Radiation: Facts, Risks 

and Realities

EPA-402-K-10-008 

April 2012

Office of Air and Radiation

Office of Radiation and Indoor Air
Table of Contents

Introduction 1

What is Radiation? 2

Types of Radiation 3

Understanding Radiation Risks 6

Naturally Occurring (Background) Radiation 7

Man-Made Radiation 8

Radiation in Industry and Commerce 9

Exposure to Ionizing Radiation 11

Regulating Radiation Use 12

Suggested Reading 14
1

Introduction

While radiation is a term that most people have heard, the 

basic facts about radiation are much less familiar. The U.S. 

Environmental Protection Agency (EPA) is responsible for 

advising the government on radiation hazards and 

regulating certain sources of radioactivity in the 

environment. This booklet provides basic facts about 

radiation science, as well as information on the risks and 

realities of radiation exposure.
2

What is Radiation?

Radiation is energy. It can come from unstable atoms or it can be produced by 

machines. Radiation travels from its source in the form of energy waves or 

energized particles.

There are actually two kinds of radiation, and one is more energetic than the other. It 

has so much energy it can knock electrons out of atoms, a process known as ionization. 

This ionizing radiation can affect the atoms in living things, so it poses a health risk by 

damaging tissue and DNA in genes. While there are other, less energetic, types of nonionizing radiation (including radio waves, microwaves—and visible light), this booklet is 

about ionizing radiation. 

In the late 1800s, Marie and Pierre Curie were among the first to study certain elements 

that gave off radiation. They described these elements as radio-actif, the property 

that is now called “radioactivity.” As scientists studied radioactivity more closely, they 

discovered that radioactive atoms are naturally unstable. In order to become stable, 

radioactive atoms emit particles and/or energy waves. This process came to be known 

as radioactive decay. The major types of ionizing radiation emitted during radioactive 

decay are alpha particles, beta particles and gamma rays. Other types, such as x-rays, 

can occur naturally or be machine-produced.

Scientists have also learned that radiation sources are naturally all around us. Radiation 

can come from as far away as outer space and from as near as the ground that you are 

standing on. Because it is naturally all around us, we cannot eliminate radiation from our 

environment. We can, however, reduce our health risks by controlling our exposure to it.
3

Types of Ionizing Radiation 

Alpha Particles

Some unstable atoms emit alpha particles (α). 

Alpha particles are positively charged and made up 

of two protons and two neutrons from the atom’s 

nucleus, as shown in the illustration at the right. 

Alpha particles come from the decay of the heaviest 

radioactive elements, such as uranium, radium and polonium. Even though alpha 

particles are very energetic, they are so heavy that they use up their energy over short 

distances and are unable to travel very far from the atom.

The health effect from exposure to alpha particles depends greatly on how a person is 

exposed. Alpha particles lack the energy to penetrate even the outer layer of skin, so 

exposure to the outside of the body is not a major concern. Inside the body, however, 

they can be very harmful. If alpha-emitters are inhaled, swallowed, or get into the body 

through a cut, the alpha particles can damage sensitive living tissue. The way these 

large, heavy particles cause damage makes them more dangerous than other types of 

radiation. The ionizations they cause are very close together--they can release all their 

energy in a few cells. This results in more severe damage to cells and DNA.

Beta Particles

Beta particles (β) are small, fast-moving particles with

a negative electrical charge that are emitted from an β

atom’s nucleus during radioactive decay. These 

particles are emitted by certain unstable atoms such 

as hydrogen-3 (tritium), carbon-14 and strontium-90.

Beta particles are more penetrating than alpha particles but are less damaging to living 

tissue and DNA because the ionizations they produce are more widely spaced. They 

travel farther in air than alpha particles, but can be stopped by a layer of clothing or 

by a thin layer of a substance such as aluminum. Some beta particles are capable of 

penetrating the skin and causing damage such as skin burns. However, as with alphaemitters, beta-emitters are most hazardous when they are inhaled or swallowed.

