nuclear medicine; and for treatment, particularly radiotherapy.

When a procedure involving ionizing radiation is medically indicated and appropriately justified, its expected clinical benefit should outweigh the radiation risk. The exposure should then be optimized so that the medical purpose is achieved while exposure is appropriately managed.

Dose limits applicable to occupational and public exposure do not apply to the medical exposure of patients because applying such limits could restrict the intended clinical benefit. Radiation protection of patients is therefore based particularly on justification and optimization.

Patients who have concerns about a procedure should discuss its expected benefits and potential risks with their health-care provider.

A radiation emergency may involve exposure to an external source of radiation, contamination with radioactive material, intake of radionuclides, or a combination of these situations.

Protective actions are selected by national authorities in charge according to the emergency situation and may be implemented to prevent or reduce radiation exposure. Public-health actions may include urgent protective actions (e.g. iodine thyroid blocking, sheltering, evacuation), and other protective measures (e.g. control of food stuffs and drinking water), individual monitoring of potentially exposed people, management of contamination, risk communication, and if needed, medical assessment and treatment of affected persons.

People should follow instructions issued by national and local authorities during a radiation or nuclear emergency. Protective actions will depend on the type and circumstances of the emergency.