Radiation is energy that comes from a source and travels through space at the speed of light. This energy has an electric field and a magnetic field associated with it, and has wave-like properties. You could also call radiation “electromagnetic waves”.

The Electromagnetic Spectrum

• There is a wide range of electromagnetic radiation in nature. Visible light is one example.
• Radiation with the highest energy includes forms like ultraviolet radiation, x-rays, and gamma rays.
• X-rays and gamma rays have so much energy that when they interact with atoms, they can remove electrons and cause the atom to become ionized.

The Ionized Atom

• Radioactive atoms have unstable blends of protons and neutrons.
• Radioactivity is the spontaneous release of energy from an unstable atom to get to a more stable state.
• Ionizing Radiation is the energy that comes out of a radioactive atom.
• Radioactive isotopes are radioactive atoms of the same element that have different numbers of neutrons.

Properties of Radioactive Isotopes

• Radioactive atoms can give off four types of ionizing radiation: alpha particles, beta particles, gamma rays, and neutrons.
• Each type of radiation has different properties that affect how we can detect it and how it can affect us.
• Radioactive decay happens when an unstable atom gives off radiation and changes into a more stable atom of a different element.
• The length of time it takes for half of the radioactive atoms in a group of radioactive isotopes to decay is called a half-life.

The most common form of radiation we are all familiar with is visible light. Light is energy that originates from a source and travels through space at the speed of … light! It has a particular wavelength and frequency that defines its energy.

We can detect this radiation with our eyes. The only difference between various colors of light, red, yellow, green, blue, and purple is in their wavelength or frequency, or in other words in their energy. Red light, for example, has less energy than purple light.

There is a wide range of electromagnetic radiation in nature. The visible part of the spectrum is only a tiny part of this wide range of energies.

As we move down in frequency from red light, there are other familiar forms of electromagnetic radiation:

• Infrared
• Microwaves
• Signals from our cell phones
• Radio waves

These are all forms of radiation that are invisible to our eyes and that have less energy than visible light.

As we move up in frequency from purple light, there are

• Ultraviolet (UV) radiation
• X-rays
• Gamma rays

These are all forms of radiation with energies much higher than visible light.

X-rays and gamma rays have enough energy that during interaction with atoms, they can remove electrons and cause the atom to become charged or ionized. That’s why we refer to these as ionizing radiation. When most people talk about radiation, they are referring to ionizing radiation.

Ionizing radiation comes from radioactive atoms, many of which occur naturally in the environment. Radioactive atoms, just like any other form of matter, can be gas, liquid, or solid.

Atoms are the building blocks of matter. Their structure determines their elemental and chemical properties.

Atoms are composed of a nucleus, containing protons and neutrons, surrounded by a cloud of electrons.

• The number of protons in the nucleus determines the identity of the atom (chemical element).
• For example, a carbon atom has six protons. If you were able to add another proton to the carbon nucleus, you wouldn’t have a carbon atom anymore: you’d have a nitrogen atom instead.

Most atoms are stable, meaning they have a good balance of neutrons and protons. But some atoms have an unstable combination blend of protons and neutrons. Isotopes are forms of elements that have a different number of neutrons.

Atoms seek to be stable; so, to get to a more stable state, the atom expels energy from the nucleus in the form of a particle or ray.

• This process is known as radioactivity, the unstable atom is said to be a radioactive atom, and the energy that’s released is radiation.
• After an atom expels energy from the nucleus, the composition of the nucleus changes, and we are left with a different element that is more stable. This process is known as radioactive decay.

To see an example of how this process works, look below:

Radioactive Decay Example

Think of the carbon atom mentioned above. Every carbon atom has six protons, and the majority of carbon atoms have six neutrons.

A carbon-12 atom has 6 protons (6P) and 6 neutrons (6N).

But some types of carbon have more than six neutrons. We call forms of elements that have a different number of neutrons, isotopes. For example, carbon-14 is a radioactive isotope of carbon that has six protons and eight neutrons in its nucleus. We call it carbon-14 because the total number of protons and neutrons in the nucleus, also known as the mass number, adds up to 14 (6+8=14).

If we wanted to abbreviate the name of an isotope, we’d use the elemental symbol and the mass number, so carbon-14 would be abbreviated C-14.

