ARCHIVED - Chronic Diseases in Canada
Volume 29 · Supplement 1 · 2010
Radiation is energy in the form of particles or electromagnetic waves. Based on the effects it can produce in matter, two classes of radiation have been defined: ionizing and non-ionizing.1a Ionizing radiation has sufficient energy to remove electrons from atoms and break atomic bonds. Both classes can alter the genetic material (DNA) of a cell. Approximately 80% of our exposure to ionizing radiation is from natural sources, usually at very low dose rates, such as cosmic rays and naturally occurring radioactive elements in the Earth’s crust and air.2a Most of the artificial (man-made) radionuclides (unstable nuclei of atoms) released into the global environment have come from nuclear weapons tests. Other artificial sources of ionizing radiation include nuclear facilities, uranium mines, mills and plants and X ray devices.
Non-ionizing radiation has lower energy than ionizing radiation and does not ordinarily have enough intensity to endanger living things from acute exposure. Exposure to non-ionizing radiation includes ultraviolet radiation (UVR) from the sun, radiofrequency radiation (radar, radio and television towers, mobile telephones) and extremely low frequency electric and magnetic fields (ELF EMF) from electrical wires and appliances. Although a portion of the ultraviolet spectrum has sufficient energy to ionize atoms, it is traditionally considered a non-ionizing form of radiation. Human exposure to ELF EMF has risen dramatically this century because of our increasing use of electricity, giving rise to concerns about the effects of long-term exposures. Also, over the past few years, the ozone layer—a thin veil of gas in the atmosphere that screens out harmful solar UVR—has become thinner, resulting in slightly more of the sun’s harmful radiation reaching the Earth’s surface.3
Sources of radiation may be taken into the body by inhalation or ingestion and some forms of electromagnetic radiation can penetrate skin to reach other organs of the body. Gamma rays penetrates skin with ease, while alpha particles do not and exposure occurs primarily through inhalation or ingestion. Energy absorbed by tissue produces reactive chemicals called free radicals, which can induce other chemical changes and ultimately biological effects.1b Radioactive sources taken into the body may persist in tissues, irradiating organs for extended periods of time.
Radiation doses can be high or low, and can be received over a short or long period of time. Effects on humans can be acute or chronic, somatic or genetic. Somatic effects are limited to the exposed individual, whereas genetic effects may also affect subsequent unexposed generations.
High doses of absorbed ionizing radiation, delivered at high dose rates, (e.g., from a nuclear accident) can produce a variety of clinical effects including localized burns, acute radiation syndrome (ARS), increased circulatory disease and death. ARS and death result from damage to critical organs and tissues, such as bone marrow and the gastrointestinal tract. Lower doses of radiation may result in effects manifested later in life as a result of damage to DNA. Usually, cellular damage is repaired through a natural process; however, if it is not adequately repaired, it may result in a viable but modified cell. After a prolonged and variable latency period, reproduction of a modified somatic cell may result in the appearance of a cancer.2b
Units used to measure radiation dose to the body reflect the damage that may result to tissues and organs from the dose received.1c Three dose measurements are used: the absorbed dose, the equivalent dose and the effective dose. The absorbed dose is the amount of energy absorbed as a result of the radiation. Its SI unit, or Système International d’Unités, is the gray (abbreviated as “Gy”). One Gy is an absorbed dose of one joule per kilogram of material irradiated. However, tissue damage also depends on the type of radiation, as some types of radiation are potentially more harmful than others. Consequently, the absorbed dose is multiplied by a weighting factor for each radiation type to obtain the equivalent dose. X rays, gamma rays and beta particles have a weighting factor of one, while neutrons and alpha particles have a factor of 20. The unit of equivalent dose is the sievert (abbreviated as “Sv”). Doses commonly received by occupational groups and the public are much smaller than a sievert and are commonly expressed in millisieverts (mSv), or one thousandth of a sievert. The average individual dose from natural sources in Canada is about 2 mSv per year.1d Thirdly, the risk to an individual also depends on which organs in the body have been exposed. The effective dose has been developed to summarize the total potential harm over different organs and is also measured in Sv or mSv. Tissue weighting factors have been developed which sum to a whole body total of 1.0. Effective doses are obtained by multiplying the equivalent dose by the organ weighting factor. As a result, the potential harm from an effective dose of 1 mSv to a specific organ should be similar to that from an effective dose of 1 mSv of whole body irradiation.1e
Units of Working Levels (WL) and Working Level Months (WLM) are often used in studies of exposure to radon and its decay products. A WL is any combination of radon progeny in one litre of air that results in the average emission of 1.3 x 105 MeV of alpha energy. A WLM is the product of a WL and time (M) in working months (170 working hours).4
Radiation risk estimates are derived largely from extrapolation of epidemiological studies of human populations that were exposed to high doses of radiation as well as by residential and occupational studies at lower levels. The main source of information on the risk of radiation-induced cancer following whole-body exposure to external ionizing radiation comes from the follow-up studies of Japanese survivors of the 1945 atomic bombings of Hiroshima and Nagasaki. Other studied populations include hard rock miners exposed to high concentrations of radon and its decay products in air, early radium dial painters who inadvertently ingested appreciable amounts of radium and patients treated in the past with high doses of medical X rays, who received repeated fluoroscopies for the management of tuberculosis or who were given radium-224, radium-226 or Thorotrast (thorium oxide) for treatment or diagnosis. While evidence is growing that there may be an effective threshold below which there are no adverse effects from low doses of radiation, the linear non-threshold assumption of biological response to dose (dose-response) continues to be used as a prudent approach to radiation protection.2c For a more detailed discussion of radiation, the reader is referred to two publications by Canada’s Atomic Energy Control Board (AECB)—Canada: Living with Radiation1f and Assessment and management of cancer risks from radiological and chemical hazards.5
The following three chapters present the epidemiological evidence relating cancer to specific types of radiation exposure. Radon, a source of ionizing radiation, is presented first, followed by two types of non-ionizing radiation—UVR and ELF EMF. Radon and ultraviolet radiation emanate primarily from the natural environment, but are mediated by the built environment and personal behaviours. Electromagnetic fields, although occurring naturally (e.g., during thunderstorms), are primarily associated with the built environment.
- a,b,c,d,e,f Atomic Energy Control Board. Canada: Living with radiation. Ottawa: Minister of Supply and Services Canada; 1995.
- a,b,c Brooks S. Environmental medicine. St. Louis, Missouri: Mosby; 1995.
- ^ Environment Canada. UV and you.
- ^ Samet JM. Diseases of uranium miners and other underground miners exposed to radon. Occup Med 1991;6:629–39.
- ^ Health Canada and Atomic Energy Control Board. Assessment and management of cancer risks from radiological and chemical hazards. Ottawa: Minister of Public Works and Government Services Canada; 1998.
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