What is radiation?

March 31st, 2015 | By Mirion Technologies


Radiation is the process by which energy is emitted as either particles or waves. Broadly, it can take the form of sound, heat, or light. However, most people generally use it to refer to radiation from electromagnetic waves, ranging from radio waves, though the visible light spectrum, and up through to gamma waves.   

What Is Radiation?


Most of the discussion about radiation, how it works, and what its effects are boil down to the interaction of radiation with atoms (and molecules) that it comes into contact with. Atoms form the basic building blocks of all matter.

They consist of a nucleus, made of positively-charged protons (and sometimes neutrally-charged neutrons), and an outer cloud of electrons, which have a negative charge.

The positive charge of a single proton is equal to the negative charge of a single electron.

Protons and neutrons have a relatively large size and atomic weight, whereas electrons are extremely small and light by comparison.

Due to the nature of opposite charges attracting, atoms tend to have an equal number of protons and electrons, leaving the atom as a whole having a net charge of zero.

However, if the atom either loses or gains an electron, it becomes an ion, and carries a charge.

It will seek bonds with other charged particles in order to regain a neutral balance, potentially leading to new molecules being formed.

What Is Radiation?


What is radiation?

Ionising radiation is produced from natural and artificial radioactive materials. It is present in the environment because naturally occurring radioactive materials such as uranium, thorium, actinium and potassium-40 exist in the material that makes up planet Earth.

This leads to exposure to alpha, beta and gamma radiation including radioactive radon gas. Natural radioactivity is present in the air we breathe, food we eat, water we drink and even in our bodies.

We are also exposed to natural ionising radiation that comes from outer space and passes through the atmosphere of the planet. This is called cosmic radiation.

There are three main sources of artificial ionising radiation. They are:

  • medical uses including diagnosis of many diseases and treatment of cancer
  • industrial uses, mainly in measurement and scientific research
  • fallout from nuclear weapons testing and accidents around the world.

The figure below shows the relative annual per capita dose to the Australian population from the various radiation sources. Currently, on average, our exposure to medical radiation is the dominant source.

2 mSv); Uranium/Thorium in the body (0.2 mSv); Atmospheric weapons testing (0.005 mSv); Medical (1.7 mSv)” width=”599″ class=”media-element file-full” src=”https://www.arpansa.gov.au/sites/default/files/legacy/images/basics/exposure.gif”>

The damaging effects of ionising radiation come from the packages of high energy that are released from radioactive sources.

Although different types of ionising radiation have different patterns of energy release and penetrating power (see ionising radiation topics), there is no general property that makes artificial ionising radiation different or more damaging than the ionising radiation that comes from natural radioactive material. While the sources differ, the types of radiation are the same which means we can directly compare doses from artificial sources of ionising radiation to those from natural sources.

Solar radiation consists of several different forms of non-ionising radiation, such as ultraviolet (UV). Many modern technologies such as power-lines, electrical equipment and mobile phone systems also produce forms of non-ionising radiation.

We cannot eliminate radiation from our environment, but by having a good understanding of radiation and how to control our exposure, we can reduce our risk.

The electromagnetic spectrum includes radio waves, microwaves, infrared rays, light rays, UV rays, X‑rays and gamma rays. All electromagnetic radiation is transmitted through empty space at the speed of light.

The different forms of electromagnetic radiation are distinguished from each other by:

  • their wavelength
  • the amount of energy they transfer.

These properties also determine their ability to travel through objects, their heating effects and their effect on living tissue.

What Is Radiation?

Sometimes electromagnetic radiation is described using its frequency rather than its wavelength. These two characteristics are related.

If we could watch a wave of electromagnetic radiation pass, the wavelength would be the distance between two adjacent wave crests, while the frequency would be the number of wave peaks that pass in a given time.

Since electromagnetic radiation travels at a constant speed in a constant environment, radiation with a longer wavelength would have fewer waves passing in a given time (lower frequency) and radiation with a shorter wavelength would have more waves passing in a given time (higher frequency). When wavelength increases, frequency decreases and vice versa.

What Is Radiation?

The hertz is the unit used to measure frequency. A measured frequency of one hertz (Hz) represents one wavelength per second or one cycle per second. The range of frequencies within the electromagnetic spectrum is large, resulting in the common use of a series of units:

  • one kilohertz(kHz) is one thousand hertz (1000 Hz)
  • one megahertz(MHz) is one million hertz (1,000,000 Hz)
  • one gigahertz(GHz) is one thousand million hertz (1,000,000,000 Hz).

