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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

This Public Health Statement is the summary 

chapter from the Toxicological Profile for Ionizing 

Radiation. It is one in a series of Public Health 

Statements about hazardous substances and their 

health effects. A shorter version, the ToxFAQsTM, is 

also available. This information is important 

because this substance may harm you. The effects 

of exposure to any hazardous substance depend on 

the dose, the duration, how you are exposed, 

personal traits and habits, and whether other 

chemicals are present. For more information, call 

the ATSDR Information Center at 1-888-422-8737. 

________________________________________ 

This public health statement tells you about ionizing 

radiation and the effects of exposure. It does not tell 

you about non-ionizing radiation, such as 

microwaves, ultrasound, or ultraviolet radiation. 

Exposure to ionizing radiation can come from many 

sources. You can learn when and where you may be 

exposed to sources of ionizing radiation in the 

exposure section below. One source of exposure is 

from hazardous waste sites that contain radioactive 

waste. The Environmental Protection Agency 

(EPA) identifies the most serious hazardous waste 

sites in the nation. These sites make up the National 

Priorities List (NPL) and are the sites targeted for 

federal cleanup. However, it's unknown how many 

of the 1,467 current or former NPL sites have been 

evaluated for the presence of ionizing radiation 

sources. As more sites are evaluated, the sites with 

ionizing radiation may increase. This information is 

important because exposure to ionizing radiation 

may harm you and because these sites may be 

sources of exposure. 

When a substance is released from a large area, 

such as an industrial plant, or from a container, such 

as a drum or bottle, it enters the environment. This 

release does not always lead to exposure. Even in 

the event that you are exposed, it does not 

necessarily mean you will be harmed or suffer longterm health effects from exposure to ionizing 

radiation. 

If you are exposed to ionizing radiation, many 

factors determine whether you'll be harmed. These 

factors include the dose (how much), the duration 

(how long), and the type of radiation. You must also 

consider the chemicals you're exposed to and your 

age, sex, diet, family traits, lifestyle, and state of 

health. 

1.1 WHAT IS IONIZING RADIATION? 

To explain what ionizing radiation is, we will start 

with a discussion of atoms, how they come to be 

radioactive, and how they give off ionizing 

radiation. Then, we will explain where radiation 

comes from. Finally, we will describe the more 

important types of radiation to which you may be 

exposed. Of the different types and sources of 

ionizing radiation, this profile will discuss the three 

main types: alpha, beta, and gamma radiation. 

The Atom. Before defining ionizing radiation, it is 

useful to first describe an atom. Atoms are the basic 

building blocks of all elements. We have models of 

an atom that are supported by measurements. An 

atom consists of one nucleus, made of protons and 

neutrons, and many smaller particles called 

electrons. The electrons normally circle the nucleus 

much like the planets or comets circle the sun. The 

number of protons in the atom's nucleus determines 

which element it is. For example, an atom with one 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

proton is hydrogen and an atom with 27 protons is 

cobalt. Each proton has a positive charge, and 

positive charges try to push away from one another. 

The neutrons neutralize this action and act as a kind 

of glue that holds the protons together in the 

nucleus. The number of protons in an atom of a 

particular element is always the same, but the 

number of neutrons may vary. Neutrons add to the 

weight of the atom, so an atom of cobalt that has 27 

protons and 32 neutrons is called cobalt-59 because 

27 plus 32 equals 59. If one more neutron were 

added to this atom, it would be called cobalt-60. 

Cobalt-59 and cobalt-60 are isotopes of cobalt. 

Isotopes are forms of the same element, but differ in 

the number of neutrons within the nucleus. Since 

cobalt-60 is radioactive, it is called a radionuclide. 

All isotopes of an element, even those that are 

radioactive, react chemically in the same way. 

Atoms tend to combine with other atoms to form 

molecules (for example, hydrogen and oxygen 

combine to form water). Radioactive atoms that 

become part of a molecule do not affect the way the 

molecule behaves in chemical reactions or inside 

your body. 

What Ionizing Radiation Is. Ionizing radiation is 

energy that is carried by several types of particles 

and rays given off by radioactive material, x ray 

machines, and fuel elements in nuclear reactors. 

Ionizing radiation includes alpha particles, beta 

particles, x rays, and gamma rays. Alpha and beta 

particles are essentially small fast moving pieces of 

atoms. X rays and gamma rays are types of 

electromagnetic radiation. These radiation particles 

and rays carry enough energy that they can knock 

out electrons from molecules, such as water, 

protein, and DNA, with which they interact. This 

process is called ionization, which is why it is 

named "ionizing radiation." We cannot sense 

ionizing radiation, so we must use special 

instruments to learn whether we are being exposed 

to it and to measure the level of radiation exposure. 

The other types of electromagnetic radiation include 

radiowaves microwaves, ultrasound, infrared 

radiation, visible light, and ultraviolet light. These 

types of radiation do not carry enough energy to 

cause ionization and are called non-ionizing 

radiation. This profile will only discuss ionizing 

radiation. 

What Ionizing Radiation Is Not. Ionizing 

radiation is not a substance like salt, air, water, or a 

hazardous chemical that we can eat, breathe, or 

drink or that can soak through our skin. However, 

many substances can become contaminated with 

radioactive material, and people can be exposed to 

ionizing radiation from these radioactive 

contaminants. 

