RADIATION 

PROTECTION

and the NRC
Radiation is all around us. 

It is naturally present in our environment and has been since before the

birth of this planet. Radiation occurs in nature, but it can also be produced artificially, as in medical x-rays and microwaves for cooking.

Radiation can be either beneficial or harmful, depending on its use and

control. Therefore, regulation of certain radioactive sources is necessary

so that people are protected from unnecessary or excessive exposures.

Radiation exists naturally everywhere on earth.

U . S . N U C L E A R R E G U L ATO RY C O M M I S S I O N 1
WHAT IS RADIATION?

Radiation is energy given off by matter in the form of rays or

high-speed particles. There are many forms of radiation that are

familiar to us. For example, we use light, heat, and microwaves

every day. Doctors use x-rays to see inside our bodies. Radio and

television waves bring us our favorite shows. All of these are forms

of radiation.

Radiation is either ionizing or non-ionizing, depending on how it

affects matter. Non-ionizing radiation includes visible light, heat,

radar, microwaves, and radio waves. It deposits energy in materials

it passes through. Conversely, ionizing radiation, such as x-rays and

cosmic rays, is more energetic than non-ionizing radiation. When

ionizing radiation passes through material, enough energy is deposited to break molecular bonds and create charged particles. These charged

particles can damage plant, animal,

and human cells, but they can also be

used beneficially. For example, we

use ionizing radiation to treat cancer,

sterilize medical equipment, and in

smoke detectors. The U.S. Nuclear

Regulatory Commission is the

Federal agency that regulates nuclear

materials that produce ionizing radiation.

WHERE DOES RADIATION COME FROM?

Although we receive ionizing radiation from both natural and artificial sources, most people are not aware of all sources of 

radiation in their environment.

Smoke detectors with a small

amount of radioactive material.

The NRC is the Federal agencyresponsible for regulating the safe

uses of radioactive materials for civilian purposes in the United States to

ensure the protection of public health

and safety and the environment.

NRC Headquarters

2 R A D I AT I O N P R OT E C T I O N
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The sun and stars send a constant stream of cosmic radiation to

Earth, much like a steady drizzle of rain. Differences in elevation,

atmospheric conditions, and the Earth’s magnetic field can change

the amount of cosmic radiation that we receive.

The Earth itself is also a source of terrestrial radiation. Radioactive materials

uranium, radium and thorium exist naturally in soil and rock. Essentially all air

contains radon, water contains small

amounts of dissolved uranium and thorium, and all organic matter, both plant

and animal, contains radioactive carbon

and potassium. All people have internal

radiation, mainly from radioactive potassium-40 and carbon-14, inside their bodies from birth. In addition to natural

sources, there are artificial or man-made

sources of radiation. These include medical sources (diagnostic x-rays and nuclear medicine procedures),

consumer products (televisions, smoke detectors, luminous watches,

and some ceramics), and industrial sources (nuclear power plants

and industrial radiography).

WHAT TYPES OF RADIATION ARE THERE?

As we mentioned before, radiation can be ionizing or non-ionizing. We are typically more concerned about

ionizing radiation because even though it has

beneficial uses, it is also potentially harmful if

not used correctly. Because of this potential hazard, the NRC strictly regulates commercial and

institutional uses of nuclear material, including

nuclear power plants.

There are five major types of ionizing radiation:

 alpha particles

 beta particles

 gamma rays

 x-rays

 neutrons

Alpha particles are charged particles with very little penetrating

power. They can be blocked by a sheet of paper, skin or a few inches

of air. Alpha particles do not usually make anything radioactive.

Nuclear medicine

Nuclear-power plant

U . S . N U C L E A R R E G U L ATO RY C O M M I S S I O N 3
Materials that emit alpha particles are potentially dangerous when

inhaled or swallowed, not because of external exposure. Alpha particles are emitted from naturally-occurring materials such as uranium,

thorium, and radium and from man-made elements such as plutonium

and americium. Very small amounts of alpha emitters are used in items

such as smoke detectors. 