Because carbon-14 has six protons, it is still carbon, but the two extra neutrons make the nucleus unstable. In order to reach a more stable state, carbon-14 releases a negatively charged particle from its nucleus that turns one of the neutrons into a proton.

This new configuration of the nucleus – with seven protons and seven neutrons – leaves the atom more stable, but it is no longer a carbon atom. Since one of the neutrons turned into a proton, the atom is now a nitrogen atom.

In the carbon-14 example, the atom gives off radiation from the nucleus to reach a more stable state. In doing so, the composition of the nucleus changes, and we are left with a different element that is more stable. This process is known as radioactive decay.

To summarize, here are some key terms:

• Radioactivity is the spontaneous release of energy from an unstable atom.
• Radioactive material is a solid, liquid, or gas that gives off radiation.
• Radiation is the energy that comes out of a radioactive atom.
• Radioactive isotopes, also known as radionuclides, are radioactive atoms of the same element that have different numbers of neutrons.
• Radioactive decay is the change from an unstable atom to a more stable atom by the emission of radiation.

Different radioactive isotopes give off different kinds of radiation, and decay at different rates. 

Glossary

Atom — The smallest particle of an element that can enter into a chemical reaction.

Electron — An elementary particle with a negative electrical charge and a mass 1/1837 that of the proton. Electrons surround the nucleus of an atom because of the attraction between their negative charge and the positive charge of the nucleus. A stable atom will have as many electrons as it has protons. The number of electrons that orbit an atom determine its chemical properties. See also neutron

Ionizing radiation — Any radiation capable of displacing electrons from atoms, thereby producing ions. High doses of ionizing radiation may produce severe skin or tissue damage. See also alpha particle, beta particle, gamma ray, neutron, x-ray.

Isotope — A nuclide of an element having the same number of protons but a different number of neutrons.

Neutron — A small atomic particle possessing no electrical charge typically found within an atom’s nucleus. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. A neutron has about the same mass as a proton. See also alpha particle, beta particle, gamma ray, nucleon, x-ray.

Nucleus — The central part of an atom that contains protons and neutrons. The nucleus is the heaviest part of the atom.

Proton — A small atomic particle, typically found within an atom’s nucleus, that possesses a positive electrical charge. Even though protons and neutrons are about 2,000 times heavier than electrons, they are tiny. The number of protons is unique for each chemical element. See also nucleon.

Radiation — Energy moving in the form of particles or waves. Familiar radiations are heat, light, radio waves, and microwaves. Ionizing radiation is a very high-energy form of electromagnetic radiation.

Radioactive decay — Disintegration of the nucleus of an unstable atom by the release of radiation.

Radioactivity — The process of spontaneous transformation of the nucleus, generally with the emission of alpha or beta particles often accompanied by gamma rays. This process is referred to as decay or disintegration of an atom.

Radionuclide — An unstable and therefore radioactive form of a nuclide.

Radioactive decay is the process in which a radioactive atom spontaneously gives off radiation in the form of energy or particles to reach a more stable state.  It is important to distinguish between radioactive material and the radiation it gives off.

Types of Radiation

There are four types of radiation given off by radioactive atoms:

• Alpha particles
• Beta particles
• Gamma rays
• Neutrons

Radioactive atoms give off one or more of these types of radiation to reach a more stable state.  Additionally, each type of radiation has different properties that affect how we can detect it and how it can affect us.

Alpha Particles

• Alpha particles are large particles that travel up to an inch in the air. 
• Alpha particles are very easy to block, even with something as thin as a sheet of paper. 
• Alpha particles do not present an external hazard to people because they can’t get through our outer layer of dead skin cells. 
• However, they can be very damaging to cells inside our bodies if we breathe or eat alpha-emitting radioactive material or if the radioactive material is introduced through an open wound.

Beta Particles

• Beta particles are smaller particles that travel several feet in air. 
• Beta particles can be blocked effectively with a few inches of plastic, or even a layer of clothing.
• However, beta particles carry enough energy to cause burns on exposed skin and present an internal hazard if we breathe or eat beta-emitting radioactive material or if the radioactive material is introduced through an open wound.