How Does an Atom Become Radioactive? An 

atom is either stable (not radioactive) or unstable 

(radioactive). The ratio of neutrons to protons 

within the nucleus determines whether an atom is 

stable. If there are too many or too few neutrons, the 

nucleus is unstable, and the atom is said to be 

radioactive. There are several ways an atom can 

become radioactive. An atom can be naturally 

radioactive, it can be made radioactive by natural 

processes in the environment, or it can be made 

radioactive by humans. Naturally occurring 

radioactive materials such as potassium-40 and 

uranium-238 have existed since the earth was 

formed. Other naturally occurring radioactive 

materials such as carbon-14 and hydrogen-3 

(tritium) are formed when radiation from the sun 

and stars bombards the earth's atmosphere. The 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

elements heavier than lead are naturally radioactive 

because they were originally formed with too many 

neutrons. Human industry creates radioactive 

materials by one of two different processes. In the 

first process, a uranium or a plutonium atom 

captures a neutron and splits (undergoes nuclear 

fission) into two radioactive fission fragments plus 

two or three neutrons. In a nuclear reactor, one of 

these "fission neutrons" is captured by another 

uranium atom, and the fission process is repeated. 

In the second process, stable atoms are bombarded 

either by neutrons or by protons that are given a lot 

of energy in a machine called an accelerator. The 

stable atoms capture these bombarding particles and 

become radioactive. For example, stable cobalt-59, 

found in the steel surrounding a nuclear reactor, is 

hit by neutrons coming from the reactor and can 

become radioactive cobalt-60. Any material that 

contains radioactive atoms is radioactive material. 

How Does a Radioactive Atom Give off Ionizing 

Radiation? Because a radioactive atom is unstable, 

at some time in the future, it will transform into 

another element by changing the number of protons 

in the nucleus. This happens because one of several 

reactions takes place in the nucleus to stabilize the 

neutron-proton ratio. If the atom contains too many 

neutrons, a neutron changes into a proton and 

throws out a negative "beta" (pronounced bay' tah) 

particle. If the atom contains too many protons, 

normally a proton changes into a neutron and 

throws out a positive "beta" particle. Some atoms 

that are more massive than lead, such as radium, 

transform by emitting an "alpha" (pronounced al'­

fah) particle. Any excess energy that is left can be 

released as "gamma" rays, which are the same as x 

rays. Other reactions are also possible, but the final 

result is to make a radioactive atom into a stable 

atom of a different element. For example, each atom 

of cobalt-60 is radioactive because it has too many 

neutrons. At some time in the future, one of its 

neutrons will change into a proton. As it changes, 

the atom gives off its radiation, which is a negative 

beta particle and two gamma rays. Because the 

atom now has 28 protons instead of 27, it has 

changed from cobalt into nickel. In this way, 

unstable atoms of radioactive cobalt-60 give off 

radiation as they transform into stable atoms of 

nickel-60. 

How Long Can Radioactive Material Give Off 

Ionizing Radiation? Theoretically, it gives off 

ionizing radiation forever. Practically, however, 

after 10 half-lives, less than 0.1% of the original 

radioactivity will be left and the radioactive 

material will give off infinitesimally small amounts 

of ionizing radiation. The half-life is the time it 

takes one-half of the radioactive atoms to transform 

into another element, which may or may not also be 

radioactive. After one half-life, 1⁄2 of the radioactive 

atoms remain; after two half-lives, half of a half or 

1/4 remain, then 1/8, 1/16, 1/32, 1/64, etc. The halflife can be as short as a fraction of a second or as 

long as many billions of years. Each type of 

radioactive atom, or radionuclide, has its own 

unique half-life. For example, technetium-99m and 

iodine-131, which are used in nuclear medicine, 

have 6-hour and 8-day half-lives, respectively. The 

naturally occurring radionuclide, uranium-235, 

which is used in nuclear reactors, has a half-life of 

700 million years. Naturally occurring potassium40, which is present in the body, has a half-life of 

13 billion years and undergoes about 266,000 

radioactive transformations per minute in the body. 

Thus, technetium-99m will remain radioactive for 

60 hours, and iodine-131 will remain radioactive for 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

3 months. On the other hand, long-lived naturally 

occurring uranium and potassium will remain, 

practically speaking, radioactive forever. 

What Are the Three Types of Radiation? The 

three main types of ionizing radiation are called 

alpha, beta, and gamma radiation. These are named 

for letters of the Greek alphabet, and they are often 

symbolized using the Greek letters alpha (α), beta 

(β), and gamma (γ). 

Alpha Radiation (or Alpha Particles). This type of 

radiation can be called either alpha radiation or 

alpha particles. Alpha radiation is a particle, 

consisting of two protons and two neutrons, that 

travels very fast and thus has a good deal of kinetic 

energy or energy of motion. The two protons and 

neutrons make an alpha particle identical to a 

helium atom, but without the electrons. Although it 

is much too small to be seen with the best 

microscope, it is large compared to a beta particle. 

The protons give it a large positive charge that pulls 

hard at the electrons of other atoms it passes near. 