α
4

ng 

ys and X-Rays

Gamma Rays

Gamma rays (γ) are weightless packets of energy 

called photons. Unlike alpha and beta particles, which 

have both energy and mass, gamma rays are pure 

energy. Gamma rays are similar to visible light, but 

have much higher energy. Gamma rays are often 

emitted along with alpha or beta particles during 

radioactive decay. 

Gamma rays are a radiation hazard for the entire body. They can easily penetrate 

barriers, such as skin and clothing that can stop alpha and beta particles. Gamma 

rays have so much penetrating power that several inches of a dense material like lead 

or even a few feet of concrete may be required to stop them. Gamma rays can pass 

completely through the human body easily; as they pass through, they can cause 

ionizations that damage tissue and DNA.

ALPHA Particles 

Stopped by a sheet of 

paper and cannot penetrate

the outer dead layer of skin

BETA Particles

Stopped by a layer of clothi

or by a thin sheet of a 

substance such as aluminum

GAMMA Rays and X-Rays

Stopped by several feet of 

concrete or a few inches 

of lead

Penetrating Powers of Alpha Particles, Beta Particles, Gamma Ra

γ
5

A CT scan uses multiple 

x-rays to give doctors a threedimensional image that they 

can use to diagnose patients.

X-Rays

Because of their use in medicine, almost everybody has heard of x-rays. X-rays are 

similar to gamma rays in that they are photons of pure energy. X-rays and gamma 

rays have the same basic properties but come from different parts of the atom. 

X-rays are emitted from processes outside the nucleus, but gamma rays originate 

inside the nucleus. They also are generally lower in energy and, therefore, less 

penetrating than gamma rays. X-rays can be produced naturally or artificially by 

machines using electricity. 

Literally thousands of x-ray machines are used daily in medicine. Computerized 

tomography, commonly known as CT or CAT scans, uses special x-ray equipment 

to make detailed images of bones and soft tissue in the body. Medical x-rays are 

the single largest source of man-made radiation exposure. X-rays are also used in 

industry for inspections and process controls. 
6

Understanding Radiation Risks

Radiation can damage living tissue by changing cell structure and damaging DNA. 

The amount of damage depends upon the type of radiation, its energy and the total 

amount of radiation absorbed. Also, some cells are more sensitive to radiation. Because 

damage is at the cellular level, the effect from small or even moderate exposure may 

not be noticeable. Most cellular damage is repaired. Some cells, however, may not 

recover as well as others and could become cancerous. Radiation also can kill cells.

The most important risk from exposure to radiation is cancer. Much of our knowledge 

about the risks from radiation is based on studies of more than 100,000 survivors of 

the atomic bombs at Hiroshima and Nagasaki, Japan, at the end of World War II. Other 

studies of radiation industry workers and studies of people receiving large doses of 

medical radiation also have been an important source of knowledge. Scientists learned 

many things from these studies. 

The most important are:

The higher the radiation dose, the greater the chance of developing cancer.

The chance of developing cancer, not the seriousness of the cancer, increases as 

the radiation dose increases.

Cancers caused by radiation do not appear until years after the radiation exposure. 

Some people are more likely to develop cancer from radiation exposure than others.

Radiation can damage health in ways other than cancer. It is less likely, but damage 

to genetic material in reproductive cells can cause genetic mutations, which could be 

passed on to future generations. Exposing a developing embryo or fetus to radiation 

can increase the risk of birth defects.

Although such levels of exposure rarely happen, a person who is exposed to a large 

amount of radiation all at one time could become sick or even die within hours or days. 

This level of exposure would be rare and can happen only in extreme situations, such 

as a serious nuclear accident or a nuclear attack. 
7

Determining Radiation Limits

Current science suggests there is some risk from any exposure to radiation. However, 

it is very hard to tell whether a particular cancer was caused by very low doses of 

radiation or by something else. While experts disagree over the exact definition and 

effects of “low dose,” U.S. radiation protection standards are based on the premise that 

any radiation exposure carries some risk.

Naturally Occurring (Background) 

Radiation

Radon

Radon is a colorless, odorless, tasteless radioactive gas that comes from the decay of 

radium, which is present in nearly all rocks and soils. Most of our exposure to 

naturally occurring radiation is from indoor radon. Since radon gas emits alpha 

particles, inhaling it can cause cancer. Radon can seep into buildings from the ground 

through cracks and other openings in floors or walls. Accumulated radon in buildings 

can pose a health hazard. 