Gamma Rays

• Gamma rays can travel many yards in air. 
• Gamma rays are primarily an external hazard because of their ability to go through material.
• It takes a few inches of lead or other dense substance to block gamma rays.
• Gamma rays also can be an internal hazard if we breathe or eat gamma-emitting radioactive materials, or if the radioactive material is introduce through an open wound, but the damage they do to cells inside our bodies is not as severe as that done by alpha and beta particles.
• The best way to protect yourself from a gamma-emitter is to increase the distance between yourself and the source.

Neutrons

Neutrons are neutral particles with no electrical charge that can travel great distances in the air.

• As neutrons travel through matter, they crash with atoms.  These atoms can become radioactive.
• Neutrons are more effective at damaging cells of the body than are other forms of ionizing radiation, such as x-rays or gamma rays.
• The best way to protect against neutron radiation is by providing shielding with thick, heavy materials such as lead, concrete, rock, or dirt.

Radionuclides can give off more than one kind of radiation, so it’s not uncommon to have a radionuclide that gives off both beta and gamma radiation, for example.

Half-Life

Another feature of each radionuclide is its half-life.  Half-life is the length of time it takes for half of the radioactive atoms of a specific radionuclide to decay.  A good rule of thumb is that, after seven half-lives, you will have less than one percent of the original amount of radiation.

Depending on the radionuclide, this process could be fast or take a very long time – radioactive half-lives can range from milliseconds to hours, days, sometimes millions of years. 

Radionuclides used in nuclear medicine procedures, have short half-lives. 

• For example, technetium-99m, one of the most common medical isotopes used for imaging studies, has a half-life of 6 hours. 
• The short half-life of technetium-99m helps keep the dose to the patient low. After 24 hours, the radioactivity from the procedure will be reduced by more than 90%.

Uranium is a radionuclide that has an extremely long half-life. 

• Naturally occurring uranium-238 present in the Earth’s crust has a half-life of almost 4.5 billion years. 
• If you take a soil sample anywhere in the world, including your backyard, you will find uranium atoms that date back to when the Earth was formed.

A Closer Look at Half-Life

Let’s take a closer look at half-life.

If you start with 100 atoms, after one half-life you’ll have 50 radioactive atoms.

After two half-lives, you’ll have 25 radioactive atoms.

And after a third half-life, you’ll have 12 radioactive atoms. 

Then 6, then 3, then 1, until eventually, all of the radioactive atoms in that population will reach their more stable state.

Radioactive Decay Chains

Some radionuclides go through a series of transformations before they reach a stable state.  For example, uranium-238 ultimately transforms into a stable atom of lead.  But in the process, several types of radioactive atoms are generated.  This is called a decay chain. When uranium-238 decays, it produces several isotopes of:

• Thorium
• Radium
• Radon
• Bismuth

Glossary

Alpha particles — The nucleus of a helium atom, made up of two neutrons and two protons with a charge of +2. Certain radioactive nuclei emit alpha particles. Alpha particles generally carry more energy than gamma or beta particles, and deposit that energy very quickly while passing through tissue. Alpha particles can be stopped by a thin layer of light material, such as a sheet of paper, and cannot penetrate the outer, dead layer of skin. Therefore, they do not damage living tissue when outside the body. When alpha-emitting atoms are inhaled or swallowed, however, they are especially damaging because they transfer relatively large amounts of ionizing energy to living cells. See also beta particle, gamma ray, neutron, x-ray.

Atom — The smallest particle of an element that can enter into a chemical reaction.

Beta Particles —  Electrons ejected from the nucleus of a decaying atom. Although they can be stopped by a thin sheet of aluminum, beta particles can penetrate the dead skin layer, potentially causing burns. They can pose a serious direct or external radiation threat and can be lethal depending on the amount received. They also pose a serious internal radiation threat if beta-emitting atoms are ingested or inhaled. See also alpha particle, gamma ray,neutron, x-ray.

Decay Chain (Decay Series) — The series of decays that certain radioisotopes go through before reaching a stable form. For example, the decay chain that begins with uranium-238 (U-238) ends in lead-206 (Pb-206), after forming isotopes, such as uranium-234 (U-234), thorium-230 (Th-230), radium-226 (Ra-226), and radon-222 (Rn-222).