When the alpha particle passes near an atom, it 

excites its electrons and can pull an electron from 

the atom, which is the process of ionization. Each 

time the alpha particle pulls an electron off from an 

atom in its path, the process of ionization occurs. 

With each ionization, the alpha particle loses some 

energy and slows down. It will finally take two 

electrons from other atoms at the end of its path and 

become a complete helium atom. This helium has 

no effect on the body. Because of their large mass 

and large charge, alpha particles ionize tissue very 

strongly. If the alpha particle is from radioactive 

material that is outside the body, it will lose all its 

energy before getting through the outer (dead) layer 

of your skin. This means that you can only be 

exposed to alpha radiation if you take radioactive 

material that produces alpha radiation into your 

body (for example, if you breathe it in or swallow it 

in food or drink). Once inside the body, this 

radioactive material can be mixed in the contents of 

the stomach and intestines, then absorbed into the 

blood, incorporated into a molecule, and finally 

deposited into living tissue such as the bone matrix. 

The alpha particles from this radioactive material 

can cause damage to this tissue. 

Beta Radiation (or Beta Particles). This type of 

radiation can be called either beta radiation or beta 

particles. Beta particles are high-energy electrons 

that some radioactive materials emit when they 

transform. Beta particles are made in one of two 

ways, depending on the radioactive material that 

produces them. As a result, they will have either a 

positive charge or a negative charge. Most beta 

particles are negatively charged. They are much 

lighter and much more penetrating than alpha 

particles. Their penetrating power depends on their 

energy. Some, such as those from tritium, have very 

little energy, and can't pass through the outer layer 

of dead skin. Most have enough energy to pass 

through the dead outer layer of a person's skin and 

irradiate the live tissue underneath. You can also be 

exposed to beta radiation from within if the betaemitting radionuclide is taken into the body. A beta 

particle loses its energy by exciting and ionizing 

atoms along its path. When all of its kinetic energy 

is spent, a negative beta particle (negatron) becomes 

an ordinary electron and has no more effect on the 

body. A positive beta particle (positron) collides 

with a nearby negative electron, and this electronpositron pair turns into a pair of gamma rays called 

annihilation radiation, which can interact with other 

molecules in the body. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

Gamma Radiation (or Gamma Rays). This type of 

radiation can be called either gamma radiation or 

gamma rays. Unlike alpha and beta radiation, 

gamma radiation is not a particle, but is a ray. It is a 

type of light you cannot see, much like radio waves, 

infrared light, ultraviolet light, and x rays. When a 

radioactive atom transforms by giving off an alpha 

or a beta particle, it may also give off one or more 

gamma rays to release any excess energy. Gamma 

rays are bundles of energy that have no charge or 

mass. This allows them to travel very long distances 

through air, body tissue, and other materials. They 

travel so much farther than either alpha or beta 

radiation that the source of the gamma rays doesn't 

have to be inside the body or near the skin. The 

gamma ray source can be relatively far away, like 

the radioactive materials in nearby construction 

materials, soil, and asphalt. A gamma ray may pass 

through the body without hitting anything, or it may 

hit an atom and give that atom all or part of its 

energy. This normally knocks an electron out of the 

atom (and ionizes the atom). This electron then uses 

the energy it received from the gamma ray to ionize 

other atoms by knocking electrons out of them as 

well. Since a gamma ray is pure energy, once it 

loses all its energy it no longer exists. 

1.2 HOW DOES RADIOACTIVE MATERIAL 

ENTER AND SPREAD THROUGH THE 

ENVIRONMENT? 

Radioactive material can be released to the air as 

particles or gases as a result of natural forces and 

from human industrial, medical, and scientific 

activities. Everyone, with no exception, is exposed 

to ionizing radiation that comes from natural 

sources, such as cosmic radiation from space and 

terrestrial radiation from radioactive materials in the 

ground. Ionizing radiation can also come from 

industrially produced radioactive materials (such as 

iridium-192); nuclear medicine (such as thyroid 

cancer treatment with iodine-131 and thyroid scans 

using iodine-125, or bone scans using technetium99m); biological and medical research using 

carbon-14, tritium, and phosphorus-32; the nuclear 

fuel cycle (producing fission products such as 

cesium-137 and activation products such as cobalt60); and production and testing of nuclear weapons. 

Radioactive material released into the air is carried 

by the wind and is spread by mixing with air. It is 

diluted in the atmosphere and can remain there for a 

long time. When the wind blows across land 

contaminated with radioactive materials, radioactive 

particles can be stirred up and returned to the 

atmosphere. Radioactive material on the ground can 

be incorporated into plants and animals, which may 

later be eaten by people. 

Water can contain man-made and naturally 

occurring radioactive materials that it dissolves 

from the soil it passes over or through. Rain and 

snow also wash man-made and naturally occurring 

radioactive material out of the air. Radioactive 

material may be added to water through planned or 

accidental releases of liquid radioactive material 

from sources such as hospitals, research 

universities, manufacturing plants, or nuclear 

facilities. Radioactive material can also reach 

surface waters when airborne radioactive materials 

settle to the earth or are brought down by rain or 

snow, and when soil containing radioactive material 

is washed away into a river or lake. The movement 

of liquid radioactive material is limited by the size 

of the bodies of water into which the radioactive 

materials have drained. Like silt, some radioactive 

material may settle along the banks or in the 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

bottoms of ponds and rivers. In public health and 

ecological contexts, it is sometimes important to 

distinguish between dissolved radioactivity and 

radioactivity bound to suspended or settled solid 

particles. Radioactive material may also concentrate 

in aquatic animals and plants. Eventually, 

radioactive material in liquid runoff that goes into 

rivers and streams may reach the oceans (there are 

approximately one million radioactive 

transformations per minute of the naturally 

occurring radioactive potassium in one cubic meter 

of ocean water). 

