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Basic Concepts

What is an Atom?
Everything in the world, and everybody in the word, is composed of different types of matter (chemical elements). The smallest part of each element is called the “Atom”. An atom is so small that it can be seen only with the most powerful microscope. But the atom is the core of every substance in the universe.

Are there different types of atoms?
The type of atom is unique for each element, such as gold, silver, lead, tin, radium, carbon, and thorium. Our bodies are made mostly of hydrogen, carbon, oxygen, and calcium atoms. The light fixtures in this room are made of some type of metal atom. The air, as you know, is made of oxygen, hydrogen, nitrogen, and other atoms. Water is hydrogen and oxygen.

What is radioactivity?
Sometimes the center of an atom (its nucleus) has too much energy in it. An atom cannot hold this energy forever. Sooner or later, the atom must get rid of the excess energy and return to its normal (stable) state. Atoms with too much energy in their nuclei are called “radioactive”. They get rid of their excess energy by emitting radiation. Some radioactive atoms exist naturally; others are made artificially.

What is radiation?
A radioactive atom gives off radiation to get rid of excess energy. The radiation can be in the form of particles moving at high speeds, or pure energy.

Radiation is a broad term which includes such things as heat, light, radiowaves, microwaves and other familiar forms of energy. When radiations are emitted from an atom they are traveling at very high speeds. This means that they have a lot of “energy”. When the radiations collide with something, they deposit some or all of that energy in the thing with which they have collided.

You can compare the radiations from an atom with rays from the sun. When the rays from the sun reach our bodies they deposit their energy and the warmth we feel is the sun’s rays’ energy. When radiation from a radioactive atom penetrates an object, it deposits its energy in that object just like the sun’s rays’ deposit energy in our bodies.

What is a “unit”?
A unit is a way in which we express measurements. For example, an “inch” is a unit of length. A “second” is a unit of time. A “pound” is a unit of weight.

What is a “rem”?
A “rem” is a unit of radiation dose. It is typically used to describe how much radiation “energy” is deposited in someone or something. If our body absorbs radiation energy equivalent to about two-millionths of a calorie, we say we have received a “rem” of radiation dose.

Is a rem a large radiation dose?
When compared to the types of radiation doses people commonly receive every day of their lives from natural background radiation, a rem is a large dose . . . just like a “ton” is a large weight when we’re talking about toothpicks, and a “mile” is a large length when we’re talking about hot dogs.

Are there smaller units than a rem?
Yes. That unit is the “millirem”. A millirem is one thousandth (1/1,000) of a rem. An even smaller, but frequently more practical unit is the “microrem”, which is one millionth (1/1,000,000) of a rem.

What does the term “rate” mean?
A rate is the amount of a particular unit of measurement that occurs over some specific time period. For example, “miles per hour” is a rate of speed, or the distance traveled in one hour. Likewise, “millirems per hour” is a dose rate, or the amount of radiation energy deposited in a one hour period of time.

What is a “curie”?
A “curie” is a unit of radioactivity. It tells us how many radioactive atoms in particular collection of atoms are giving off radiation. Just like a “ream” of paper in a drawer tells us that there are 500 sheets in the drawer, a curie of radium in a container tells us that there are 37,000,000,000 radium atoms giving off radiation.

Is a curie a lot of radioactivity?
Yes. Compared to the amount of naturally-occurring radioactivity in our bodies, it is a very large amount — about ten million times larger. Therefore, it is sometimes more convenient to use units like “picocuries”. A picocurie is one trillionth of a curie.

Is there an easy way to distinguish “millirems” from “picocuries”?
Yes there is. A fireplace with a nice fire burning in it is a good way to explain the difference between these two terms. In a fireplace, the burning wood or coals radiate heat. In this case, the amount of burning wood (fuel) in the fireplace is analogous to the number of picocuries of radioactivity. The amount of heat (energy) given off by the fireplace is analogous to the number of millirems of radiation energy.

