eMedicine Specialties > Clinical Procedures > none
Ionizing Radiation Exposure, Medical
Edward B Holmes, MD, MPH, MSc, Owner, Holmes Company Consulting, LLC
Occupational Medicine and Medical Toxicology; Chief Medical Consultant for the State of
Utah, Disability Determination Services for Social Security; Chief VA Compensation and
Pension Medical Evaluator, Salt Lake City Veterans Affairs Medical Center
George L White Jr, PhD, MSPH, Professor and Director, Public Health Program, School of
Nursing and Health Sciences, Westminster College.; David K Gaffney, MD, PhD, Professor
of Radiation Oncology, University of Utah
Updated: Dec 2, 2010
Medical diagnostic procedures used to define and diagnose medical conditions are currently
the greatest manmade source of ionizing radiation exposure to the general population.
However, even these sources are generally quite limited compared to the general background
radiation on Earth.
The risks and benefits of radiation exposure due to medical imaging and other sources must
be clearly defined for clinicians and their patients. This article is a general overview for the
medical practitioner, who should understand the fundamentals of medical ionizing radiation
and the general associated risks. This article also acquaints the practitioner with relative doses
of common radiographic procedures as well as natural background radiation.
The use of ionizing radiation in medicine began with the discovery of x-rays by Roentgen in
1895. Ionizing radiation is the portion of the electromagnetic spectrum with sufficient energy
to pass through matter and physically dislodge orbital electrons to form ions. These ions, in
turn, can produce biological changes when introduced into tissue. Ionizing radiation can exist
in 2 forms: as an electromagnetic wave, such as an x-ray or gamma ray, or as a particle, in the
form of an alpha or beta particle, neutron, or proton.[1 ]X-rays are machine-generated,
whereas gamma rays are electromagnetic waves that are emitted from the nucleus of an
unstable atom. Different forms of ionizing radiation have differing abilities to generate
biologic damage. The order of ionization effect of these forms can be found in Table 1
below.[2 ]
Table 1. Relative Mass and Radiation Weighting of Ionizing Radiation Types (Greatest
Effect to Least Effect)
Radiation Type
Type of particle or ray
Alpha Neutron Beta Gamma ray or x-ray
Atomic mass
Radiation weighting factor (RWF) or quality (Q) 20
A clear understanding of the measurement units of radiation and radioactivity is required to
better communicate with colleagues or patients. Different units are used to describe
radioactivity by energy (erg), decay activity rate (curie [Ci] or becquerel [Bq]), effect in air
(roentgen [R]), ability to be absorbed (radiation-absorbed dose [rad] or gray [Gy]), or
biologic effect (roentgen equivalent man [rem] or sievert [Sv]). See Table 2 below for a
comparison of these terms.
Table 2. Comparison of Terms Used to Define Radiation and Dose
Conventional Units
Curie (Ci)
Absorbed dose
Rad (rad)
Rem (rem)
3.7 X 1010 disintegrations/s
100 ergs/g of absorbing
rad x Q factor or RWF
System International (SI)
Unit Name
Gray (Gy)
100 rad
Sievert (Sv)
100 rem
The rad is the amount of radiation absorbed per unit mass. The current preferred term for
absorbed dose is gray (Gy). One rad equals 0.01 Gy or 1 centigray. However, different tissues
can have different absorbed doses and, therefore, unequal biologic effects, depending on the
tissue and the source of radiation. For example, 1 Gy of alpha radiation can be more harmful
than 1 Gy of beta radiation because alpha particles are much larger than beta particles and
carry a greater charge.
The rem is a unit that describes the equivalent dose, which accounts for the actual biological
effect of radiation. The rem is calculated by multiplying the absorbed dose (rad) by a quality
(Q) factor or the radiation weighting factor (RWF), which reflects the differences in the
amount of potential biological effect for each type of radiation. For example, beta particles,
gamma rays, and x-rays have a RWF of 1.0, making their effects on tissue largely equivalent.
Alpha particles, however, have a RWF of 20, which indicates a biologic effect that is
potentially 20 times greater than that of beta particles, gamma rays, or x-rays.
The sievert (Sv) is the unit for equivalent dose in the System International (SI) nomenclature.