Radioactive material moves very slowly in soil 

compared to its speed of movement in air and water. 

Radioactive material will often stick to the surface 

of the soil. The organic material in soils can bind 

radioactive material, which slows its movement 

through the environment. If crops are watered with 

water containing radioactive material, the 

radioactive material may be taken up through the 

roots of the plant or may contaminate the outside of 

the plant. The plants may then be eaten by both 

animals and people. Radioactive materials that 

occur naturally in the soil (uranium, radium, 

thorium, potassium, tritium, and others) are also 

taken up by plants, and become available for intake 

by animals and people. 

1.3 HOW MIGHT I BE EXPOSED TO 

IONIZING RADIATION? 

The earth is continually irradiated with low levels of 

ionizing radiation, so all animals, plants, and other 

living creatures are exposed to small amounts of 

ionizing radiation from several sources every day. 

Figure 1 shows that most of your radiation dose 

comes naturally from the environment. Smaller 

portions come from medicine, consumer products 

and other sources. 

Figure 2 is another breakdown of the sources of 

radiation dose to the average American. The natural 

background levels (82%) shown in Figure 1-1 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

include the radon, terrestrial, cosmic, and natural 

internal sources shown in Figure 1-2. Most of your 

daily radiation dose is from radon (55%), which is 

found in all air. Higher levels are normally found 

indoors (especially in the basement). 

Figure 3 shows that indoor levels of radon vary 

depending on where you live. Higher levels can be 

found in underground areas, such as mines. 

You are always exposed to radiation from cosmic 

sources (mostly from outer space, some from the 

sun, 8%), terrestrial sources (rocks and soil, 8%), 

and natural internal sources (radioactive material 

normally inside your body, 10%). You may also be 

exposed to radiation from x ray exams (11%), 

nuclear medicine exams such as thyroid scans (4%), 

and consumer products including TV and smoke 

detectors (3%), as well as other sources. 

Less than 1% of the total ionizing radiation dose to 

people living in the United States comes from their 

jobs, nuclear fallout, the nuclear fuel cycle, or other 

exposures. However, people in some types of jobs 

may have higher doses (pilots and flight attendants, 

astronauts, industrial and nuclear power plant 

workers, x ray personnel, medical personnel, etc.). 

Some groups of people have been exposed to 

higher-than- normal levels of ionizing radiation 

from weapons testing, and some individuals from 

accidents at nuclear facilities or in industry. 

Not everyone will be exposed to every source or the 

same percentage of radiation shown in Figure 2. 

Since the percentages shown in Figure 2 are 

averages, half of the population will receive greater 

doses and half will receive smaller doses from the 

several sources shown in the figure. For example, if 

you are not regularly x rayed, you may receive less 

total radiation dose than what is shown. However, if 

you live in a town or city at a high altitude, you may 

receive a greater radiation dose from outer space 

cosmic rays than someone who lives in a town or 

city near the ocean at sea-level. Table 1 shows you 

that where you live and what you do determines 

how much ionizing radiation you will receive. 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

Table 1-1. Approximate Doses of Ionizing Radiation 

to Individuals 

Activity Approximate 

doses of 

radiation 

received 

Comments 

Average American exposure to ionizing radiation 

Total yearly dose 360 mrem/yr 

(3.6 mSv/yr) 

From natural sources 300 mrem/yr 

(3.0 mSv/yr) 

From man-made 

sources 

60 mrem/yr 

(0.6 mSv/yr) 

From nuclear power Less than 1 

mrem/yr 

 (<0.01 mSv/yr) 

Approximate doses of ionizing radiation (cosmic + terrestrial) for 

different locations 

Kerala, India, 

resident 

1300 mrem/yr 

(13 mSv/yr) 

Concentrated radioactive 

material in the soil 

Colorado state 

resident 

179 mrem/yr 

(1.79 mSv/yr) 

High altitude above sea 

level 

Boston, 

Massachusetts, 

resident 

100 mrem/yr 

(1 mSv/yr) 

Louisiana state 

resident 

92 mrem/yr 

(0.92 mSv/yr) 

Low altitude above sea 

level 

Approximate doses of ionizing radiation above background radiation 

and some activities 

Anyone near a 

patient released after 

a nuclear medicine 

test. 

Less than 500 

mrem/patient 

(5 mSv/patient) 

Guidance for medical 

facilities. Quantity 

depends on the quantity 

of radioactive material. 

A person who works 

inside a nuclear 

power plant 

< 300 mrem/yr 

(<3 mSv/yr) 

A person who gets a 

full set of dental x 

rays 

40 mrem 

(0.4 mSv) 

A flight attendant 

flying from New York 

to Los Angeles 

5 mrem/flight 

(0.05 mSv/flight) 

Watching a color TV 

set 

2-3 mrem/yr 

(0.02-0.03 mSv/yr) 

"Dose" is a broad term that is often used to mean 

either absorbed dose, or dose equivalent, depending 

on the context. The absorbed dose is measured in 

both a traditional unit called a rad and an 

International System (S.I.) unit called a gray (Gy). 