Is there another example that is an analog to the term “picocurie”?
Yes. This time, picture yourself sitting in a stadium watching a sporting event. When something exciting happens, you are likely to see a lot of flashes coming from the stands where people are taking pictures. If you could somehow count the number of flashes over a particular time period – say 10 minutes – you would know the “flash rate” from all the cameras that are in the stadium. This measurement is similar to how the amount of radioactivity in a particular collection of atoms is determined. In this case we count the bursts of radiation (flashes) being given off by the atoms (cameras) per unit time (10 minutes). When we see 22 bursts in 10 minutes, we know we have measured a picocurie of radioactivity.

How does the term “millirems” fit into this analogy?
Let’s say that while you are in the stadium, you take out your light meter and measure how much light is coming from the flashes in the stand over a one hour period. The amount of light measured by the meter is a measurement of the amount of “energy” coming from the cameras in the stadium. This measurement is similar to the radiation dose (energy) from a collection of atoms (cameras) per unit time (one hour). The units of this measurement would be “millirems per hour”.

What does the term “picocurie per gram” mean?
This refers to the amount of radioactivity in a particular solid substance. Picture a one-ton batch of concrete that contains 1,000 pounds of gravel, 500 pounds of cement, and 500 pounds of water. To describe this particular mix of concrete, one might say it contains “500 pounds per ton” of cement. This means that for every pound of concrete, there will also be a quarter of a pound of cement present. Similarly, if you wished to describe the amount of radioactivity that typically exists in soil throughout the United States, you would say that it contains about “one picocurie per gram” of radium, one picocurie per gram of thorium, and a host of other radioactive elements. This means that for every gram (about 0.002 pounds) of soil, there will also be one picocurie of radium and one picocurie per gram of thorium present, along with the rest of the radioactive elements commonly found in soil.

What does the term “picocurie per liter” mean?
This refers to the amount of radioactivity in a liter (about a quart) of liquid substance, such as water.

Water directly out of the tap contains about 0.01 “picocuries per liter” each of uranium, radium, and radioactive lead. It may also contain between 100 and 400 picocuries per liter of radioactive hydrogen, between 100 and 500 picocuries per liter of radioactive carbon, between 10 and 30 picocuries per liter of radioactive beryllium. and a variety of other radioactive elements such as aluminum, chlorine, silicon, lead, bismuth, polonium, and argon.  It can contain several hundred to several thousand picocuries per liter of radon gas, particularly if you get your drinking water from a well.

Is there radioactivity in the world around us?
Absolutely.  The earth has always been radioactive. Everyone and everything that has ever lived has been radioactive. In fact, the natural radioactivity in the environment is just about the same today as it was at the beginning of the Neolithic Age, more than 10,000 years ago.

Is there radioactivity in our bodies?
Yes. During our lifetime, our bodies harbor measurable amounts (billions) of radioactive atoms. About half of the radioactivity in our bodies comes from Potassium-40, a naturally-radioactive form of potassium. Potassium is a vital nutrient and is especially important for the brain and muscles. Most of the rest of our bodies’ radioactivity is from radioactive carbon and hydrogen.

We have about 120,000 picocuries of radioactivity in our bodies. These naturally-occurring radioactive substances expose our bodies to about 25 “millirem” per year, abbreviated as “mrem/yr”.

Most radioactive substances enter our bodies as part of food, water or air. Our bodies use the radioactive as well as the nonradioactive forms of vital elements such as iodine and sodium. Radioactivity can be found in all foods. As we said before, it is even in our drinking water. In a few areas of the United States, the naturally-occurring radioactivity in the drinking water can result in a dose of more than 1,000 millirem in one year.

Are there other sources of natural radiation?
Another type of natural radiation is cosmic radiation given off by the sun and stars in outer space. Because the earth’s atmosphere absorbs some of this radiation, people living at higher altitudes receive a greater dose than those at lower altitudes. In Ohio, for example, the average resident receives a dose of about 40 millirem in one year from cosmic radiation. In Colorado, it is about 180 millirem in one year. Generally, for each 100-foot increase in altitude, there is an increased dose of one (1) millirem per year.

