Ionizing Radiation Profile


IARC Monograph Vol. 75, 2000 (Group 1)
IARC Monograph Vol. 78, 2001 (Group 1)
IARC Monograph Vol. 100D, 2011 (Group 1)

Ionizing Radiation Profile

General Information

Ionizing radiation consists of particles or rays emitted by natural and artificial radioactive materials, nuclear reactions, and radiation producing machines.[1] An atom is said to be radioactive when the number of neutrons within its nucleus is too small or too large to match the number of protons; this mismatch results in an unstable nucleus. Radiation particles and rays are defined as “ionizing” when they carry sufficient energy to knock out electrons from molecules, such as water, protein, and DNA.[1] For a list of isotopes and radioactive compounds, please refer to synonyms listed in the Hazardous Substances Data Bank (HSDB).[2]

Ionizing radiation includes:

  1. X and gamma [γ] rays – packets of energy (photons) having neither charge nor mass. These are generally classified as external hazards since they penetrate the skin and internal organs.[1,3,4]
  2. Alpha [α] and beta [β] particles – small, fast moving particles which are primarily an internal hazard when deposited in the body through inhalation, ingestion, or injection.[3,5] They are carried by dust, suspension solutions, or gases.[5]

Radiation dose is described in three ways:

  1. Absorbed dose (given in Grays, Gy) – the amount of energy left behind when radiation passes through matter.[6]
  2. Equivalent dose (given in Sieverts, Sv) – absorbed dose multiplied by a radiation weighting factor dependent on the type and amount of radiation.[6]
  3. Effective dose (given in Sieverts, Sv) – equivalent dose multiplied by a tissue weighting factor, dependent on the sensitivity of a given tissue or organ.[6]

DNA is the most critical molecule for radiation damage, although other important cellular components are reportedly affected by exposure to radiation.[1] Overexposure to large amounts of ionizing radiation may result in health effects such as skin burns, hair loss, birth defects, cancer, intellectual disability, and death.[1] Exposure to substantial amounts of ionizing radiation to a fetus during pregnancy may cause negative pregnancy and birth outcomes, such as miscarriage, birth defects, hereditary effects, and a higher risk of childhood cancer and cancer in adult life.[7]

Ionizing radiation has been classified by the International Agency for Research on Cancer (IARC) as Group 1, carcinogenic to humans, in three separate monographs: Vol. 75: X and gamma radiation, Vol. 78: alpha, beta and neutrons, and, most recently, Vol. 100D, part of a volume reviewing all Class 1 carcinogens.[3,5,8] Epidemiological evidence has confirmed a strong association between ionizing radiation and leukemia, as well as cancers of the thyroid, breast, salivary gland, esophagus, bone, stomach, colon, skin, brain and central nervous system, kidney, and lung.[4] Many other cancer sites also show links with ionizing radiation exposure. The risk of developing cancer can be influenced by factors such as dose, dose rate, age, sex, genetics, lifestyle, environmental exposures, and time post exposure.[3,4]

For more information on radon, a naturally occurring radioactive gas, please refer to the CAREX Carcinogen Profile for Radon.

Regulations and Guidelines

Occupational Exposure Limits (OEL)

All nuclear facilities and devices, such as nuclear plants, medical radiation devices and uranium mines, are regulated federally by the Canadian Nuclear Safety Commission (CNSC).[9] The CNSC was established under the Nuclear Safety and Control Act in 2000 to replace the former Atomic Energy Control Board (AECB).[10] Under the Nuclear Safety and Control Act, the Radiation Protection Regulations established occupational exposure limits for ionizing radiation in Canada (shown in following table).[11] In exposure situations where the CNSC does not have jurisdiction, such as where radiation is used in healthcare settings, provinces and territories are responsible for setting and enforcing occupational exposure limits for ionizing radiation.

Canadian dose limits for exposure to licensed sources of radiation[11]

Population Dose Limit (mSv) Time Period and Type of Exposure
Nuclear energy worker 50 Annual effective dose
100 5-year cumulative effective dose
4 Effective dose during pregnancy
150 Annual equivalent dose, to the lens of the eye
500 Annual equivalent dose, to the skin, hands and feet
Non-nuclear energy worker 1 Annual effective dose
15 Annual equivalent dose, to the lens of the eye
50 Annual equivalent dose, to the skin, hands and feet

The American Conference of Industrial Hygienists (ACGIH, 2020) also have guidelines for exposure to ionizing radiation[12]:

Dose Limit (mSv) Time Period and Type of Exposure
50 Annual effective dose
100 5-year cumulative effective dose
10 x age in years Lifetime cumulative effective dose
150 Annual equivalent dose to the lens of the eye
500 Annual equivalent dose to the skin, hands and feet
0.5 Monthly equivalent dose for embryo/fetus
mSv = milliSievert

Canadian drinking water guidelines[13]

Naturally Occurring Radionuclides MAC
Total uranium 0.02 mg/L
Lead-210 0.2 Bq/L
Radium-226 0.5 Bq/L
Artificial Radionuclides MAC
Tritium 7000 Bq/L
Strontium-90 5 Bq/L
Iodine-131 6 Bq/L
Cesium-137 10 Bq/L
MAC = maximum allowable concentration
NOTE: The radiological effects of multiple radionuclides in the same drinking water source are considered to be additive, therefore the sum of ratios of the observed concentration to the MAC for each radionuclide present should not exceed 1.[13]
mg/L = milligrams per litre
Bq/L = Bequerels per litre (bequerels: a measure of radioactivity)