It indicates what is received by each irradiated organ and relative sensitivity. The equivalent
dose expressed in rem or Sv gives an index of potential harm to a particular tissue or organ
from exposure to different radiation types (see Table 2 above for comparison of terms).[2,3 ]
Biological Effects of Ionizing Radiation
Radiation damages the cell by damaging DNA molecules directly through ionizing effects on
DNA molecules or indirectly through free radical formation. A lower dose delivered through
a long period of time theoretically allows the body the opportunity to repair itself. Radiation
damage may not cause any outward signs of injury in the short term; effects may appear
much later in life.
Deterministic effects, such as cell killing, can be more immediate and have a threshold above
which severity increases with radiation dose. However, the threshold is not necessarily the
same in each individual or tissue. While healing may ensue, necrosis and fibrotic changes in
internal organs, acute radiation sickness, cataracts, and sterility may also occur. For acute
deterministic effects, large doses are usually required, such as 1-2 Gy or 1-2 Sv (with x-ray
exposure RWF of 1).[4 ]
Stochastic effects, such as mutations, can result in cancer and hereditary effects. Cancer
induction can have a long latency period. Estimating cancer risks associated with diagnostic
x-rays using epidemiological tools is difficult because of extrapolation to low radiation doses,
recall bias, and different x-ray energies used at various institutions. Most low-dose human
ionizing radiation risk estimates come from the atomic bomb survivors in Japan. Other
sources of information include laboratory cellular mutation studies and studies on various
strains of mice; of course, the applicability to humans remains to be seen.
Significant debate is ongoing in the scientific community regarding the effects of low-dose
radiation, whether the dose-response curve is linear or nonlinear at low doses, and whether or
not a threshold of adverse effect exists. Recent studies have led the Committee on Health
Effects of Exposure to Low levels of Ionizing Radiations (BEIR VII) to conclude that
"biologic data are emerging on phenomena that could affect the shape of the dose-response
curve at low doses."[5 ]The latency period to cancer induction from human ionizing radiation
exposure varies from several years to more than 20 years, if it occurs at all.[4 ]
Radiation-induced malformations during pregnancy are important illustrations of
deterministic effect. Studies on atomic bomb survivors show that the period of organogenesis
(3rd -8th week) is a particularly vulnerable window. Exposure between the 8th and 15th week
can lead to malformations of the forebrain, resulting in mental retardation. The threshold dose
during these periods of pregnancy is much lower, potentially at 100-200 mSv. However, high
doses to the embryo or fetus can result in death or gross malformations at 0.1 Sv to 1 Sv.
Fetal radiation exposure can increase the risk of cancer in later childhood. Pregnant women
should avoid all ionizing radiation, if possible, since x-rays to one site on the body provide
some scatter dose to the fetus.[4 ]Of course, medical necessity may require x-ray imaging of
pregnant women in some circumstances.
The other main sequelae of radiation are hereditary effects. Radiation damage to the gonads
during the reproductive period of life produces mutations to the gametes. Inherited diseases
can encompass a range of mild disorders to serious consequences, including death or severe
mental defects. However, no human population studies have shown hereditary effects from
typical background ionizing radiation doses. Furthermore, some studies of the offspring of
atomic bomb survivors have not shown statistically significant increases in hereditary defects
or cancers.[6 ]
Sources of Ionizing Radiation
Most human exposure to ionizing radiation comes from natural sources inherent to life on
Earth. The annual average dose for the world population is approximately 2.8 mSv (3.0 mSv
in the United States); 85% of this comes from natural sources. The remaining proportion
(15%) of the annual ionizing radiation dose comes from artificial sources, which are almost
exclusively provided by medical ionizing radiation. The combined radiation exposure from
nuclear fuel, Chernobyl fallout, and nuclear testing fallout accounts for less than 0.3% of the
annual radiation dose (see Table 3).[7 ]
Table 3. Average Annual Radiation Dose Sources
Source of Radiation Average Annual Dose, mSv
Natural sources
Gamma rays
Artificial sources
Nuclear testing
Nuclear power
All sources
Medical Uses of Radiation
The vast majority of artificial exposure to ionizing radiation in the general population comes
from uses in medicine or allied health for diagnosis and therapy. Medical ionizing radiation
contributes 0.4 mSv to the annual average dose of radiation (>14%). The most frequently
used modality of radiation is diagnostic x-ray examinations. Examinations of the chest
account for over 25% of all x-ray examinations.[8 ]The most common radiographic test is the
chest x-ray, and it has a wide range of effective dose—approximately 0.02-0.67 mSv,
depending upon the individual and equipment settings.