Gamma Rays — High-energy electromagnetic radiation emitted by certain radionuclides when their nuclei transition from a higher to a lower energy state. These rays have high energy and a short wave length. All gamma rays emitted from a given isotope have the same energy, a characteristic that enables scientists to identify which gamma emitters are present in a sample. Gamma rays penetrate tissue farther than do beta or alpha particles, but leave a lower concentration of ions in their path to potentially cause cell damage. Gamma rays are very similar to x-rays. See also neutron.

Isotope — A nuclide of an element having the same number of protons but a different number of neutrons.

Neutron — A small atomic particle possessing no electrical charge typically found within an atom’s nucleus. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. A neutron has about the same mass as a proton. See also alpha particle, beta particle, gamma ray, nucleon, x-ray.

Radioactive Decay — Disintegration of the nucleus of an unstable atom by the release of radiation.

Radiation — Energy moving in the form of particles or waves. Familiar radiations are heat, light, radio waves, and microwaves. Ionizing radiation is a very high-energy form of electromagnetic radiation.

Radioactive Material — Material that contains unstable (radioactive) atoms that give off radiation as they decay.

Radionuclide — An unstable and therefore radioactive form of a nuclide.

Radioactive contamination and radiation exposure could occur if radioactive materials are released into the environment as the result of an accident, an event in nature, or an act of terrorism. Such a release could expose people and contaminate their surroundings and personal property.

Types of Contamination

Internal Contamination

Internal contamination occurs when people swallow or breathe in radioactive materials, or when radioactive materials enter the body through an open wound or are absorbed through the skin. Some types of radioactive materials stay in the body and are deposited in different body organs. Other types are eliminated from the body in blood, sweat, urine, and feces.

Radioactive Contamination

Radioactive contamination occurs when radioactive material is deposited on or in an object or a person. Radioactive materials released into the environment can cause air, water, surfaces, soil, plants, buildings, people, or animals to become contaminated. A contaminated person has radioactive materials on or inside their body.

How Radioactive Contamination Is Spread

People who are externally contaminated with radioactive material can contaminate other people or surfaces that they touch. For example, people who have radioactive dust on their clothing may spread the radioactive dust when they sit in chairs or hug other people.

People who are internally contaminated can expose people near them to radiation from the radioactive material inside their bodies. The body fluids (blood, sweat, urine) of an internally contaminated person can contain radioactive materials. Coming in contact with these body fluids can result in contamination and/or exposure.

External Contamination

External contamination occurs when radioactive material, in the form of dust, powder, or liquid, comes into contact with a person’s skin, hair, or clothing. In other words, the contact is external to a person’s body. People who are externally contaminated can become internally contaminated if radioactive material gets into their bodies.

Radiation Exposure

Radioactive materials give off a form of energy that travels in waves or particles. This energy is called radiation. When a person is exposed to radiation, the energy penetrates the body. For example, when a person has an x-ray, he or she is exposed to radiation.

How Your Home Could Become Contaminated

People who are externally contaminated can spread the contamination by touching surfaces, sitting in a chair, or even walking through a house. Contaminants can easily fall from clothing and contaminate other surfaces.

Homes can also become contaminated with radioactive materials in body fluids from internally contaminated people. Making sure that others do not come in contact with body fluids from a contaminated person will help prevent contamination of other people in the household.

How You Can Limit Contamination

Since radiation cannot be seen, smelled, felt, or tasted, people at the site of an incident will not know whether radioactive materials were involved. You can take the following steps to limit your contamination.

1. Get out of the immediate area quickly. Go inside the nearest safe building or to an area to which you are directed by law enforcement or health officials.
2. Remove the outer layer of your clothing. If radioactive material is on your clothes, getting it away from you will reduce the external contamination and decrease the risk of internal contamination. It will also reduce the length of time that you are exposed to radiation.
3. If possible, place the clothing in a plastic bag or leave it in an out-of-the-way area, such as the corner of a room. Keep people away from it to reduce their exposure to radiation. Keep cuts and abrasions covered when handling contaminated items to avoid getting radioactive material in them.
4. Wash all of the exposed parts of your body using lots of soap and lukewarm water to remove contamination. This process is called decontamination. Try to avoid spreading contamination to parts of the body that may not be contaminated, such as areas that were clothed.
5. After authorities determine that internal contamination may have occurred, you may be able to take medication to reduce the radioactive material in your body.