Radon causes an estimated 20,000 lung cancer deaths each year. The Surgeon 

General has warned that radon is the second leading cause of lung cancer in the 

United States. Only smoking causes more lung cancer deaths. A smoker living in a 

home with high radon levels has an especially high risk of lung cancer.

Radon in the air is measured in picocuries per liter (pCi/L). When radon levels reach 4 

pCi/L or higher, the EPA and the U.S. Surgeon General recommend that homeowners 

take action to reduce them. It is estimated that nearly one in 15 American homes has 

a radon level that should be reduced. The only way to find out about the radon level in 

any home is to test for it.

Controlling the Risks from Radon Exposure

Testing for radon is easy. There are many kinds of low-cost, “do-it-yourself” radon test 

kits available by phone, online and in many stores. It takes only a few minutes to set

TOTAL AVERAGE ANNUAL DOES 

FROM MEDICAL PROCEDURES

300 mrem

e Annual Dose

g Background

tion) 311 mrem

Computed Tomography 

(CT Scans)

147 mrem

Nuclear Medicine 

77 mrem

Surgical Fluoroscopy 

43 mrem

Xrays and Fluoroscopes 

33 mrem

For more information 

about radon, its risks, and 

what you can do to protect 

yourself, or to obtain a free 

copy of EPA’s “A Citizen’s 

Guide to Radon,” visit 

www.epa.gov/radon, call 

the National Radon Hotline 

at 1-800-SOS-RADON or 

contact your state’s 

radon office.
8

TOTAL AVERAGE ANNUAL DOES 

FROM MEDICAL PROCEDURES

300 mrem

l Average Annual Dose

rrounding Background

ral Radiation) 311 mrem

halation

 Thoron)

28 mrem

m Space

33 mrem

ngestion

diation in

d Water)

29 mrem

rrestrial

and Soil)

21 mrem

Computed Tomography 

(CT Scans)

147 mrem

Nuclear Medicine 

77 mrem

Surgical Fluoroscopy 

43 mrem

Xrays and Fluoroscopes 

33 mrem

TOTAL AVERAGE ANNUAL DOES 

FROM MEDICAL PROCEDURES

300 mrem

ose

und

rem

Computed Tomography 

(CT Scans)

147 mrem

Nuclear Medicine 

77 mrem

Surgical Fluoroscopy 

43 mrem

Xrays and Fluoroscopes 

33 mrem

up the kit and then send it in for analysis. Homeowners can also hire a professional to 

do the testing. High radon levels in a home can be reduced in a variety of ways. The 

preferred method is called an active soil depressurization system, which is basically a 

vent pipe with a fan that vents radon (and other soil gases) from beneath the house.

Radiation from the Ground and from Space

Radon is not the only source of naturally occurring radiation. Some exposure to natural 

radiation comes from other elements in Earth’s crust, such as thorium and potassium. 

The radiation dose from these sources depends on the makeup of the soil and rocks in 

the local area. Another natural source is cosmic (space) radiation. Earth is constantly 

exposed to radiation created by processes occurring in the sun, other stars and in 

outer space. 

Exposure to cosmic radiation depends largely on elevation. Exposure increases as 

you rise farther above sea level to where the atmosphere is thinner. For example, 

people who live in Denver, Colorado, which is more than 5,000 feet above sea level, 

are exposed to more cosmic radiation than people living in Chicago, Illinois, which is 

approximately 700 feet above sea level.

Man-Made Radiation

Radiation in Medicine

Today, nearly half of the exposure of the U.S. population to radiation comes from 

medical sources according to the National Council on Radiation Protection and 

Measurements (NCRP). Most medical exposure comes from the use of standard x-rays 

and CT scans to diagnose injuries and diseases in patients. Drugs with radioactive 

material attached, known as radiopharmaceuticals, also are used to diagnose some 

diseases. These procedures are an important tool to help doctors save lives through 

quick and accurate diagnoses.