In conventional radiography, the effective dose that a patient receives depends on several
factors. First, it depends on beam energy and filtration, which increase the average energy to
result in an acceptable image. Second, collimation in radiography allows exposure to the area
of interest and reduces scatter and unnecessary exposure to other tissues. Third, grids are also
used to reduce scatter. Both collimation and grids act to improve radiographic images.
Fourth, patient size dictates the amount of incident radiation, because the thicker the tissue in
the area of interest, the higher the x-ray energy required for penetration.[9 ]
With these factors in mind, the fact that different people may have varying doses for the same
commonly performed test is not surprising. Furthermore, different institutions were shown to
have a wide range of doses for various diagnostic tests.[10,11 ]In Table 4, doses for common
radiographic procedures are given in ranges, which are due to variations in technique and
body habitus, as reported in the literature. Interventional radiology has the highest doses of
radiation, followed by computed tomography (CT) and then plain-film radiography.
For a detailed listing of the radiation doses of medical imaging procedures, see Table 4 and
the image below. The effective dose associated with most diagnostic imaging modalities in
medicine covers a wide range, from less than 0.03 to more than 70 mSv.
Average radiation dose of common radiographic procedures.
Some authors have concluded, consistent with the analysis by the authors of this article, that
CT scans of the abdomen vary in effective radiation dose by as much as 13-fold, depending
upon the technique and device used. Smith-Bindman et al reported that "within each type of
CT study, effective dose varied significantly within and across institutions, with a mean 13fold variation between the highest and lowest dose for each study type."[12 ]
Given the tremendous variability in dose depending upon the facility, machine, and technique
used to perform the imaging, the resulting variations in radiation exposure and potential
cancer risk are also great. Presumably using the lowest estimated chest x-ray dose and the
highest segment of the range of dose for CT scans of the abdomen, the FDA recently stated
that "the radiation dose associated with a CT abdomen scan is the same as the dose from
approximately 400 chest x-rays."[13 ]Based upon their analysis, the FDA plans an initiative to
reduce unnecessary radiation exposure from CT, nuclear medicine studies, and fluoroscopy.
The initiative focuses on these types of medical imaging because "these procedures are the
greatest contributors to total radiation exposure within the U.S. population and use much
higher radiation doses than other radiographic procedures."[13 ]
Table 4. Radiation Doses of Medical Imaging Procedures[8,9,14,15,16,17,18,19,20,21 ]
Dose Range, Average Dose,
Chest X-ray
Equivalent Dose
Abdomen, kidneys,
ureters, bladder
Barium enema
Intravenous pyelogram
Upper GI tract
CT scans
CT has seen increased use, encompassing up to 40% of all radiographic studies. Nuclear
medicine is used for treatment as well as diagnostic studies. The radionuclide technetium99m in nuclear medicine has a short half-life of 6 hours. As shown in Media file 1 above and
Table 5 below, the radiation doses from technetium scans are comparable to those of CT
scans. Radiotherapy specifically uses radiation to kill cancer cells when trying to cure the
cancer. To be effective, such doses typically require 20-60 Gy (or 20-60 Sv for x-ray
Table 5. Technetium Scan Radiation Doses
Radiation Dose, mSv
Thyroid, lung 1
Liver, kidney 1
One growing concern in the field of medical imaging is the current trend in patient-procured
whole-body CT scans.[22 ]These scans are marketed in shopping centers directly to the general
public as screening tests. These scans are sometimes routinely repeated. The positive and
negative predictive values of these whole-body scans for disease detection have not been
determined by quality studies to date. The American College of Radiology currently
condemns the screening of healthy patients with whole-body CT scans. Radiological
procedures are medically prescriptive and should "only be used for specific purposes when
patient benefit outweighs potential risk."[23 ]
Studies have consistently shown that physicians who are not radiologists but who operate
their own imaging equipment and have the opportunity to self-refer use imaging substantially
more than do physicians who refer their patients to radiologists for imaging.[24 ]A viable
concern has been raised by many practitioners regarding the routine and repeated use by
chiropractors of relatively high gonadal dose lumbar spine x-rays. Many chiropractors
regularly perform repeat spine imaging on young healthy individuals, including women and
children. The practice of routinely performing x-rays on women of childbearing age and
children should be highly discouraged in this setting.