What is Radiation?

Radiation may bring to mind the superheroes and monsters of comic books and movies, but radiation is very real and all around us! In fact, you are currently being bombarded by radiation.

It might be coming from the sun, various electronic devices you own, or even the food in your kitchen. If you have ever eaten a banana, you have eaten a radioactive material.

The good news is that the vast majority of radiation you are exposed to is relatively harmless.

Whether or not radiation can harm you depends on the type of radiation, the dosage you come in contact with, and the length of the exposure. Here we'll go over the different types of radiation, their causes, uses to us, and dangers.

Before we get started, you need to know what exactly radiation is in general. Radiation can be defined as the transmission of energy from a body in the form of waves or particles.

This can encompass anything from dangerous radiation created by a nuclear power plant to the harmless light created by a flashlight.

Ionizing and Non-Ionizing Radiation

Before we go any further, let's cover some basic terms. Ionization is the process in which an atom either loses or gains an electron.

Since electrons are negatively charged, this process will take an atom, which normally has no charge, and give it either a positive or negative charge depending on whether it lost or gained electrons.

An atom that has a charge to it is called an ion.

So the difference between ionizing radiation and non-ionizing radiation is that ionizing radiation has enough energy to strip electrons off of atoms, and non-ionizing radiation does not have enough energy to strip electrons off of atoms.

One of the easiest ways to visualize the difference between these two is to look at the frequency spectrum for light. As the frequency goes up, so does energy, so we can see the energy cut off for light where it goes from non-ionizing to ionizing radiation is within the ultraviolet light spectrum.

Now let's look at the causes of both types of radiation.

Causes of Radiation

Non-ionizing radiation is limited to the lower energy range electromagnetic radiation, which is more commonly known as light. However, the light we can see with our eyes, visible light, is only a small section of the electromagnetic radiation spectrum as seen here.

Electromagnetic radiation spectrum

What Is Radiation?

What is Radiation?

Radiation is energy that travels in invisible waves or rays. Exposure to radiation is an everyday occurrence – in fact, it has always been a part of life on Earth.

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Radiation can be natural or man-made.


  • The sun emits ultraviolet rays that can cause sunburn.
  • Granite, a common rock used in kitchen counters, is a natural source of radiation.


  • Doctors use X-rays and MRIs to see inside patients with broken bones and other problems.
  • A microwave uses a form of radiation to cook food.

Understanding Types of Radiation

There are two types of radiation: non-ionizing (low frequency) and ionizing (high frequency). Both types can be harmful in excessive amounts. Fortunately, scientists, nuclear engineers and doctors understand radiation and know how to harness its benefits and protect us from its dangers.

Non-ionizing radiation emits enough energy to move or “excite” atoms. For example, microwave ovens use non-ionizing radiation to cook food. The radiation vibrates water contained in food, which creates heat. That heat cooks the food.

Ionizing radiation emits enough energy to change the structure of an atom, which can damage biological cells. For instance, a sunburn is a type of radiation damage.

In nuclear facilities, technicians focus on four types of ionizing radiation: alpha, beta, gamma and neutrons. Alpha radiation is too weak to penetrate most objects. Beta radiation is stronger, while gamma radiation is the strongest. Neutrons can penetrate many objects, but are slowed by water.

What Is Radiation?

Measuring Radiation

Radiation doses are measured in an international unit called a Sievert (Sv). Typically, radiation doses are so low that they are measured in milliSieverts (mSv) or one-thousandth of a Sievert.

Because exposure to radiation happens every day, it is helpful to understand the average amount of radiation that people receive from natural and man-made sources.

For instance, the average annual radiation dose that a person receives from food and water is nearly 0.3 mSv. At the same time, the average annual radiation dose that the public receives from nuclear power is 0.0002 mSv.

Managing Radiation in Nuclear Energy Plants

The nuclear energy industry follows international best practices and standards to protect the public, workers and the environment. Modern nuclear energy plants use many barriers to protect people from radiation.

Every barrier provides another layer of protection. In addition, the intensity of radiation decreases with distance from the source. Nuclear energy plants add distance from radioactive sources by incorporating large open spaces around the facility that the public cannot enter.