Flying in an airplane increases our exposure to cosmic radiation. A coast-to-coast round trip gives us a dose of about six millirem.

In Ohio, radiation in soil and rocks contributes about 60 millirem in one year to our exposure. In Colorado, it is about 105 millirem per year. In Kerala, India, this radioactivity from soil and rocks can be 3,000 millirem per year, and at a beach in Guarapari, Brazil, it is over 5 millirem in a single hour. Some of the residents who use that beach receive doses approaching 1,000 millirem per year.

If you live in a wood house, the natural radioactivity in the building materials gives you a dose of 30 to 50 millirem per year. In a brick house, the dose is 50 to 100 millirem per year. And, if your home is so tightly sealed that the leakage of outside air into the home is small, natural radioactive gases (radon) can be trapped for a longer period of time and thus increase your dose.

Is there anything else?
Yes. Typical members of the US population may receive the following types of radiation exposures:

  • 2,000 millirem per Angiography of the heart by computerized tomography
  • 1,300 millirem per year for the average cigarette smoker
  • 1,000 millirem per whole-body computerized tomography screening
  • 700 millirem per computerized tomorgraphy of the chest
  • 650 millirem per nuclear medicine examination of the brain
  • 509 millirem per nuclear medicine examination of the thyroid
  • 405 millirem per barium enema
  • 245 millirem per upper gastrointestinal tract series
  • 200 millirem per computerized tomography of the head
  • 150 millirem per nuclear medicine examination of the lung
  • 7.5 millirem per year to spouses of recipients of certain cardiac pacemakers
  • 6.1 millirem from each one way flight from Los Angeles to London
  • 6 millirem per dental x-ray
  • 5 millirem per year from foods grown on lands in which phosphate fertilizers are used.
  • 4 millirem per year from highway and road construction materials
  • 2.8 millirem from each one way flight from New York to Seattle
  • 2 millirem per year from the use of gas mantles
  • 1.9 millirem from each one way flight from Washington, DC to Los Angeles
  • 1 to 6 millirem per year from domestic water supplies
  • 1 millirem per year from television receivers
  • 0.5 millirem from eating one-half pound of Brazil nuts.
  • 0.3 millirem per year from combustible fuels, (i.e., coal, natural gas, and liquefied petroleum)
  • 0.2 millirem from drinking a quart of Gatorade each week
  • 0.1 millirem per year from sleeping with one’s spouse (or “significant other”)

So, almost everything is radioactive, right?
Yes, radiation is everywhere. Our bodies and the world around us are radioactive. But there is no cause for alarm. These very small but detectable levels of radioactivity are natural . . . as natural as life itself. We are exposed to a constant stream of radiation from the sun and outer space. Radioactivity is in the ground, the air, the buildings we live in, the food we eat, the water we drink, and the products we use. The average person in the United States receives a dose of about 620 millirem per year from these natural sources of radioactivity as well as from typical medical radiation exposures such as diagnostic x-rays, nuclear medicine and CT studies.

Is a radiation dose of 620 millirem in a year harmful?
No. No effects have ever been observed at doses below 5,000 millirem delivered over a one year period. In fact, effects seen when humans are exposed to 100,000 millirem over a short time period are temporary and reversible. It takes a short-term dose of well over 500,000 millirem to cause a fatality.

Where can I obtain more Information about Radiation and Radioactivity?
There are a number of organizations and agencies that can provide you with additional information on this topic. The following is just a short listing:

Health Physics Society – (703) 790-1745
American National Standards Institute – (212) 642-4900
The American Academy of Health Physics – (703) 790-1745
Conference on Radiation Control Program Directors – (502) 227-4543
American Nuclear Society – (708) 579-8265
National Council on Radiation Protection and Measurements – (301) 657-2652
United Nations Scientific Committee on the Effects of Atomic Radiation – (800) 274-4888