Sources of  radiation  are all around us all the time. Some are natural and some are man-made. The amount of radiation absorbed by a person is measured in dose. A dose is the amount of radiation energy absorbed by the body.

Background Radiation

Background radiation  is present on Earth at all times. The majority of background radiation occurs naturally from minerals and a small fraction comes from man-made elements. Naturally occurring radioactive minerals in the ground, soil, and water produce background radiation. The human body even contains some of these naturally-occurring radioactive minerals. Cosmic radiation from space also contributes to the background radiation around us. There can be large variances in natural background radiation levels from place to place, as well as changes in the same location over time.

Cosmic Radiation

Cosmic radiation comes from extremely energetic particles from the sun and stars that enter Earth’s atmosphere. Some particles make it to the ground, while others interact with the atmosphere to create different types of radiation. Radiation levels increase as you get closer to the source, so the amount of cosmic radiation generally increases with elevation. The higher the altitude, the higher the dose. That is why those living in Denver, Colorado (altitude of 5,280 feet) receive a higher annual radiation dose from cosmic radiation than someone living at sea level (altitude of 0 feet).

Radioactive Materials in the Earth and in Our Bodies

Uranium and thorium naturally found in the earth are called  primordial   radionuclides  and are the source of terrestrial radiation. Trace amounts of uranium, thorium and their decay products can be found everywhere. Terrestrial radiation levels vary by location, but areas with higher concentrations of uranium and thorium in surface soils generally have higher dose levels. 

Traces of radioactive materials can be found in the body, mainly naturally occurring potassium-40. Potassium-40 is found in the food, soil, and water we ingest.  Our bodies contain small amounts of radiation because the body metabolizes the non-radioactive and radioactive forms of potassium and other elements in the same way.

Man-made Sources

A  small fraction of background radiation comes from human activities. Trace amounts of radioactive elements have dispersed in the environment from nuclear weapons tests and accidents like the one at the Chernobyl nuclear power plant in Ukraine. Nuclear reactors emit small amounts of radioactive elements. Radioactive materials used in industry and even in some consumer products are also a source of small amounts of background radiation.

Average U.S. Doses and Sources

All of us are exposed to radiation every day, from natural sources such as minerals in the ground, and man-made sources such as medical x-rays. According to the National Council on Radiation Protection and Measurements (NCRP), the average annual radiation dose per person in the U.S. is  6.2 millisieverts (620  millirem) . The pie chart below shows the sources of this average dose.

Most of our average annual dose comes from natural  background radiation  sources:

• The radioactive gases radon and thoron, which are created when other naturally occurring elements undergo radioactive decay.
• Space (cosmic radiation).
• Naturally occurring radioactive minerals:
  • Internal (in your body).
  • Terrestrial (in the ground).

Another 48 percent of the average American’s dose comes from medical procedures. This total does not include the dose from radiation therapy used in the treatment of cancer, which is typically many times larger.

Scientists measure radiation in different ways. Sometimes, they measure the dose that a person receives from a radioactive source, and sometimes they measure the amount of radioactivity in water, or in soil, or in the air. These measurements are taken to determine if safety actions are needed. 

There are different but interrelated units for measuring radioactivity and estimating health effects.

Radioactivity

Radioactivity is a measure of the ionizing radiation released by a radioactive material. Different types of ionizing radiation have the potential to damage human tissue. 

A material's radioactivity is measured in becquerels (Bq, international unit) and curies (Ci, U.S. unit). Because a curie is a large unit, radioactivity results are usually shown in picocuries (pCi). A picocurie is one trillionth of a curie. The higher the number, the more radiation released by the material.

Examples:

• The natural radium-226 level of surface water generally ranges from 0.0037 to 0.0185 becquerels per liter (Bq/L), or 0.1 to 0.5 picocuries per liter (pCi/L).¹ 
• The radium limit in drinking water for daily consumption is 0.185 becquerels per liter (Bq/L), or 5.0 picocuries per liter (pCi/L).²

Unit Conversions and Calculations

Absorbed Dose

Absorbed dose describes the amount of energy deposited per unit mass in an object or person. The units for absorbed dose are gray (Gy, international unit) and rad (rad, U.S. unit).  