Also, other procedures, such as radiation therapy, use radiation to treat patients. 

One-third of all successful cancer treatments involve radiation. Carefully targeted 

radiation beams and certain radiopharmaceuticals destroy cancerous cells while limiting 

damage to nearby healthy cells. 
9

Controlling the Risks of Medical Radiation

The Nuclear Regulatory Commission (NRC), the Food and Drug Administration (FDA) 

and other federal and state agencies issue regulations and guidelines to ensure that 

technicians and equipment meet standards for minimizing radiation exposure. When 

prescribed appropriately, the benefits of medical radiation outweigh the risks.

Radiation in Industry and Commerce 

Nuclear Power

Nuclear power reactors, which use uranium as fuel, supply the United States with about 

20 percent of its electricity. Nuclear power plant operations are tightly controlled, making 

nuclear energy responsible for only a very small part of the public’s overall exposure 

to radiation.

Industrial and Commercial Uses of Radiation 

Industries use radiation in a variety of ways. For example, industrial radiography uses 

x-rays to check for weak points in metal parts and welds before products are sold. Other 

examples of the use of radiation in industry include irradiators (machines used to kill 

bacteria and other pathogens in food and other items), devices that test the density of 

highway and construction materials, research reactors, and security screening at airports 

and shipping ports.
10

Controlling the Risks from Nuclear Materials in Industry 

and Commerce

Several agencies regulate the use of radioactive materials in industry. NRC and 

authorized state radiation programs issue licenses to companies to use radioactive 

materials and require special safety measures for their use, storage and disposal. 

The Occupational Safety and Health Administration (OSHA) issues regulations and 

standards to help protect workers from unsafe handling of radioactive material or 

equipment that creates radiation such as x-ray machines.

In the event of an emergency involving radioactive material, state and local 

governments have the first responsibility for protecting the public and the environment. 

Several federal agencies would respond as well, including EPA; NRC; and the 

Departments of Homeland Security, Energy, Agriculture, and Health and Human 

Services. In addition, U.S. nuclear power plants and the communities that surround 

them must have emergency plans for protecting the public from radiation exposure in 

the event of an accidental release of radioactive material into the environment.

Radioactive Waste

Any activity that produces or uses radioactive material generates radioactive waste. 

Examples include: nuclear power generation, defense weapons production, nuclear 

medicine, mining, oil and gas production and scientific research. Depending on the 

material it contains, this waste can remain radioactive for periods ranging from a few 

days to billions of years.
11

Sources of Radiation Exposure

From: NCRP Report No. 160

Reprinted with permission of the National 

Council on Radiation Protection and 

Measurements, http://NCRPonline.org

Industrial (< 0.1%)

Occupational (< 0.1%)

Space

(background)

Internal (5%)

(background)

(5%)

Terrestrial

(background)

(3%)

Computed 

tomography

(medical)

(24%)

Radon & thoron

(background)

(37%)

Consumer (2%)

Conventional 

radiography/fluoroscopy

(medical) (5%)

Nuclear medicine

(medical) (12%) Interventional 

fluoroscopy

(medical) (7%)

Controlling Radioactive Waste

Radioactive wastes must be managed and disposed of properly. Federal agencies 

and some states control the risks that come with radioactive waste by setting limits 

and regulations that disposal facilities must follow. EPA is responsible for setting 

environmental standards that are used by other federal and state agencies in 

regulations for the disposal of radioactive waste.

Exposure to Ionizing Radiation

In 2009 the National Council on Radiation Protection and Measurements (NCRP) 

published a study of the U.S. population’s exposure to radiation. The chart below shows 

sources that contributed to an annual average dose of 620 millirem (6.2 millisieverts) 

per person (millirem and millisievert are units of radiation dose). This is a national 

average: individual exposures will vary depending on factors such as altitude (space), 

local soils (radon and thoron), and the number of nuclear medicine procedures or 

x-rays received.
12

Regulating Radiation Use

States

The states have agencies responsible for regulating the use of radiation and for 

responding to radiation questions and problems. State agencies are the best, first 

source of information about radiation issues. States also regulate the use of x-ray 

machines. Some also are responsible for regulating other sources of radiation within the 

state on behalf of federal agencies such as the NRC.