  • Radiation Protection
  • All restricted areas of the plant are clearly marked.
  • In addition, there are three simple ways to limit exposure to radiation.
  • Create a barrier: Barriers made of steel, concrete or water provide protection from radiation. This is why the reactor is inside several layers of thick walls made of steel and concrete. It is also why used fuel is stored in concrete and steel-lined pools of water. 
  • Minimize time: The less time a person spends near a source of radiation, the less radiation they receive.
  • Increase distance: The farther away a person is from a source of radiation, the less radiation they receive. This is one of the reasons why there are restricted areas of the plant.

What is Ionizing Radiation?

The purpose of this section is to provide information on the basics of ionizing radiation for everyone.

Energy emitted from a source is generally referred to as radiation. Examples include heat or light from the sun, microwaves from an oven, X rays from an X-ray tube, and gamma rays from radioactive elements

What Is Radiation?

Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized.

Here we are concerned with only one type of radiation, ionizing radiation, which occurs in two forms – waves or particles. More information on Non-Ionizing radiation.

Forms of electromagnetic radiation. These differ only in frequency and wave length.

  • Heat waves
  • Radiowaves
  • Infrared light
  • Visible light
  • Ultraviolet light
  • X rays
  • Gamma rays

Longer wave length, lower frequency waves (heat and radio) have less energy than shorter wave length, higher frequency waves (X and gamma rays). Not all electromagnetic (EM) radiation is ionizing. Only the high frequency portion of the electromagnetic spectrum which includes X rays and gamma rays is ionizing.

What is radiation?

Radiation can be described as energy or particles from a source that travel through space or other mediums. Light, heat, and the microwaves and radio waves used for wireless communications are all forms of radiation.

Radiation includes particles and electromagnetic waves that are emitted by some materials and carry energy. The kind of radiation discussed below is called ionising radiation because it can produce charged particles (or ions) in matter. X-rays, gamma-rays, alpha particles, beta particles and neutrons are all examples of ionising radiation.

Natural background radiation

Australians are constantly exposed to ionising radiation from a variety of natural and artificial sources.

The sun is a major source of cosmic radiation, or radiation originating from space. Airline flights and skiing at high altitudes are activities that will increase exposure to this cosmic radiation. Many buildings also emit ionising radiation simply because the materials that were used to build them, such as clay bricks and granite, are naturally radioactive.

Average exposures to background radiation

  • In Australia, people receive about 1,500 to 2,000 μSv of ionising radiation every year. This is the level of natural background radiation in Australia. Low exposure to ionising radiation at this background level is not harmful.
  • Australia's level of natural background radiation is quite low compared to many other parts of the world. As a typical example, in the county of Cornwall, UK, people receive about 7,800 μSv of ionising radiation every year. Again, this low level of background radiation is not harmful.
  • Passengers on high-altitude flights receive more exposure to cosmic radiation than they would experience at ground level. For example, if you flew return from Sydney to Los Angeles, you would receive an extra 160 μSv of background radiation dose.
  • As granite emits more radiation than other materials, a home would granite tiles would exposure the occupants to an extra 1,000 μSv of background radiation annually.
  • Some medical X-rays and similar procedures provide radiation doses, with a typical X-ray or CT scan of your head exposing you to 2,600 μSv of ionising radiation.
  • Due to its high potassium content, eating a banana will provide you with a dose of 0.1 μSv from the naturally-occurring potassium-40 isotope. 

Frequently-asked questions

A radioactive atom is unstable because it contains extra energy, or an unbalanced number of particles, in its nucleus. When this atom ‘decays’ to a more stable atom, it releases the extra energy and/or particles as ionising radiation.

Radiation in Everyday Life

» Types of Radiation | Radiation Dose | Radiation Protection | At What Level is Radiation Harmful? | Risks and Benefits

Radioactivity is a part of our earth – it has existed all along. Naturally occurring radioactive materials are present in its crust, the floors and walls of our homes, schools, or offices and in the food we eat and drink. There are radioactive gases in the air we breathe. Our own bodies – muscles, bones, and tissue – contain naturally occurring radioactive elements.

Man has always been exposed to natural radiation arising from the earth as well as from outside the earth. The radiation we receive from outer space is called cosmic radiation or cosmic rays.