Examples:

• A dose to the lens of the eyes from a brain CT scan is about 60 milligray (mGy) or 6 rad.
• A dose to the thyroid from a chest CT scan is about 10 milligray (mGy) or 1 rad.³

Unit Conversions and Calculations

Effective Dose

Effective dose takes the absorbed dose (see above) and adjusts it for radiation type and relative organ sensitivity. The result is an indicator for the potential for long-term health effects (i.e., cancer and hereditary effects) from an exposure. It is used to set regulatory limits that protect against long-term health effects in a population. It also allows experts to compare anticipated health effects from different exposure situations. Because this value is a calculated approximation, not a physical quantity, it cannot be used to predict individual health effects. The units for effective dose are sievert (Sv, international unit) and rem (rem, U.S. unit).   

Examples:

• The annual radiation dose limit for workers is 0.05 sieverts (Sv) or 5 rem.⁴  
• During an emergency, the guidance for when to evacuate or shelter in place is when the total projected dose exceeds 10-50 millisieverts (mSv) or 1-5 rem over the course of four days.⁵

Unit Conversions and Calculations

Scientists have been studying the effects of radiation for over 100 years; so we know quite a bit about how radiation interacts with living tissue, and its effect on the body. Because we can measure radiation and because we understand its health effects, we can work safely around it.

It’s All About the Dose!

• As with other types of toxins, “the dose makes the poison”.
• We receive low doses of radiation from our natural environment every day.
• We know that radiation at high doses can cause cancer, could harm fetuses, and can even lead to death.

Factors that Affect Dose

• A dose received over a long period of time is less harmful than the same dose received all at once.
• A dose to a part of the body is less harmful than a dose to the whole body.
• Children and young adults are more sensitive to the effects of radiation.

How Radiation Affects Your Body

• Radiation can damage the DNA in our cells.
• High doses of radiation can cause Acute Radiation Syndrome (ARS) or Cutaneous Radiation Injuries (CRI).
• High doses of radiation could also lead to cancer later in life.

Radiation exposure is one of the best-understood health hazards. We have been studying the effects of radiation for over 100 years, so we know quite a bit about how radiation interacts with living tissue.

As with other toxins, “the dose makes the poison.”

It is the radiation dose, or the amount of radiation, that is the critical issue in determining health consequences. It is helpful to put radiation dose in perspective.

We receive low doses of radiation from our natural environment.

However, we know that radiation at high doses can be lethal. We know that radiation can cause cancer, and we also know radiation can be harmful to the fetus at various stages of pregnancy. And, although we haven’t seen it in humans, radiation can cause hereditary effects in lab animals.

Other Factors that Influence Health Effects

How fast the dose is received

This is known as the dose rate. If a person receives a dose over an extended period of time, the impact on health won’t be as severe as if the same dose were received all at once.

Where the dose is received

If the dose is received by only a portion of the body, the impact on health won’t be as severe as if the dose were delivered to the entire body.

How sensitive the body is to radiation

Individual sensitivity to radiation is also a factor. A developing fetus is the most vulnerable to the effects of radiation. Infants, children, the elderly, pregnant women, and people with compromised immune systems are more vulnerable to health effects than healthy adults.

For more information on radiation and pregnancy, click here

The risk from radiation is higher for younger people mainly because:

• Younger people have more cells that are dividing rapidly and tissues that are growing
• Younger people have a longer lifespan ahead of them, giving cancers more time to develop

Also, some people within the same age group may have different sensitivity to radiation.

How Radiation Affects Your Body

Like with many other contaminants or toxins, our genetic material, or DNA, is the primary target.

Radiation can interact with DNA directly and cause damage by breaking bonds in the DNA or indirectly by breaking water molecules surrounding the DNA. When these water molecules are broken, they produce free radicals–unstable oxygen molecules that can damage cells and organs.

Once a cell is damaged, three things can happen.

1. The cell repairs itself. The cell would then go back to normal.

2. The cell damage is not repaired or is incorrectly repaired, so the cell is changed. This change may eventually lead to cancer.