U.S. Environmental Protection Agency (EPA)

EPA issues standards and guidelines to limit human exposure to radiation. EPA works 

directly with the public and industry, the states, and other government agencies to 

inform people about radiation’s risks and to promote actions that reduce human 

exposure. EPA measures environmental levels of radiation and assesses radiation’s 

effects on people and the environment.

U.S. Nuclear Regulatory Commission (NRC)

NRC develops regulations based on EPA’s standards for protecting the public from 

radiation. NRC regulates the civilian uses of nuclear materials in the United States 

by licensing facilities that possess, use, or dispose of nuclear materials; establishing 

standards; and inspecting licensed facilities. NRC regulates nuclear power plants and 

other users of nuclear materials, including hospitals, educational institutions, research 

institutions, and industrial equipment such as gauges and testing equipment.

U.S. Department of Homeland Security (DHS)

DHS has the primary responsibility for ensuring that emergency response professionals 

are prepared to respond to a terrorist attack, natural disaster or other large-scale 

emergency. DHS coordinates the comprehensive federal response to any large-scale 

crisis and mounts a recovery effort. Additionally, DHS educates citizens about preparing 

themselves, their families, and their homes for major emergencies.
13

U.S. Department of Health and Human Services (HHS)

The HHS Food and Drug Administration’s (FDA) Center for Devices and 

Radiological Health establishes safety standards for x-ray machines and other 

radiation-producing devices.

U.S. Department of Energy (DOE)

DOE is responsible for the development of the disposal system for spent nuclear fuel 

from the nation’s civilian nuclear power plants. This activity is fully funded by a tax 

paid by the users of nuclear-generated electricity. DOE is also responsible for the 

management and disposal of nuclear waste and other radioactive materials associated 

with nuclear weapons production at federally owned facilities. DOE is cooperating with 

state governments and private industry in working to clean up its present and former 

nuclear sites. DOE provides technical advice and assistance to states and the private 

sector in the management and disposal of low-level radioactive waste.

U.S. Department of Defense (DOD)

While the DOE is responsible for the safe handling of radioactive material at defense 

production facilities, the DOD is responsible for the safe handling and storage of nuclear 

weapons in its custody and for other military uses of nuclear energy.

U.S. Department of Transportation (DOT)

The DOT, in cooperation with the NRC and the states, governs the packaging 

and transport of radioactive materials. The DOT also regulates carriers of 

radioactive materials.

Occupational Safety and Health Administration (OSHA)

OSHA, a division of the U.S. Department of Labor, develops and enforces radiation 

protection regulations to protect workers not covered by other agencies.
14

Suggested Reading

Explore “RadTown” at www.epa.gov/radtown. RadTown is an interactive, virtual 

community. Each place in RadTown helps you learn about radiation sources or 

radiation-treated items you might find there.

The following books and websites provide more information on the health effects of 

radiation exposure.

Basic Information

Cember, H. Introduction to Health Physics. 4th ed., McGraw-Hill Medical, 2008.

 “Conference of Radiation Control Program Directors, Inc.” 2011. Conference of 

 Radiation Control Program Directors. www.crcpd.org

“Health Physics Society.” 2011. Health Physics Society. www.hps.org

 Martin, A. and Harbison, S.A., An Introduction to Radiation Protection. 3rd ed., 

 London: Chapman and Hall, 1987.

 “Radiation Protection.” 2011. U.S. Environmental Protection Agency. 

 www.epa.gov/radiation

 Shapiro, J. Radiation Protection. 4th ed., Harvard University Press, 2002. 

 Turner, J.F. Atoms, Radiation, and Radiation Protection 3rd ed. Wiley, 2007.

Advanced Reading

 Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 

 The National Academies Press, Washington, DC, 2006.

 Ionizing Radiation Exposure of the Population of the United States (NCRP

 Report No. 160): National Council on Radiation Protection and Measurements, 2009. 
For additional radiation information, please visit our website: www.epa.gov/radiation