We also receive exposure from man-made radiation, such as X-rays, radiation used to diagnose diseases and for cancer therapy. Fallout from nuclear explosives testing, and small quantities of radioactive materials released to the environment from coal and nuclear power plants, are also sources of radiation exposure to man.

Radioactivity is the term used to describe disintegration of atoms. The atom can be characterized by the number of protons in the nucleus. Some natural elements are unstable.

Therefore, their nuclei disintegrate or decay, thus releasing energy in the form of radiation. This physical phenomenon is called radioactivity and the radioactive atoms are called nuclei.

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The radioactive decay is expressed in units called becquerels. One becquerel equals one disintegration per second.

The radionuclides decay at a characteristic rate that remains constant regardless of external influences, such as temperature or pressure. The time that it takes for half the radionuclides to disintegrate or decay is called half-life.

This differs for each radioelement, ranging from fractions of a second to billions of years. For example, the half-life of Iodine 131 is eight days, but for Uranium 238, which is present in varying amounts all over the world, it is 4.5 billion years.

Potassium 40, the main source of radioactivity in our bodies, has a half-life of 1.42 billion years.

Types of Radiation

The term “radiation” is very broad, and includes such things as light and radio waves. In our context it refers to “ionizing” radiation, which means that because such radiation passes through matter, it can cause it to become electrically charged or ionized. In living tissues, the electrical ions produced by radiation can affect normal biological processes.

There are various types of radiation, each having different characteristics. The common ionizing radiations generally talked about are:

  • Alpha radiation consists of heavy, positively charged particles emitted by atoms of elements such as uranium and radium. Alpha radiation can be stopped completely by a sheet of paper or by the thin surface layer of our skin (epidermis). However, if alpha-emitting materials are taken into the body by breathing, eating, or drinking, they can expose internal tissues directly and may, therefore, cause biological damage.
  • Beta radiation consists of electrons. They are more penetrating than alpha particles and can pass through 1-2 centimetres of water. In general, a sheet of aluminum a few millimetres thick will stop beta radiation.
  • Gamma rays are electromagnetic radiation similar to X-rays, light, and radio waves. Gamma rays, depending on their energy, can pass right through the human body, but can be stopped by thick walls of concrete or lead.
  • Neutrons are uncharged particles and do not produce ionization directly. But, their interaction with the atoms of matter can give rise to alpha, beta, gamma, or X-rays which then produce ionization. Neutrons are penetrating and can be stopped only by thick masses of concrete, water or paraffin.

Although we cannot see or feel the presence of radiation, it can be detected and measured in the most minute quantities with quite simple radiation measuring instruments.

Radiation Dose

Sunlight feels warm because our body absorbs the infra-red rays it contains. But, infra-red rays do not produce ionization in body tissue.

In contrast, ionizing radiation can impair the normal functioning of the cells or even kill them.

The amount of energy necessary to cause significant biological effects through ionization is so small that our bodies cannot feel this energy as in the case of infra-red rays which produce heat.

The biological effects of ionizing radiation vary with the type and energy. A measure of the risk of biological harm is the dose of radiation that the tissues receive. The unit of absorbed radiation dose is the sievert (Sv).

Since one sievert is a large quantity, radiation doses normally encountered are expressed in millisievert (mSv) or microsievert (µSv) which are one-thousandth or one millionth of a sievert. For example, one chest X-ray will give about 0.

2 mSv of radiation dose.

On average, our radiation exposure due to all natural sources amounts to about 2.4 mSv a year – though this figure can vary, depending on the geographical location by several hundred percent. In homes and buildings, there are radioactive elements in the air.

These radioactive elements are radon (Radon 222), thoron (Radon 220) and by products formed by the decay of radium (Radium 226) and thorium present in many sorts of rocks, other building materials and in the soil.

By far the largest source of natural radiation exposure comes from varying amounts of uranium and thorium in the soil around the world.

The radiation exposure due to cosmic rays is very dependent on altitude, and slightly on latitude: people who travel by air, thereby, increase their exposure to radiation.

We are exposed to ionizing radiation from natural sources in two ways:

  • We are surrounded by naturally-occurring radioactive elements in the soil and stones, and are bathed with cosmic rays entering the earth's atmosphere from outer space.
  • We receive internal exposure from radioactive elements which we take into our bodies through food and water, and through the air we breathe. In addition, we have radioactive elements (Potassium 40, Carbon 14, Radium 226) in our blood or bones.