Main Uses

Since x-rays were discovered in 1895, ionizing radiation has been used medically for diagnosing and treating benign or malignant disease. It was used in 80-90% of all imaging procedures in 1996.[2] Diagnostic procedures using x-rays include positron emission tomography,[4] radiographic imaging (most widespread use), fluoroscopic imaging, and computed tomography.[2,4]

Ionizing radiation is used to examine welded joints in structures; it is used in the oil industry to determine geological structures in bore holes.[4] Ionizing radiation may also be used to sterilize and preserve food products.[2]

Countries using ionizing radiation in the form of nuclear power include Canada (where 17% of the country’s electricity production share is nuclear), Germany (14%), Sweden (34%), France (76%) and the United States (20%).[14] In the military, materials and processes that emit x-radiation and gamma radiation are used to produce, test, and apply nuclear weapons.[4]

Sources of ionizing radiation may be found in various consumer products, such as smoke detectors, televisions, and radioluminescent clocks and watches.[4]

Production and Industry

Canada opened its first Canada deuterium uranium (CANDU) reactor in 1971.[15] Today, there are 22 operational commercial nuclear power reactors across five plants located in Ontario and New Brunswick.[16] Two nuclear reactors are proposed for Ontario, and one in Saskatchewan.[17]

Canada is an international supplier of isotopes, including cobalt-60, heavy water, and tritium.[18] Canada was also the world’s largest producer of uranium until 2009, when it was overtaken by Kazakhstan; as of 2020, Canada accounts for 8% of world production of uranium, amounting to 3,885 tonnes. This uranium is produced at a Northern Saskatchewan mine.[19]

Environmental Exposures Overview

Over 80% of the general population’s annual dose is estimated to occur from exposure to natural background radiation, including cosmic radiation (from the sun and stars), radioactive elements in the earth’s crust (i.e. radon), and naturally occurring radionuclides such as potassium-40.[1,20]

Ionizing radiation from human-made sources such as medical applications (x-rays, gamma rays in cancer treatments, CT Scanners, MRIs) and dental x-rays comprise approximately 15% of the general population’s annual dose.[1,20] Consumer goods, nuclear power plant operation, and fallout from nuclear weapons testing or accidents make up the remaining fraction of exposure.[1,20]

An individual’s background radiation dose (from cosmic rays and naturally occurring radioactive material) may vary depending on geographic location and altitude.[1] The average annual radiation dose in Canada falls between 2-3 mSv.[21]

Consumer products, such as televisions and radioluminescent clocks and watches, may emit ionizing radiation, although there are restrictions on the maximum allowable radioactivity in these products.[4]

Occupational Exposures Overview

Inhalation, ingestion, wound contamination, and dermal absorption are all potential routes of intaking ionizing radiation.[2]

CAREX Canada estimates that 35,000-86,000 Canadians are exposed to ionizing radiation in their workplaces. The two largest industrial groups exposed are healthcare, and professional and scientific services industries. In terms of occupation, the largest exposed groups were nurses and aircrew members, followed by ward aids and orderlies.

Occupational exposure to ionizing radiation may also occur in the military, in research laboratories, in other industries using radiography or radioactive materials, and as a result of industrial accidents.[1,4,22] Occupations exposed to natural sources of radiation above typical background levels include uranium, coal, and other underground miners, airline crews, and astronauts.[1,4]

Canadians working in occupations involving exposure to radiation have been monitored by the National Dose Registry (NDR), which is maintained by the Radiation Protection Bureau of Health Canada, since the 1940s.[23] Although exposure levels vary across occupations, the average annual radiation exposure of a monitored Canadian worker is approximately 0.2 mSv.[24]

For more information, see the occupational exposure estimate for ionizing radiation.


1. Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Ionizing Radiation (1999) (PDF)
2. US National Library of Medicine. Hazardous Substances Data Bank (HSDB) (Search term: ‘Ionizing radiation’)
3. International Agency for Research on Cancer (IARC). Monograph summary, Volume 75 Part 1 (2000)
4. National Toxicology Program (NTP). 15th Report on Carcinogens for Ionizing Radiation (2021) (PDF)
5. International Agency for Research on Cancer (IARC). Monograph summary, Volume 78 Part 2 (2001)
6. Health Canada. Fact Sheet, Radiation Doses (2009)
8. International Agency for Research on Cancer (IARC). Monograph summary, Volume 100 Part D (2009)
9. Canadian Nuclear Safety Commission. Annual Report (2012-2013) (2013) (PDF)
10. Ministry of Justice. Nuclear Safety and Control Act (c.9) (1997)
12. American Conference of Governmental Industrial Hygienists (ACGIH). 2020 TLVs and BEIs. (2020)
14. International Atomic Energy Agency (IAEA). Power Reactor Information System – Country Statistics (2015)
15. World Nuclear Association. Nuclear Power in Canada (2021)
16. Canadian Nuclear Safety Commission. Nuclear power plants (2022)
17. Canadian Nuclear Safety Commission. Nuclear facilities in Canada (2021)
18. Canadian Nuclear Isotope Council. Canadian isotope landscape (2022)
19. World Nuclear Association. World Uranium Mining Production (2021)
20. Health Canada. Fact Sheet, Ionizing Radiation (2005)
22. Health Canada. Facts – Radiation in Dental Medicine (2007) (PDF)


Other Resources

  1. Health Canada. Technical Reports and Publications for Radiation (2008)
  2. National Research Council. The National Academies Report in Brief on Biological Effects of Ionizing Radiation (BEIR) VII: Health Risks from Exposure to Low Levels of Ionizing Radiation (2006)

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