R A D I A T I O N 

P R O T E C T I O N

a n d t h e N R C

U . S . N uc l ear R e g u l ator y C o m m ission 1 

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.
2 R adiation P rotection

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.

The NRC is the Federal agency responsible 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

Smoke detectors with a small 

amount of radioactive material.
U . S . N uc l ear R e g u l ator y C o m m ission 3

+

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-ioniz- ing. 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
4 R adiation P rotection

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.
U . S . N uc l ear R e g u l ator y C o m m ission 5

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 radioac- tive 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 on living 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 Skin Wood Cement
6 R adiation P rotection

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 0.31 

rems (310 milli- rems) each year 

from natural or 

background radia- tion. Most of this 

exposure comes 

from radon in the 

air with lesser 

amounts from cos- mic rays and the 

Earth itself. We also receive about 0.31 rems (310 mrem) yearly from 

man-made sources of radiation, including medical, commercial, and 

industrial sources. Nuclear medical procedures provide the largest 

amount of human exposure to man-made radiation. For example, a chest 

x-ray typically gives a dose of about 0.01 rems (10 mrems) and a CT 

(full body) scan gives a dose of about 1 rem (1,000 mrem). 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 25 

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 impos- sible to completely avoid exposure. We can, however, minimize 

A yearly dose of 0.62 rems (620 millirems) from all radiation sources has 

not been shown to cause humans any harm.

Sources of Radiation Exposure in the United States

Natural Sources - 50%

~310 millirem (0.31 rem)

Source: NCRP Report No.160(2009) 

Full report is available on the NCRP Web site at www.NCRPpublications.org.

Industrial and

Occupational - .1%

Consumer 

Products - 2%

Nuclear 

Medicine - 12%

Internal - 5%

Terrestrial 

(Soil) - 3%

Cosmic 

(Space) - 5%

Manmade Sources - 50%

~310 millirem (0.31 rem)

Radon and

Thoron - 37%

Medical

Procedures -

36%
U . S . N uc l ear R e g u l ator y C o m m ission 7

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 or minimizing 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.

2Distance: Increasing the distance from the radiation source will 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. 

3Shielding: Inserting a solid material or shield between yourself and a radiation source will greatly reduce the radia- tion 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.
8 R adiation P rotection

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 mrems) in a year. Adults 

working with radioactive material must be protected so as not to 

receive more than 5 rems (5,000 mrems) 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.

NUREG/BR-0322, Rev. 1

June 2010

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555-0001

Office of Public Affairs