Additionally, we are exposed to varying amounts of radiation from sources such as dental and other medical X-rays, industrial uses of nuclear techniques and other consumer products such as luminized wrist watches, ionization smoke detectors, etc. We are also exposed to radiation from radioactive elements contained in fallout from nuclear explosives testing, and routine normal discharges from nuclear and coal power stations.

Radiation Protection

It has long been recognized that large doses of ionizing radiation can damage human tissues. Over the years, as more was learned, scientists became increasingly concerned about the potentially damaging effects of exposure to large doses of radiation.

The need to regulate exposure to radiation prompted the formation of a number of expert bodies to consider what is needed to be done. In 1928, an independent non-governmental body of experts in the field, the International X-ray and Radium Protection Committee was established.

It later was renamed the International Commission on Radiological Protection (ICRP). Its purpose is to establish basic principles for, and issue recommendations on, radiation protection.

These principles and recommendations form the basis for national regulations governing the exposure of radiation workers and members of the public.

They also have been incorporated by the International Atomic Energy Agency (IAEA) into its Basic Safety Standards for Radiation Protection published jointly with the World Health Organization (WHO), International Labour Organization (ILO), and the OECD Nuclear Energy Agency (NEA). These standards are used worldwide to ensure safety and radiation protection of radiation workers and the general public.

An intergovernmental body was formed in 1955 by the General Assembly of the United Nations as the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).

UNSCEAR is directed to assemble, study and disseminate information on observed levels of ionizing radiation and radioactivity (natural and man-made) in the environment, and on the effects of such radiation on man and the environment.

Radiation Exposure

URL of this page: https://medlineplus.gov/radiationexposure.html

Radiation is energy. It travels in the form of energy waves or high-speed particles. Radiation can occur naturally or be man-made. There are two types:

  • Non-ionizing radiation, which includes radio waves, cell phones, microwaves, infrared radiation and visible light
  • Ionizing radiation, which includes ultraviolet radiation, radon, x-rays, and gamma rays

What are the sources of radiation exposure?

Background radiation is all around us all the time. Most of it forms naturally from minerals. These radioactive minerals are in the ground, soil, water, and even our bodies. Background radiation can also come from outer space and the sun. Other sources are man-made, such as x-rays, radiation therapy to treat cancer, and electrical power lines.

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What are the health effects of radiation exposure?

Radiation has been around us throughout our evolution. So our bodies are designed to deal with the low levels we're exposed to every day. But too much radiation can damage tissues by changing cell structure and damaging DNA. This can cause serious health problems, including cancer.

The amount of damage that exposure to radiation can cause depends on several factors, including

  • The type of radiation
  • The dose (amount) of radiation
  • How you were exposed, such as through skin contact, swallowing or breathing it in, or having rays pass through your body
  • Where the radiation concentrates in the body and how long it stays there
  • How sensitive your body is to radiation. A fetus is most vulnerable to the effects of radiation. Infants, children, older adults, pregnant women, and people with compromised immune systems are more vulnerable to health effects than healthy adults.

Being exposed to a lot of radiation over a short period of time, such as from a radiation emergency, can cause skin burns. It may also lead to acute radiation syndrome (ARS, or “radiation sickness”). The symptoms of ARS include headache and diarrhea. They usually start within hours.

Those symptoms will go away and the person will seem healthy for a little while. But then they will get sick again. How soon they get sick again, which symptoms they have, and how sick they get depends on the amount of radiation they received.

In some cases, ARS causes death in the following days or weeks.

Exposure to low levels of radiation in the environment does not cause immediate health effects. But it can slightly increase your overall risk of cancer.

What are the treatments for acute radiation sickness?

Before they start treatment, health care professionals need to figure out how much radiation your body absorbed.

They will ask about your symptoms, do blood tests, and may use a device that measures radiation.

They also try get more information about the exposure, such as what type of radiation it was, how far away you were from the source of the radiation, and how long you were exposed.

Treatment focuses on reducing and treating infections, preventing dehydration, and treating injuries and burns. Some people may need treatments that help the bone marrow recover its function.

If you were exposed to certain types of radiation, your provider may give you a treatment that limits or removes the contamination that is inside your body.

You may also get treatments for your symptoms.

How can radiation exposure be prevented?