3. There is too much damage to the cell, and the cell dies. Cell death is not always a bad option.

• If a few radiation-damaged cells die, your body will recover and you do not have the risk of those cells potentially turning into cancer.
• However, widespread cell death, such as that caused by high radiation doses, can lead to organ failure and, ultimately, death.

To learn more about Radiation Emergencies and Your Health, Click Here

Glossary

Dose rate — The radiation dose delivered per unit of time.

Radiation — Energy moving in the form of particles or waves. Familiar radiations are heat, light, radio waves, and microwaves. Ionizing radiation is a very high-energy form of electromagnetic radiation.

Radiation dose — Radiation absorbed by person’s body. Several different terms describe radiation dose. For more information, see “Primer on Radiation Measurement”.

Ionizing radiation  has sufficient energy to affect the atoms in living cells and thereby damage their genetic material (DNA). Fortunately, the cells in our bodies are extremely efficient at repairing this damage. However, if the damage is not repaired correctly, a cell may die or eventually become cancerous.

Exposure to very high levels of radiation, such as being close to an atomic blast, can cause acute health effects such as skin burns and acute radiation syndrome (“radiation sickness"). It can also result in long-term health effects such as cancer and cardiovascular disease. Exposure to low levels of radiation encountered in the environment does not cause immediate health effects, but is a minor contributor to our overall cancer risk.

Acute Radiation Syndrome from Large Exposures

A very high level of radiation exposure delivered over a short period of time can cause symptoms such as nausea and vomiting within hours and can sometimes result in death over the following days or weeks. This is known as acute radiation syndrome, commonly known as “radiation sickness.”

It takes a very high radiation exposure to cause acute radiation syndrome—more than 0.75  gray  (75  rad)  in a short time span (minutes to hours). This level of radiation would be like getting the radiation from 18,000 chest x-rays distributed over your entire body in this short period. Acute radiation syndrome is rare, and comes from extreme events like a nuclear explosion or accidental handling or rupture of a highly radioactive source.

Radiation Exposure and Cancer Risk

Exposure to low-levels of radiation does not cause immediate health effects, but can cause a small increase in the  risk  of cancer over a lifetime. There are studies that keep track of groups of people who have been exposed to radiation, including atomic bomb survivors and radiation industry workers. These studies show that radiation exposure increases the chance of getting cancer, and the risk increases as the dose increases: the higher the dose, the greater the risk. Conversely, cancer risk from radiation exposure declines as the dose falls: the lower the dose, the lower the risk.

Radiation doses are commonly expressed in millisieverts (international units) or  rem  (U.S. units). A dose can be determined from a one-time radiation exposure, or from accumulated exposures over time. About 99 percent of individuals would not get cancer as a result of a one-time uniform whole-body exposure of 100 millisieverts (10 rem) or lower. At this dose, it would be extremely difficult to identify an excess in cancers caused by radiation when about 40 percent of men and women in the U.S. will be diagnosed with cancer at some point during their lifetime.

Risks that are low for an individual could still result in unacceptable numbers of additional cancers in a large population over time. For example, in a population of one million people, an average one-percent increase in lifetime cancer risk for individuals could result in 10,000 additional cancers. The EPA sets regulatory limits and recommends emergency response guidelines well below 100 millisieverts (10 rem) to protect the U.S. population, including sensitive groups such as children, from increased cancer risks from accumulated radiation dose over a lifetime.

Exposure Pathways

Understanding the type of radiation received, the way a person is exposed (external vs. internal), and for how long a person is exposed are all important in estimating health effects.

The risk from exposure to a particular  radionuclide  depends on:

• The energy of the radiation it emits.
• The type of radiation (alpha, beta, gamma, x-rays).
• Its activity (how often it emits radiation).
• Whether exposure is external or internal:
  • External exposure is when the radioactive source is outside of your body. X-rays and gamma rays can pass through your body, depositing energy as they go.
  • Internal exposure is when radioactive material gets inside the body by eating, drinking, breathing or injection (from certain medical procedures). Radionuclides may pose a serious health threat if significant quantities are inhaled or ingested.
• The rate at which the body metabolizes and eliminates the radionuclide following ingestion or inhalation.
• Where the radionuclide concentrates in the body and how long it stays there.