Beta particles, which are similar to electrons, are lighter and more

penetrating than alpha particles, can travel a few feet in the air, and can

penetrate the skin. Like alpha particles, they do not usually make

things radioactive. A thin sheet of metal or plastic or a block of wood

can stop a beta particle. Beta emitters, such as strontium-90, are used

in medical applications such as treating

eye disease.

Gamma rays and x-rays consist of high

energy waves that can travel great distances at the speed of light and are very

penetrating. In general, neither x-rays nor

gamma rays have the ability to make anything radioactive. Several feet of concrete

or a few inches of more dense material,

such as lead, are needed to block these

types of radiation. Because of their penetration powers, gamma rays, such as from

cobalt-60, are often used in medical applications to treat cancer and to sterilize medical instruments. X-rays are

typically used to provide images of stationary parts of the body such as

teeth and bones and are also used in industry to find defects in welds.

Neutrons are nuclear particles that are also

very penetrating. Neutrons are the only

one of the five types discussed here that

can make objects radioactive. This process,

called neutron activation, is used to produce many of the radioactive sources used

in commercial, academic, and medical

applications. Neutrons can travel great distances in the air and require very thick

hydrogen-containing material (such as concrete or water) to stop them. Neutron radiation occurs primarily inside a nuclear

reactor where many feet of water provide

effective shielding. Neutrons are also used

in medicine research, and oil exploration.

X-rays penetration.

Nuclear power plant reactor core.

4 R A D I AT I O N P R OT E C T I O N
HOW IS RADIATION MEASURED?

Whether it emits alpha or beta particles, gamma rays or neutrons,a quantity of radioactive material is expressed in terms of its

radioactivity, or simply its activity, and

is measured in curies. Activity represents

how many atoms in the material decay

in a given time period.

The activity of a radioactive material

decreases with time. The time it takes

for the activity of a radioactive material

to decrease by half is called the radioactive half-life. After one half-life, the

remaining activity is one half of the

original activity. After two half-lives, the remaining activity is one

fourth; after three half-lives, one eighth; and so on. Some radioactive materials, such as iodine-131, have short half-lives measured in

days. Others, such as uranium-238, have half-lives measured in billions of years.

WHAT HAPPENS WHEN LIVING THINGS ARE 

EXPOSED TO RADIATION?

We tend to think of radiation exposure in terms of its effect onliving cells. For low levels of radiation exposure, the effects

are so small they may not be detected, because a body can repair

damage caused by radiation. The health effects of radiation on living

cells may result in three outcomes: (1) injured or damaged cells

repair themselves, resulting in no net damage; (2) cells die, much

like millions of body cells do every day, being replaced through normal biological processes; or (3) cells incorrectly repair themselves

resulting in a physical change. The exact effect depends on the specific type and intensity of the radiation exposure.

The amount of energy deposited in material by ionizing radiation is

called dose. Biological doses are measured in units called rems.

Geiger counter.

Penetrating Power of Radiation

Paper Wood Cement Skin

U . S . N U C L E A R R E G U L ATO RY C O M M I S S I O N 5
Where curies tell us the radioactivity of a substance, rems tell us the

amount of energy that radiation has deposited in an area of living

tissue. The likelihood of effects caused by radiation is related to the

dose of radiation received, regardless of whether the radiation is of

natural or artificial origin.

On average,

Americans receive a

dose of about 0.3 rems

(300 millirems) each

year from natural or

background radiation.

Most of this exposure

comes from radon in

the air with lesser

amounts from cosmic

rays and the Earth

itself. We also receive

about 0.06 rems (60

millirems) yearly from

man-made sources of

radiation, including

medical, commercial, and industrial sources. Medical procedures provide the largest amount of human exposure to human-made radiation.

For example, a chest x-ray typically gives a dose of about 0.01 rems (10

millirems). The average dose is regularly re-evaluated. 