There are steps you can take to prevent or reduce radiation exposure:

  • If your health care provider recommends a test that uses radiation, ask about its risks and benefits. In some cases, you may be able to have a different test that does not use radiation. But if you do need a test that uses radiation, do some research into the local imaging facilities. Find one that monitors and uses techniques to reduce the doses they are giving patients.
  • Reduce electromagnetic radiation exposure from your cell phone. At this time, scientific evidence has not found a link between cell phone use and health problems in humans. More research is needed to be sure. But if you still have concerns, you can reduce how much time you spend on your phone. You can also use speaker mode or a headset to place more distance between your head and the cell phone.
  • If you live in a house, test the radon levels, and if you need to, get a radon reduction system.
  • During a radiation emergency, get inside a building to take shelter. Stay inside, with all of the windows and doors shut. Stay tuned to and follow the advice of emergency responders and officials.

Environmental Protection Agency

  • Radiation Protection (Environmental Protection Agency)
  • RadTown USA: Basic Information (Environmental Protection Agency)
  • X-Rays, Pregnancy and You (Food and Drug Administration)
  • Radiation sickness (Medical Encyclopedia) Also in Spanish

Radiation, how much is considered safe for humans?

Editor's Note: The information below compares 1. the radiation exposures to the whole body which are the established federal standard for various activities (Note: The first federal standard for fetuses of pregnant radiation workers went into effect Jan. 1.); 2.

amounts of natural background radiation; 3. common sources of additional radiation; 4. amounts from medical treatment (very high radiation to a limited part of the body), and 5. amounts from diagnostic research (low levels from radioactive tracer elements).

The source of this information is Francis Masse, director of the MIT Radiation Protection Office. Dr.

Masse is a past president of the Health Physics Society and served in 1987-89 as chairman of the National Academy of Sciences panel which reviewed the exposure of soldiers to radiation from atmospheric testing in the 1940s and 1950s.

Astronauts: 25,000 Millirems

The highest recommended limit for radiation exposures is for astronauts-25,000 millirems per Space Shuttle mission, principally from cosmic rays. This amount is beyond the average 300+ millirems of natural sources of radiation and any medical radiation a person has received.

25,000 millirems per year level was the federal occupational limit during World War II and until about 1950 for radiation workers and soldiers exposed to radiation. The occupational limit became 15,000 millirems per year around 1950. In 1957, the occupational limit was lowered to a maximum of 5,000 millirems per year.

Average Natural Background: 300 Millirems

The average exposure in the United States, from natural sources of radiation (mostly cosmic radiation and radon), is 300 millirems per year at sea level. Radiation exposure is slightly higher at higher elevations-thus the exposure in Denver averages 400 millirems per year.

(A milliRem is 1/1000th of a Rem.

According to McGraw-Hill's Dictionary of Scientific and Technical Terms, a Rem is a unit of ionizing radiation equal to the amount that produces the same damage to humans as one roentgen of high-voltage x-rays.

The name is derived from “Roentgen equivalent man.” Wilhelm Roentgen discovered ionizing radiation in 1895 at about the same time that Pierre and Marie Curie discovered radium.)

All of these limits are for the amount of radiation exposure in addition to background radiation and medical radiation.

Adult: 5,000 Millirems

The current federal occupational limit of exposure per year for an adult (the limit for a worker using radiation) is “as low as reasonably achievable; however, not to exceed 5,000 millirems” above the 300+ millirems of natural sources of radiation and any medical radiation.

Radiation workers wear badges made of photographic film which indicate the exposure to radiation. Readings typically are taken monthly.

A federal advisory committee recommends that the lifetime exposure be limited to a person's age multiplied by 1,000 millirems (example: for a 65-year-old person, 65,000 millirems).

Minor: 500 Millirems

The maximum permissible exposure for a person under 18 working with radiation is one-tenth the adult limit or not to exceed 500 millirems per year above the 300+ millirems of natural sources, plus medical radiation. This was established in 1957 and reviewed as recently as 1990.

Fetus: 500 Millirems Or 50 Per Month (New Rule Jan. 1, 1994)

New federal regulations went into effect New Year's Day, establishing for the first time an exposure limit for the embryo or fetus of a pregnant woman exposed to radiation at work. The limit for the gestation period is 500 millirems, with a recommendation that the exposure of a fetus be no more than 50 millirems per month.

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