Both grays and rads are physical units (1 Gy = 100 

rad) that measure the concentration of absorbed 

energy. The absorbed dose is the amount of energy 

absorbed per kilogram of absorber. Physical doses 

from different radiations are not biologically 

equivalent. For this reason, a unit called the dose 

equivalent, which considers both the physical dose 

and the radiation type, is used in radiation safety 

dosimetry. The unit of dose equivalent is called the 

rem in traditional units and the sievert (Sv) in S.I. 

units (1 rem = 0.01 Sv). For beta and gamma 

radiation, 1 rad = 1 rem (1 gray = 1 sievert). For 

alpha radiation, however, 1 rad = 20 rem (1 gray = 

20 sievert). Small radiation doses can be expressed 

using small dose units such as the millirem (mrem) 

and the millisievert (mSv), where 1 mrem = 0.001 

rem and 1 mSv = 0.001 Sv. 

A person who lives 1 mrem/yr 

directly outside of a (0.01 mSv/yr) 

nuclear power plant 

A person who lives in ~1 mrem/yr for Difference between Los 

a multi-storied each 5 stories Angeles and Denver = 

apartment building above the ground 87 mrem/5000 feet = 2 

floor mrem/100 feet = 1 

(<0.01 mSv/yr) mrem/5 stories 

A person who Less than 0.1 

watches a truck mrem/truck 

carrying nuclear (0.001 mSv) 

waste pass by 

The average annual dose to a person in the United 

States is about 360 mrem (3.6 mSv). An individual's 

exact dose depends on several factors, such as the 

natural background where the person lives, and the 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

person's medical history and occupational 

experience with sources of radiation. 

1.4 HOW CAN IONIZING RADIATION 

ENTER AND LEAVE MY BODY? 

Ionizing radiation exposure can occur from a 

radiation source outside of the body. Exposure can 

also occur as a result of taking radioactive material 

into your body. The answer to the question of how 

you can be exposed to ionizing radiation can be 

broken into two parts. The first paragraph below 

describes ionizing radiation that comes from a 

source outside your body and some distance away 

(external radiation). The second paragraph describes 

ionizing radiation that comes from a source inside 

your body (internal radiation). 

External radiation comes from natural and manmade sources of ionizing radiation that are outside 

your body. Part of the natural radiation is cosmic 

radiation from space. The rest is given off by 

radioactive materials in the soil and building 

materials that are around you. As a result of human 

activities, higher levels of natural radioactive 

material are left in products or on the land. 

Examples of such activities are manufacturing 

fertilizer, burning coal in power plants, and mining 

and purifying uranium. Ionizing radiation from 

human activities adds to your external radiation 

exposure. Some of this radiation is given off by x 

ray machines, televisions, radioactive sources used 

in industry, and patients who have had recent 

nuclear medicine tests and therapy. The rest is given 

off by man-made radioactive materials in consumer 

products, industrial equipment, atom bomb fallout, 

and to a smaller extent by hospital waste and 

nuclear reactors. Gamma rays are the main type of 

ionizing radiation that are of concern when you are 

exposed to external sources of ionizing radiation. 

Gamma rays (like x rays) are special bundles of 

light energy that you cannot see, feel, or smell. 

Gamma rays from natural and man-made sources 

pass through your body just like x rays do, at the 

speed of light. Gamma rays may pass directly 

through your body without hitting anything. When 

one gamma ray hits a cell, it leaves a small bit of 

energy behind that can cause damage. Other types 

of ionizing radiation, like alpha and beta particles, 

hit your body but normally do not have enough 

energy to get inside to harm you. Your external 

radiation dose depends on the amount of energy that 

ionizing radiation gives to your body as it passes 

through. Exposure to external radiation does not 

make you radioactive. The average yearly dose 

from external radiation in the United States is about 

100 mrem per person (1 mSv/person). 

Internal radiation is ionizing radiation that natural 

and man-made radioactive materials give off while 

they are inside your body. You take radioactive 

materials into your body every day since they are in 

the air you breathe, the food you eat, and the water 

you drink. Examples of natural radioactive 

materials that enter, reside in, and leave your body 

every day include potassium-40, carbon-14, radium, 

and radon. Man-made radioactive materials also get 

into your body from the decreasing amounts of 

fallout from past nuclear weapons testing. 

Sometimes, natural conditions or industrial 

activities concentrate radioactive materials. If you 

are exposed to these, you will take in more 

radioactive material. Low amounts of material that 

act as sources of ionizing radiation may also be put 

into your body for medical purposes to test for or 

treat some types of disease, such as cancer. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

Scientists and clinicians have made sure that the 

benefits of exposing you to ionizing radiation far 

outweigh any bad health effects you may get from 

the ionizing radiation by itself. (Medical tests use 

small amounts of radiation or radioactive material, 

but some radiotherapy uses large doses that are 

beneficial to the patient.) Hospitals, coal-fired 

electricity generating plants, and nuclear reactors 

release radioactive materials in ways that keep your 

dose low. Radioactive materials build up in your 

body if you take them in faster than they leave in 

urine and feces and by radioactive transformation. If 

the internally deposited radioisotope is short lived 

and decays before the body eliminates it, then, of 

course, it will disappear faster from the body than 

by biological elimination alone. Thus, retention or 

elimination of internally deposited radioisotopes is 

measured by the effective half-life, which considers 

the combined effect of biological elimination and 

radioactive decay. 