Because radiation from nuclear material is strictly regulated, large

doses (~50 rems) of radiation to humans seldom occur. However,

high radiation doses, particularly over a short period of time, kill

many cells and can damage tissues and organs. Although radiation

affects different people in different ways, it is believed that humans

exposed to about 500 rems of radiation all at once would likely die

without medical treatment. A single dose of 100 rems to a human

may cause nausea or skin reddening, but recovery is likely. About

twenty-five rems of radiation can cause temporary sterility in men.

If these doses are spread out over time, instead of delivered all at

once, their effects tend to be less severe.

HOW CAN RADIATION EXPOSURE BE REDUCED?

Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. We can, however, minimize

Natural Background Radiation (~0.3 rems) 

RADON: 55%

COSMIC: 8%

TERRESTRIAL: 8%

INTERNAL: 11%

OTHER: <1%

CONSUMER 

PRODUCTS: 3%

NUCLEAR 

MEDICINE: 4%

MEDICAL 

X-RAYS: 11%

A yearly dose of 0.36 rems (360 millirems) from all radiation sources has

not been shown to cause humans any harm.

Man-Made Radiation (~0.06 rems)

6 R A D I AT I O N P R OT E C T I O N
exposure from discrete sources of radiation in three basic ways.

They work much the same way as protection against overexposure

to the sun.

1Time: The concept of time works in two ways. Limiting orminimizing the amount of exposure time will reduce the dose

from the radiation source. Also, allowing radioactive material as

much time as possible to decay before exposure will minimize

radiation exposure when that material needs to be handled.

2 Distance: Increasing the distance from the radiation sourcewill sharply decrease exposure. The intensity of radiation

decreases the further you are from the source in the same way that

the heat from a fire is less intense the further away you are from

it. 

3 Shielding: Inserting a solid material or shield betweenyourself and a radiation source will greatly reduce the radiation dose. Barriers of lead, concrete or water provide protection

from radiation from gamma rays and neutrons. This is why certain

radioactive materials are stored under water or in concrete or leadlined rooms, and why dentists place a lead blanket on patients

receiving x-rays of their teeth.

HOW DOES NRC HELP PROTECT THE PUBLIC 

FROM RADIATION EXPOSURE?

Radiation and its risks command considerable public attention.However, a system of radiation protection has been developed

over many decades as the effects of radiation have become better

understood. Specifically, the NRC ensures that users of radioactive

materials keep radiation exposures within its dose limits and as low as

reasonably achievable. Users must obtain a license from the NRC and

be inspected by the NRC to make sure they are following regulations

and using radioactive materials safely.

Less time spent 

near source: less radiation received.

Behind shielding from

source: less radiation 

received.

Greater distance from

source: less radiation

received.

U . S . N U C L E A R R E G U L ATO RY C O M M I S S I O N 7
In its standards for protection against radiation, the NRC has requirements for:

 Dose limits for radiation workers and members of the public

 Monitoring and labeling radioactive materials

 Posting signs in and around radiation areas

 Reporting the theft or loss of radioactive material

In addition, there are penalties for not following NRC regulations.

The NRC requires that radioactive materials be used in a way that

limits radiation exposure of individual members of the public to a

dose that does not exceed 0.1 rem (100 millirems) in a year. Adults

working with radioactive material must be protected so as not to

receive more than 5 rems (5,000 millirems) per year. Because workers

are exposed to various radiation sources, they are carefully monitored

with the use of small instruments called dosimeters.

If certain conditions are met, the NRC may enter into an agreement

with the governor of a state to turn over authority for regulating

radioactive materials to that state. States that meet these conditions

and agree to regulate materials using the same standards as the NRC

are called Agreement States. Typically, Agreement States regulate all

sources of radiation in the state except nuclear power plants, large

quantities of certain nuclear material, and any high-level radioactive

waste stored in the state. Currently, 34 states have such agreements

with the NRC.

For more information on radiation protection, please visit our website

at: www.nrc.gov and click on the second blue box, Radiation

Protection, on the left-hand side of the screen.

8 R A D I AT I O N P R OT E C T I O N
NUREG/BR-0322

February 2006

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555-0001

Office of Public Affairs