Sensitive Populations

Children and fetuses are especially sensitive to radiation exposure. The cells in children and fetuses divide rapidly, providing more opportunity for radiation to disrupt the process and cause cell damage. EPA considers differences in sensitivity due to age and sex when revising radiation protection standards.

Three common measurements of radiation are the amount of radioactivity, ambient radiation levels, and radiation dose. But, to get accurate and reliable measurements, we need to have both the right instrument and a trained operator. It is important to maintain radiation detection equipment to ensure it is working properly.

It’s All About the Energy!

• When working with radiation, we are concerned about the amount of energy the material is emitting. The size, weight, and volume of the material do not necessarily matter.
• A small amount of material may give off a lot of radiation.
• On the other hand, a large amount of radioactive material may give off a small amount of radiation.

Measuring the Amount of Radioactivity

• We measure the amount of radioactivity by finding out how many radioactive atoms decay every second. These atoms may be giving off alpha particles, beta particles, and/or gamma rays.
• The amount of radioactivity is reported in Becquerel (Bq), which is the international unit, or the Curie (Ci), which is the unit used in the United States.
• Geiger counters are commonly used to measure the amount of radioactivity, but there are other types of detectors that may be used.

Measuring Ambient Radiation Levels

• Ambient radiation levels measure how much radiation is in the environment around us.
• Ambient radiation levels are reported in Gray per hour (Gy/h) or Sievert per hour (Sv/h), which are the international units. In the United States, we use Roentgen per hour (R/h) or rem per hour (rem/h).
• Instruments called pressurized ionization chambers are best suited for measuring ambient radiation levels.

Measuring Radiation Dose

• Radiation dose is the amount of radiation absorbed by the body.
• Radiation doses are reported in Gray (Gy) or Sievert (Sv), which are international units. In the U.S., we use rad or rem
• Alarming dosimeters can be used by first responders and safety officers to monitor dose in real time. There are also specialized instruments used by hospitals and laboratories that can measure dose.

Radiation is part of our life.  Background radiation , coming primarily from natural minerals, is around us all the time. Fortunately, there are very few situations where an average person is exposed to uncontrolled sources of radiation above background. Nevertheless, it is wise to be prepared and know what to do if such a situation arises.

One of the best ways to be prepared is to understand the radiation protection principles of time, distance and shielding. During a radiological emergency (a large release of radioactive material into the environment), we can use these principles to help protect ourselves and our families.

Time, Distance and Shielding

Time, distance, and shielding actions minimize your exposure to radiation in much the same way as they would to protect you against overexposure to the sun:
 

• Time: For people who are exposed to  radiation  in addition to natural background radiation, limiting or minimizing the exposure time reduces the dose from the radiation source.
• Distance: Just as the heat from a fire reduces as you move further away, the dose of radiation decreases dramatically as you increase your distance from the source.
• Shielding: Barriers of lead, concrete, or water provide protection from penetrating  gamma rays  and  x-rays . This is why certain radioactive materials are stored under water or in concrete or lead-lined rooms, and why dentists place a lead blanket on patients receiving x-rays of their teeth. Therefore, inserting the proper shield between you and a radiation source will greatly reduce or eliminate the dose you receive.

Radiation Emergencies

In a large scale radiological release, such as a nuclear power plant accident or terrorist incident, the following advice has been tested and proven to provide maximum protection.

If a radiation emergency occurs, you can take actions to protect yourself, your loved ones and your pets: Get Inside, Stay Inside and Stay Tuned. Follow the advice of emergency responders and officials.

Get Inside

In a radiation emergency you may be asked to get inside a building and take shelter for a period of time.

• This action is called " sheltering in place ."
• Get to the middle of the building or a basement, away from doors and windows.
• Bring pets inside.

Stay Inside

Staying inside will reduce your exposure to radiation.

• Close windows and doors.
• Take a shower or wipe exposed parts of your body with a damp cloth.
• Drink bottled water and eat food in sealed containers.

Stay Tuned

Emergency officials are trained to respond to disaster situations and will provide specific actions to help keep people safe.

• Get the latest information from radio, television, the Internet, mobile devices, etc.
• Emergency officials will provide information on where to go to get screened for contamination.