Internally deposited radioisotopes may emit gamma 

rays, beta particles, or alpha particles, depending on 

the isotope. Many gamma rays escape your body 

without hitting anything. When a gamma ray does 

hit a cell, it transfers energy to the cell. When all 

their energy is transferred, they vanish. Alpha and 

beta particles travel short distances, giving energy 

to cells they hit. They lose energy and quickly come 

to a stop. Their energy is totally absorbed inside 

your body. When alpha particles come to a stop, 

they become helium that you breathe out later. 

When beta particles come to a stop, they become 

electrons and attach to atoms near them. Your 

internal dose is a measure of the energy deposited 

by all the ionizing radiation that is produced inside 

your body. The average yearly dose in the United 

States from internal radiation is about 260 mrem per 

person (2.6 mSv/person). 

1.5 HOW CAN IONIZING RADIATION 

AFFECT MY HEALTH? 

How radiation affects your health depends on the 

size of the radiation dose. Scientists have been 

studying the effects of ionizing radiation in humans 

and laboratory animals for many years. Studies so 

far have not shown that the low dose of ionizing 

radiation we are exposed to every day causes us any 

harm. We do know that exposure to massive 

amounts of ionizing radiation can cause great harm, 

so it is wise to not be exposed to any more ionizing 

radiation than necessary. 

Overexposure to high amounts of ionizing radiation 

can lead to effects like skin burns, hair loss, birth 

defects, cancer, mental retardation (a complex 

central nervous system functional abnorm ality), 

and death. The dose determines whether an effect 

will be seen and its severity. For some effects such 

as skin burns, hair loss, sterility, nausea, and 

cataracts, there is a certain minimum dose (the 

threshold dose) that must be exceeded to cause the 

effect. Increasing the size of the dose after the 

threshold is exceeded makes the effect more severe. 

Psychological stress has been documented in large 

populations exposed to small doses of radiation 

(Three Mile Island and Chernobyl). Neurological 

injury (CNS syndrome) resulting in compromised 

mental function has also been documented in 

individuals exposed to several thousand rads of 

ionizing radiation. 

Ionizing radiation is called a carcinogen because it 

may also increase your chance of getting cancer. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

Increasing the size of the dose increases your 

chance of getting cancer. Scientists base radiation 

safety standards on the assumption that any 

radiation dose, no matter how small, carries with it 

a corresponding probability of causing a cancer. 

This is called a "zero threshold" dose response 

relationship. Cancers that are actually caused by 

radiation are completely indistinguishable from 

those from other causes, so we can never be certain 

whether any individual cancer was not caused by 

radiation. To determine how likely it is that a 

certain dose of radiation will cause cancer, scientists 

measure the radiation dose to a group of exposed 

people, like the Japanese atomic bomb survivors. 

Then they compare the frequency of cancers (the 

observation period for cancer extends over decades) 

in this exposed group with a similar group of people 

who were not exposed. They also look at factors 

like age, sex, and time since the exposure ended. 

Finally, they calculate risk factors for various 

cancer types. Using these factors, it is possible to 

estimate the chance of getting cancer from a dose of 

radiation. Even though they assume a zero 

threshold, researchers have not actually seen an 

increase in cancer frequency for people in the 

exposed Japanese group who had a radiation dose 

below 20 rad (0.2 Gy). No increase in any type of 

leukemia has been found in people whose radiation 

dose was below 10–40 rad (0.1–0.4 Gy). 

The effects of internally deposited radioactive 

material are similar to those of external radiation. 

The effects depend on the size of the dose and 

factors like your sex and age when you were 

exposed. The radiation absorbed dose, in turn, 

depends on the radioactive material, the amount of 

activity, the type and energy of the radiation, the 

effective half-life of the radioactive material, its 

chemical form, how it was taken into your body, 

and how quickly it leaves your body. 

Many people are exposed to radiation and 

radioactive materials used in medical testing and 

therapy. Radiation treatments for medical reasons 

carry the same risk as radiation from other sources. 

As with any medical treatment, the potential health 

benefits should be balanced against the potential 

harmful health effects. 

One way to better understand the effects of 

radiation is to study its effects on test animals. 

Without laboratory animals, scientists would lose a 

basic method to get information needed to make 

informed decisions to protect public health. 

Scientists have the responsibility to treat research 

animals with care and compassion. Laws today 

protect the welfare of research animals, and 

scientists must comply with strict animal care 

guidelines. 

1.6 HOW CAN IONIZING RADIATION 

AFFECT CHILDREN? 

This section discusses potential health effects from 

exposures during the period from conception to 

maturity at 18 years of age in humans. Potential 

effects on children resulting from exposures of the 

parents are also considered. 