In addition to the radiation exposure risk to patients undergoing radiological procedures,
physicians and medical staff in facilities performing imaging can be exposed to ionizing
radiation. The Occupational Safety and Health Administration (OSHA) has exposure
standards for employees, and various professional organizations have recommended exposure
limits for health care workers potentially exposed to radiation. One study involving an
analysis of ionizing radiation exposure dose amongst emergency physicians revealed very
low levels of exposure, well below recommended annual doses.[25 ]On the other hand,
radiologists using fluoroscopy and other techniques may have much higher exposure doses in
the work setting.
Medical ionizing radiation has great benefits and should not be feared, especially in urgent
situations. Radiological dose and risk depends on good methodology and quality control.
Obviously, using the lowest possible dose is desired. In fact, a central principle in radiation
protection is "as low as reasonably achievable." Therefore, the prescribing physician must
justify the examination and determine relevant clinical information before referring the
patient to a radiologist. Indications and decisions should reflect the possibility of using nonionizing radiation examinations, such as MRI or ultrasonography. Repetition of examinations
should be avoided at other clinics or sites.
The International Commission on Radiological Protection (ICRP) estimates that the average
person has an approximately 4-5% increased relative risk of fatal cancer after a whole-body
dose of 1 Sv. However, other studies on multiple cohorts of radiation workers have largely
failed to establish statistically significant cancer risks. When multiple occupational cohorts
were combined and evaluated in a somewhat systematic way, a combined excess relative risk
of cancer death of just less than 1% was estimated.[26 ]
Cancer is a central public health problem. It is the leading cause of death in persons in the
United States younger than 85 years. The lifetime incidence of cancer in the United States is
45% for males and 38% for females.[27 ]The overall spontaneous risk of fatal cancer in a
lifetime in industrialized countries is 1 in 4 (25%). In pediatric populations, the potential for
the medical uses of radiation to do harm is much greater than for adults because of children's
more radiosensitive tissue and longer life expectancies.[4 ]
Using complex modeling, some authors have concluded that cancer risk from medical
imaging can be estimated. Although clearly not as conclusive or exact as risk estimates that
could be obtained from a prospective exposure study, the estimates are concerning and should
be considered. After analysis of their cancer risk modeling studies, Berrington de González et
al estimated that "approximately 29,000 (95% uncertainty limits [UL]; 15,000-45,000) future
cancers could be related to CT scans performed in the US in 2007."[28 ]
Table 6 indicates the number of days of natural background radiation necessary to expose a
person to the same amount of radiation in various numbers of chest x-rays.
Table 6. Equivalent Doses of Background Radiation and Chest X-rays
Chest X-ray
Radiation Exposure,
Natural Background Equivalents,
Table 7 shows the ionizing radiation doses to which passengers may be subjected during air
travel between various cities.
Table 7. Typical Ionizing Radiation Dose From Air Travel
Departure and Destination Cities Effective Dose, mSv
Vancouver - Honolulu
Montreal - London
London - Tokyo
Paris - San Francisco
Debate continues over the health consequences of exposure to low levels of ionizing
radiation. Most of the data were derived from estimates of exposure to the Japanese
population after the atomic bombing. Recently, a study involving over 400,000 nuclear
radiation workers showed a dose-related increase in all cancer mortality from radiation.[29 ]
Although the average annual radiation dose to the public from medical sources continues to
be low (see Table 3), the use of medical x-rays has increased dramatically over the past
couple of decades. In 1980, 3 million CT scans were performed in the United States; this has
grown to more than 62 million CT scans per year. More than 4 million CT scans are
performed annually on children. Some authors have estimated that one third of these scans
may be medically unnecessary. In some emergency departments, an increasingly large
number of patients with abdominal pain or headache are evaluated with CT scanning.