Like adults, children are exposed to small 

background amounts of ionizing radiation that 

comes from the soil around where they live, in the 

food and water that they eat and drink, in the air that 

they breathe, and from sources that reach earth from 

space. How much background radiation you receive 

depends on where you live. Some places naturally 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

have more than others. There are no reports that say 

exposure to background levels of ionizing radiation 

causes health effects in children or adults. 

If a pregnant woman is exposed to high levels of 

ionizing radiation, it is possible that her child may 

be born with some brain abnormalities. There is an 

8-week period during early pregnancy when an 

unborn child is especially sensitive to the effects of 

higher than normal levels of ionizing radiation. As 

the levels of ionizing radiation increase, so does the 

chance of brain abnormalities. These abnormalities 

may eventually result in small head size, decreased 

intelligence as measured by Intelligence Quotient 

(IQ) tests, and other defects. These effects are not 

reversible. 

A child will be exposed to small amounts of 

radiation from the environment all during prenatal 

development and throughout its life. There are no 

reports that say children suffer health effects from 

normal amounts of background radiation. If 

children are exposed to higher than background 

levels of ionizing radiation, they are likely to have 

the same possible health effects as adults exposed to 

similar levels. 

1.7 HOW CAN FAMILIES REDUCE THE 

RISK OF EXPOSURE TO IONIZING 

RADIATION? 

If your doctor finds that you have been exposed to 

significant amounts of ionizing radiation, ask 

whether your children might also be exposed. Your 

doctor might need to ask your state health 

department to investigate. 

The best way to reduce your risk of exposure to 

higher than background amounts of radiation is to 

not let yourself be exposed at all. However, this is 

not always possible or sensible. A common way to 

be exposed to ionizing radiation is by receiving an x 

ray, but a few x rays every year will not hurt you. 

When you or your children receive an x ray, be sure 

to correctly wear any protective garments that are 

provided. The technician will make sure that only 

the area that needs to be x rayed will be exposed to 

the x ray beam. 

It may be necessary to inject you with a chemical 

that has some amount of radioactive material in it to 

help a doctor diagnose or treat a disease. Many 

studies have shown that these drugs, used correctly, 

will not harm you. Be sure to follow the doctor's 

directions after you have been treated with these 

drugs. 

Many places make or use various types of 

radioactive material or ionizing radiation for 

medical or research purposes. If you visit one of 

these facilities, be sure to follow all of the 

recommended safety precautions. Do not go into 

unauthorized areas. You may be asked to wear a 

special device on your shirt that records the amount 

of ionizing radiation you are exposed to while in the 

facility. This is a safety precaution. Do not put it in 

your pocket or let someone else wear it. 

1.8 IS THERE A MEDICAL TEST TO 

DETERMINE WHETHER I HAVE BEEN 

EXPOSED TO IONIZING RADIATION? 

There are no easy or accurate medical tests to 

determine whether you have been exposed to low 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

doses of ionizing radiation, but tests are available 

for determining whether you have been exposed to 

radioactive material. 

Tests for Recent Exposure to Ionizing Radiation. A 

great degree of overexposure is necessary to cause 

the clinical signs or symptoms of radiation 

exposure. In the absence of clinical signs or 

symptoms there are two kinds of tests scientists use 

to see if you have been overexposed to ionizing 

radiation; they look for changes in blood cell counts 

and changes in your chromosomes. If you are 

exposed to no more than 10 rad (0.1 Gy) of ionizing 

radiation, there are no detectable changes in blood 

cell counts. The most sensitive measure of radiation 

exposure involves a study of your chromosomes. 

This is a special test for doses that are too low to 

produce clinically observable signs or symptoms; 

this test may be useful for doses greater than about 

3 times the maximum annual permissible dose for 

radiation workers. Changes in the white blood cell 

count may be seen in people whose doses exceeded 

about 5 times the occupational maximum 

permissible annual dose. Radiation doses at or 

above these levels can be reliably estimated using 

these two special tests. 

Tests for Radioactive Material Inside Your Body. 

Scientists can also examine your blood, feces, 

saliva, urine, and even your entire body to see if 

measurable amounts of radioactive material are 

being excreted from your body. Different tests are 

used for different types of radioactive material. 

Several types of instruments are available to look 

for each type of radiation. These instruments are not 

available at your doctor's office. They are normally 

large, heavy, and available only in laboratories. 

Equipment usually consists of a "detector," 

electrical cables, and a "processor." The detector 

contains material sensitive to one or more types of 

radiation, so the detector is chosen based on the 

type of radiation to be measured. Alpha, beta, and 

gamma radiation have different energies that 

depend upon the radioactive isotope from which 

they come. By determining the type and energy of 

the radiation, scientists can tell which radioisotope 

is on your skin or inside your body. 

1.9 WHAT RECOMMENDATIONS HAS THE 

FEDERAL GOVERNMENT MADE TO 

PROTECT HUMAN HEALTH? 

Recommendations and regulations are periodically 

updated as more information becomes available. 

For the most current information, check with the 

federal or state agency or organization that provides 

it. 

The current federal and state regulations limit 

radiation workers' doses to 0.05 Sv/year (5 

rem/year). The limit for the unborn child of a 

female radiation worker is 0.005 Sv (0.5 rem) per 9­

month gestation period. For the general public, the 

limit is 0.001 Sv/year (0.1 rem/year), with 

provisions for a limit of 0.005 Sv/year (0.5 

rem/year) under special circumstances. The public 

dose limit is set at least 10 times lower than the 

occupational limit to give the public an extra margin 

of safety. A factor of 10 is also used for public 

protection in other industries. 