X-rays (including CT scans) should be ordered judiciously. An article in the New England
Journal of Medicine notes that the evidence is "convincing" that the radiation dose from CT
scans can lead to cancer induction in adults and "very convincing" in the case of children.[22
Clinicians need to realize that doses from a typical CT scan can range from 6-35 times higher
than the dose of a standard chest x-ray examination (see Table 4 for comparisons).
Of further national and international concern is the ever-increasing threat of nuclear weapons
or radiological dispersal devices (RDDs) to potentially spread ionizing radiation sources over
large population areas. A basic understanding of ionizing radiation terms and relative dosages
of various exposure sources may ultimately prove useful for medical practitioners faced with
such exposure situations. Health physicists are trained in estimating exposure. These
professionals would be highly valuable in the event of a radiation emergency but may not be
readily available.
Radiation exposure cannot be entirely avoided on this planet. Taking into account how much
radiation people receive from natural sources, medical ionizing radiation accounts for only a
small proportion of the annual average dose for the average patient. The proper use of
medical ionizing radiation can greatly benefit patients. A better understanding of medical
ionizing radiation allows practitioners to better communicate the risks and benefits to their
Media file 1: Average radiation dose of common radiographic procedures.
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radiation, radiation exposure, CT scan radiation, CT radiation exposure, ionizing radiation,
radioactivity, x-ray, X-ray, gamma ray, alpha particle, imaging, nuclear weapons, dirty bomb,
radiological dispersal device, RDD, terrorism, medical imaging, REM, rem, RAD, rad,
radiation absorbed dose, radiation average dose, average radiation exposure, x-ray exposure,
Gray, gray, Sievert, sievert, Curie, radiograph, radioactivity by energy, erg, decay activity
rate, curie, Ci, becquerel, Bq, effect in air, roentgen, R, radiation-absorbed dose, biologic
effect of radiation, roentgen equivalent man
Contributor Information and Disclosures
Edward B Holmes, MD, MPH, MSc, Owner, Holmes Company Consulting, LLC
Occupational Medicine and Medical Toxicology; Chief Medical Consultant for the State of
Utah, Disability Determination Services for Social Security; Chief VA Compensation and
Pension Medical Evaluator, Salt Lake City Veterans Affairs Medical Center
Edward B Holmes, MD, MPH, MSc is a member of the following medical societies:
American College of Occupational and Environmental Medicine and European Association
of Poisons Centres and Clinical Toxicologists
Disclosure: Nothing to disclose.
George L White Jr, PhD, MSPH, Professor and Director, Public Health Program, School of
Nursing and Health Sciences, Westminster College.
George L White Jr, PhD, MSPH is a member of the following medical societies: American
Academy of Physician Assistants, American Public Health Association, Association of
Military Surgeons of the US, Sigma Xi, Society for Epidemiologic Research, and Southern
Medical Association
Disclosure: Nothing to disclose.
David K Gaffney, MD, PhD, Professor of Radiation Oncology, University of Utah
David K Gaffney, MD, PhD is a member of the following medical societies: American
Cancer Society, American Society for Therapeutic Radiology and Oncology, and Phi Beta
Disclosure: Nothing to disclose.
Medical Editor
Erik D Schraga, MD, Staff Physician, Department of Emergency Medicine, Mills-Peninsula
Emergency Medical Associates
Disclosure: Nothing to disclose.
Pharmacy Editor
Mary L Windle, PharmD, Adjunct Associate Professor, University of Nebraska Medical
Center College of Pharmacy; Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.
Chief Editor
Rick Kulkarni, MD, Assistant Professor of Surgery, Section of Emergency Medicine, YaleNew Haven Hospital
Rick Kulkarni, MD is a member of the following medical societies: Alpha Omega Alpha,
American Academy of Emergency Medicine, American College of Emergency Physicians,
American Medical Association, American Medical Informatics Association, Phi Beta Kappa,
and Society for Academic Emergency Medicine
Disclosure: WebMD Salary Employment
The authors would like to acknowledge the assistance of occupational medicine resident
Kathy Chang, MD, who provided input and assistance in compiling sources and tables and
reviewing material as this article was being developed.
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Sources of Ionizing Radiation