We have seen health effects from very high doses of 

ionizing radiation, but not at normal everyday 

levels. To be cautious, scientists and regulating 

agencies assume that there could be some harmful 

effects at any dose, no matter how small. Because 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

ionizing radiation has the potential to cause harmful 

health effects in overexposed people, regulations 

and guidelines have been established for ionizing 

radiation by state, national, and international 

agencies. The basic philosophy of radiation safety is 

to allow only a reasonable risk of harm using the 

concept of "as low as reasonably achievable" 

(ALARA). Some regulations and recommendations 

for ionizing radiation include the following: 

Radiation protection standards for radiation workers 

and members of the public are recommended by the 

International Commission on Radiological 

Protection (ICRP) and the National Council on 

Radiation Protection and Measurements (NCRP). 

These standards are not regulations, but they 

provide the scientific basis for the making of 

regulations by federal agencies. The ICRP and 

NCRP are authoritative bodies that analyze current 

scientific and epidemiological data and make 

recommendations to government and nongovernment organizations that set standards. ICRP 

and NCRP do not issue standards themselves. 

Federal agencies, such as the EPA, the Nuclear 

Regulatory Commission (NRC), and the 

Department of Energy (DOE), as well as individual 

states are responsible for making federal and state 

regulations about exposure to ionizing radiation. 

The NRC regulates nuclear power plant operations 

and regulates the use of radioactive material in 

research and medical applications. The DOE has 

issued employee dose limits for its facilities. 

The EPA is responsible for federal radiation 

protection guidance for environmental radiation 

standards and regulations to implement specific 

statutory requirements, such as the Safe Drinking 

Water Act and the Clean Air Act. Natural 

background radiation, of course, cannot be 

regulated but EPA recommends that the 

concentration of indoor radon not exceed 4 

picocuries per liter (4 pCi/L) of air. EPA's National 

Emission Standards for Hazardous Air Pollutants 

(NESHAPs) contain regulations that limit the dose 

from radionuclides released to the air to 0.1 

mSv/year (10 mrem/year). The EPA sets limits on 

the maximum acceptable concentration of 

radionuclides in public drinking water supplies. 

Based on the Safe Drinking Water Act, the EPA has 

issued drinking water standards for radionuclides, 

which include dose limits of 0.04 mSv/year (4 

mrem/year) for man-made sources of beta and 

gamma emitters. EPA also sets limits on several 

alpha emitters in drinking water, such as radium and 

radon. 

The NRC regulations apply to all types of ionizing 

radiation that are emitted from special nuclear 

material (such as nuclear reactor fuel) and from byproduct material (materials made radioactive in the 

use of special nuclear material), and from source 

material (material from which nuclear fuel is 

made).The NRC sets limits on the total dose of 

ionizing radiation above background from these 

sources. It also sets limits for the amounts and 

concentrations of radioactive material that will give 

these doses if taken into the body. These are called 

Annual Limits on Intake (ALI) and derived air 

concentrations (DAC). 

The NRC has also issued a standard for cleaning up 

sites contaminated with radioactive materials. It 

requires that the radiation dose to the public from 

these sites will not be more than 0.25 mSv per year 

(25 mrem per year). 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

IONIZING RADIATION 

Division of Toxicology September 1999 

Radiation doses from procedures used by licensed 

physicians in diagnosis and treatment of disease is 

not limited by regulations. However, physicians and 

medical technicians must be specially trained and 

licensed to use radiation-producing machines and 

licensed to use radioisotopes for these purposes. 

They are required to limit exposures to the members 

of the public who are inside their facilities to 100 

mrem per year, which is the same level as required 

by the NRC. Also, patients with radioactive 

materials inside their bodies from the treatment are 

kept until it is likely that they will not expose 

anyone around them to more than 0.5 mSv (500 

mrem) from that radioactive material. 

States also regulate radioactive materials and other 

sources of radiation that are not regulated by the 

NRC. These include sources of natural 

radioactivity, such as radium, and radiationproducing machines, such as x ray machines and 

radioactive material produced by particle 

accelerators. 

1.10 WHERE CAN I GET MORE 

INFORMATION? 

If you have any more questions or concerns, please 

contact your community or state health or 

environmental quality department or: 

Agency for Toxic Substances and Disease Registry 

Division of Toxicology 

1600 Clifton Road NE, Mailstop F-32 

Atlanta, GA 30333 

Information line and technical assistance: 

Phone: 888-422-8737 

FAX: (770)-488-4178 

ATSDR can also tell you the location of 

occupational and environmental health clinics. 

These clinics specialize in recognizing, evaluating, 

and treating illnesses resulting from exposure to 

hazardous substances. 

To order toxicological profiles, contact: 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

Phone: 800-553-6847 or 703-605-6000 

Reference 

Agency for Toxic Substances and Disease Registry 

(ATSDR). 1999. Toxicological profile for ionizing 

radiation. Atlanta, GA: U.S. Department of Health 

and Human Services, Public Health Service. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov