RADON (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Radon is the last member of the noble gas family. The noble gases are the elements that make up Group 18 (VIIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. The noble gases get their name because they are inactive chemically. They combine with other substances under only extreme conditions. Their tendency to avoid contact with other elements was seen by early chemists as "royal" or "noble" behavior. The noble gases are also called the inert gases.

Radon is a radioactive element. A radioactive element is one that gives off radiation and breaks down to form a different element. Radon is formed when heavier radioactive elements, like uranium and thorium, break down. In turn, radon breaks down to form lighter elements, such as lead and bismuth.

SYMBOL
Rn

ATOMIC NUMBER
86

ATOMIC MASS
222.0176

FAMILY
Group 18 (VIIIA)
Noble gas

PRONUNCIATION
RAY-don

Radon is a well-know air pollutant today. It is formed in rocks and soil where uranium is present. As a gas, radon tends to drift upward out of the ground. If a house or building has been built above soil containing uranium, radon may collect in the structure. The U.S. Environmental Protection Agency (EPA) regards the presence of radon in homes and offices as a serious health problem.

Discovery and naming

Radioactivity was discovered in 1896 by French physicist Antoine-Henri Becquerel (1852-1908). Becquerel observed that a photographic plate was exposed even in the dark when placed next to an ore called pitchblende. The explanation for this phenomenon was offered two years later by a colleague of Becquerel's, Polish-French chemist Marie Curie (1867-1934). Curie said that something in the pitchblende was giving off radiation. The radiation was similar to light in some ways. But it was also different, since it could not be seen. Curie suggested the name of radioactivity for this behavior.

Over the next decade, many scientists worked to find out more about radioactive materials. Curie and her husband, Pierre Curie (1859-1906), isolated two new radioactive elements, polonium and radium. In 1900, German physicist Friedrich Ernst Dorn (1848-1916) found a third radioactive element.

Dorn found this element because of an observation made by Curie. When radium is exposed to air, the air becomes radioactive. The Curies did not study this phenomenon further. However, Dorn did. Eventually he discovered that radium produces a gas when it breaks apart. The radioactive gas escapes into the air. The radioactivity of air exposed to radium is caused by this gas.

At first, Dorn called this radioactive gas radium "emanation". The term emanation refers to something that has been given off. Radium emanation, then, means something given off by radium. Dorn also considered the name of niton for the gas. This name comes from the Latin word nitens, which means "shining." Eventually, however, scientists decided on the modern name of radon. The name is a reminder of the source from which the gas comes, radium.

The proper location of radon in the periodic table was determined by Scottish chemist Sir William Ramsay (1852-1916). Ramsay was also involved in the discovery of three other noble gases, neon, krypton, and xenon. In 1903, Ramsay was able to determine the atomic weight of radon. He showed that it belonged beneath xenon in Group 18 (VIIIA) of the periodic table.

Credit for the discovery of radon is often given to other scientists as well. In 1899, Robert B. Owens announced the presence of a radioactive gas that he named thoron. In 1903, French chemist Andre Louis Debierne (1874-1949) made a similar discovery. He named the gas actinon. Certainly, some credit for the discovery of element 86 can be shared among all these men.

Physical properties

Radon is a colorless, odorless gas with a boiling point of -61.8°C (-79.2°F) . Its density is 9.72 grams per liter, making it about seven times as dense as air. It is the densest gas known. Radon dissolves in water and becomes a clear, colorless liquid below its boiling point. At even lower temperature, liquid radon freezes. As a solid, its color changes from yellow to orangish-red as the temperature is lowered even further. It is a dramatic sight since it also glows because of the intense radiation being produced.

Chemical properties

Radon was long thought to be chemically inert. The term inert means incapable of reacting with other substances. In the early 1960s, however, a number of chemists found ways of making compounds of the noble gases. They did so by combining a noble gas with a very active element. The element generally used was fluorine, the most active chemical element. The result was the formation of noble gas compounds. The first radon compound to be produced was radon fluoride (RnF).

Radon: the secret visitor

A dangerous stranger may be hiding in your home. You won't be able to see, smell, or hear the stranger. But it has the ability to cause cancer. That dangerous stranger is radon gas.

Radon is produced naturally when uranium breaks down. Uranium is a radioactive element that occurs naturally in the Earth's crust. It is a fairly common element and could be in the ground below your own home.

When uranium breaks down, it produces many different elements, including radium, thorium, bismuth, and lead. Hone of these elements is a threat since they all remain in the ground. But uranium also forms radon when it breaks down. And radon is a gas. It can float upward, out of the earth, and into the basement of your home.

In some respects, radon is a serious health hazard. It gives off radiation that can kill cells. But radon does not have a very long half life. It breaks down and disappears fairly quickly.

The problem is that it breaks down into elements that are solid. These include polonium-214, polonium-218, and lead-214. These elements are more of a threat to your health. If you inhale them, they may stick to the lining of your lungs. While there, they give off radiation. The radiation can kill or damage cells. The final result of radon escaping into a building can be a variety of respiratory problems. Respiratory problems are those affecting the lungs and other parts of the system used for breathing. The most serious of these respiratory problems is lung cancer.

Scientists today think that radon may cause as many as 20,000 cases of lung cancer per year. If so, that would make radon the second leading cause of this disease, after smoking. The people most in danger from radon are those who also smoke. These people are threatened both by radon and by cigarette smoke.

The EPA has studied the problem of radon in homes and offices. The agency believes that up to 8 million homes may have levels of radon that are too high. About 20 percent of all homes the agency has studied have high radon levels.

Fortunately, it's easy to find out if radon is lurking in your home. Radon test kits can be purchased easily and at low cost. Anyone can learn how to use one in a few minutes. If radon is present, some simple steps can be taken to reduce the danger the element presents. For example, any cracks in the foundation of a house can be sealed. By doing so, radon gas will be prevented from seeping into the house. Also, some method for circulating air should always be available. A fan or an air conditioner, for example, will insure that fresh air is constantly brought into a house and "stale" air (containing radon gas) is removed.

Occurrence in nature

The abundance of radon in air is too small to be estimated. Some radon is always present because it is formed during the breakdown of uranium and radium.

Isotopes

Three isotopes of radon occur in nature—radon-219, radon-220, and radon-222. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. At least 18 other radioactive isotopes of radon have been produced artificially.

All isotopes of radon have short half-lives and do not remain in the atmosphere very long. The half life of a radioactive element or isotope is the time it takes for half of a sample of the element or isotope to break down. The radon isotope with the longest half life is radon-222 at only 2.8 days. If 10 grams of radon-222 were prepared today, only 5 grams would remain 2.8 days from now. After another 2.8 days, only 2.5 grams would be left. Within a month, it would be difficult to detect any of the isotope.

Extraction

Radon is produced during the breakdown of radium. It is obtained commercially by the following method. A compound of radium is placed under water. Gases given off by the radium compound are collected in a glass tube. Oxygen, nitrogen, water vapor, carbon dioxide, and other gases are removed from the gas in the tube. The gas that remains is pure radon.

Uses

The uses for radon all depend on the radiation it gives off. That radiation cannot be seen, smelled, tasted, or detected by any other human sense. However, a number of instruments have been invented for detecting this radiation. For example, a Geiger counter is a device that makes a clicking sound or flashes a light when radiation passes through it.

As a solid, radon changes its color from yellow to orangish-red as its temperature decreases. It is a dramatic sight since it also glows because of the intense radiation being produced.

One use of radon based on this principle is in leak detection. An isotope of radon is added to a flow of gas or liquid through a tube. A Geiger counter can be passed along the outside of the tube. If radiation is present, the Geiger counter makes a sound or flashes a light. The presence of radiation indicates a leak in the tube. This principle is applied in many other systems to study materials that cannot actually be seen.

Radon was once commonly used to treat cancer too. The radiation it gives off kills cancer cells. However, the element must be used with great care because radiation can kill healthy cells as well. In fact, the bad side-effects of radiation therapy are caused by the killing of healthy cells by radiation. Today, radon is not as widely used for the treatment of cancer. More efficient isotopes have been found that are easier and safer to work with.

Compounds

Chemists are trying to make compounds of radon, but the task is difficult. One compound that has been made is radon fluoride. In any event, such compounds are laboratory curiosities and have no commercial uses.

Health effects

Because of the radiation it produces, radon is a highly dangerous material. It is used only with great caution. Radon is especially dangerous because it is inhaled, exposing fragile tissues to penetrating radiation.

Radon

Radon is a colorless and odorless radioactive gas formed from radioactive decay. Denoted by the atomic symbol, Rn, radon has an atomic number of 86. The atomic weight of its most stable and common isotope is 222. It is classified as a noble gas based on its location on the periodic table of the elements. Radon is the heaviest of the inert, or noble, gases.

The discovery of radon is credited to Friedrich Dorn (1848–1916), a German physics professor. Marie Curie’s (1867–1934) experiments stimulated Dorn to begin studying the phenomenon of radioactivity. In 1900, he showed that radium emitted a radioactive gas that was called radium emanation for several years.

The most common geologic source of radon is the decay of naturally occurring uranium. Radon is commonly found at low levels in widely dispersed crustal formations, soil, and water samples. To some extent, radon can be detected throughout the United States. Specific geologic formations, however, frequently present elevated concentration of radon that may pose a significant health risk. The Surgeon General of the United States and the Environmental Protection Agency identify radon exposure as the second leading cause of lung cancer in the United States. Cancer risk rates are based upon magnitude and duration of exposure.

Produced underground, radon moves toward the surface and eventually diffuses into the atmosphere or in groundwater. Because radon has a half-life of approximately four days, half of any size sample deteriorates during that time. Regardless, because radon can be continually supplied, dangerous levels can accumulate in poorly ventilated spaces (e.g., underneath homes, buildings, etc.). Moreover, the deterioration of radon produces alpha particleradiation and radioactive decay products that can exhibit high surface adherence to dust particles. Radon detection tests are designed to detect radon gas in picocuries per liter of air (pCi/L). The picocurie is used to measure the magnitude of radiation in terms of disintegrations per minute. One pCi, one trillionth of a Curie, translates to 2.2 disintegrations per minute. EPA guidelines recommend remedial action (e.g., improved ventilation) if long term radon concentrations exceed 4 pCi/L.

Working level units (WL) are used to measure radon decay product levels. The working level unit is used to measure combined alpha radiation from all radon decay products. Commercial test kits designed for use by the general public are widely available. The most common forms include the use of charcoal canisters, alpha track detectors, liquid scintillation detectors, and ion chamber detectors. In most cases, these devices are allowed to measure cumulative radon and byproduct concentrations over a specific period of time (e.g., 60 to 90 days) that depends on the type of test and geographic radon risk levels. The tests are usually designed to be returned to a qualified laboratory for analysis. The EPA estimates that nearly one out 15 homes in the United States has elevated radon levels.

Radon can be kept at low concentration levels by ventilation and the use of impermeable sheeting to prevent radon seepage into enclosed spaces. Radon in water does not pose nearly the health risk as does breathable radon gas. Regardless, radon removal protocols are increasingly a part of water treatment programs. Radon is removed from water by aeration or carbonfiltration systems.

Exposure to radon is cumulative. Researchers are conducting extensive research into better profiling the mutagenic risks of long term, low-level radiation exposure.

Uranium miners must take special precautions to avoid radioactive poisoning by radon.

See also Biophysics; Earth’s interior; Geophysics; Radiation detectors; Radiation exposure; Radioactive pollution.

Radon

Radon is an odorless, colorless, radioactive, though chemically unreactive gas. It has an atomic number of eighty-six, which corresponds to the number of protons found in the nucleus of any isotope of radon. There are more than thirty known isotopes of radon, and each one emits some combination of alpha , beta , and gamma radiation when undergoing radioactive transformation, commonly referred to as "decay."

Radon gas is ubiquitous in the natural environment. This is because the precursors to radon, such as the aforementioned radium isotopes, and others such as radium, thorium, and uranium isotopes, are present in some rock formations. Radon is also found in the man-made environment because many of the materials, consumer products, and foodstuffs of everyday life come from the naturally radioactive environment.

Radon is one of the few examples in nature of a gaseous element that results from the decay of a solid element and then decays into another solid element. This increases its potentially harmful effect in humans. For example, radon-222, the most common isotope of radon, is a product of the alpha decay of radium-226 atoms, found in rocks. Radon-222 atoms subsequently produce polonium-218 in a similar alpha-decay process, and it is this solid substance that can lodge in human tissue.

Solid-state radionuclides remain where created by decay processes unless they are redistributed by dissolving in groundwater or by becoming airborne. Given the chemically inert nature of radon, there are no known compounds that include this element. Thus isotopes of radon may diffuse away from their place of origin and usually end up dissolved in ground water or mixed with air above the soil and rocks that bear their solid precursors.

People's exposure to radon primarily occurs when radon seeps out of air spaces above soil or rocks and into surrounding indoor or outdoor air, such as the basements of houses built over radium-bearing rocks. It is not exposure to radon gas that actually may lead to harm, but exposure to the decay products of radon, specifically the ones with short half-lives that emit alpha radiation. Radon-222 offspring, like polonium-218 and polonium-214, become attached to dust particles that may be breathed in by people exposed to the gas and become lodged in the respiratory tract. Decay of the radon progeny while in the lungs is the means by which the radiation dose is delivered to the lungs. This dose, which is the energy of alpha particles absorbed by cells that line the lungs, is what gives rise to the potential for lung cancer associated with exposure to radon.

Radon has been labeled by the Environmental Protection Agency as the second-leading cause of lung cancer in humans (after tobacco smoke), based on mathematical risk estimates derived from many published studies of exposure of subsurface uranium miners to highly elevated levels of the gas, primarily radon-222. Many radiation health scientists have challenged such findings because of the vast difference in exposure levels between homes and buildings on the one hand, and subsurface mines on the other. However, a variety of action levels and exposure limits for radon gas exposure have been recommended or set into law for the protection of the public. The Surgeon General and the EPA recommend that radon levels of four picocuries or more inside homes be reduced. The EPA states that radon levels less than four picocuries still pose a risk, especially for smokers.

Methods to both detect and mitigate indoor radon exposure have been devised as well. Detection and measurement methods usually make use of a device to collect radon gas atoms or the offspring particles. The simplest real-time method would be a "grab sample," in which air is drawn into an evacuated flask that is then taken back to a laboratory for analysis. The most popular short-term measurement device is the activated charcoal canister, a small container of steam-treated charcoal that is opened and left at the sampling location for a prescribed time. Radon is adsorbed by the charcoal, and the decay products of the radon are analyzed after the canister has been resealed and retrieved. The simplest mitigation methods include sealing cracks and penetrations through foundations, as well as diverting the radon away from the slab or out of the ground, with vacuum or ventilation systems.

see also Cancer; Health, Human; Risk.

Bibliography

National Research Council. (1999). Health Effects of Exposure to Radon—BEIR VI. Washington, D.C.: National Academy Press.

National Research Council. (1999). Risk Assessment of Radon in Drinking Water. Washington, D.C.: National Academy Press.

other resources

U.S. Environmental Protection Agency. "Indoor Air Quality: Radon." Available from http://www.epa.gov/radon.

Ian Scott Hamilton

Radon

Radon (usually in the form of the radon-222 isotope ) is a colorless and odorless radioactive gas formed from radioactive decay . Denoted by the atomic symbol, Rn. radon has an atomic number of 86 and the atomic weight of its most stable isotope is 222. It is a colorless, odorless gas that emits radioactivity. It is classified as a noble gas based on its location on the periodic table . Radon is the heaviest element in the family of inert, or noble, gases.

The discovery of radon is credited to Friedrich Dorn (1848-1916), a German physics professor. Marie Curie's experiments stimulated Dorn to begin studying the phenomenon of radioactivity. In 1900, he showed that radium emitted a radioactive gas that was called radium emanation for several years.

The most common geologic source of radon derives from the decay of uranium . Radon is commonly found at low levels in widely dispersed crustal formations, soil , and water samples. To some extent, radon can be detected throughout the United States. Specific geologic formations, however, frequently present elevated concentration of radon that may pose a significant health risk. The Surgeon General of the United States and the Environmental Protection Agency identify radon exposure as the second leading cause of lung cancer in the United States. Cancer risk rates are based upon magnitude and duration of exposure.

Produced underground, radon moves toward the surface and eventually diffuses into the atmosphere or in groundwater . Because radon has a half-life of approximately four days, half of any size sample deteriorates during that time. Regardless, because radon can be continually supplied, dangerous levels can accumulate in poorly ventilated spaces (e.g., underneath homes, buildings, etc.). Moreover, the deterioration of radon produces alpha particle radiation and radioactive decay products that can exhibit high surface adherence to dust particles. Radon detection tests are designed to detect radon gas in picocuries per liter of air (pCi/L). The picocurie is used to measure the magnitude of radiation in terms of disintegrations per minute. One pCi, one trillionth of a Curie, translates to 2.2 disintegrations per minute. EPA guidelines recommend remedial action (e.g., improved ventilation) if long term radon concentrations exceed 4 pCi/L.

Working level units (WL) are used to measure radon decay product levels. The working level unit is used to measure combined alpha radiation from all radon decay products. Commercial test kits designed for use by the general public are widely available. The most common forms include the use of charcoal canisters, alpha track detectors, liquid scintillation detectors, and ion chamber detectors. In most cases, these devices are allowed to measure cumulative radon and byproduct concentrations over a specific period of time (e.g., 60–90 days) that depends on the type of test and geographic radon risk levels. The tests are usually designed to be returned to a qualified laboratory for analysis.

The EPA estimates that nearly one out 15 homes in the United States has elevated radon levels.

Radon can be kept at low concentration levels by ventilation and the use of impermeable sheeting to prevent radon seepage into enclosed spaces. Radon in water does not pose nearly the health risk as does breathable radon gas. Regardless, radon removal protocols are increasingly a part of water treatment programs. Radon is removed from water by aeration or carbon filtration systems.

Exposure to radon is cumulative. Researchers are presently conducting extensive research into better profiling the mutagenic risks of long term, low-level radiation exposure .

Uranium miners must take special precautions to avoid radioactive poisoning by radon. The gas can also migrate upward into the soil and leak into buildings. Radon can seep into groundwater and so may be found in public drinking supplies.

See also Biophysics; Earth's interior; Geophysics; Radiation detectors; Radiation exposure; Radioactive pollution.

RADON

Radon-222 and radon-220 (thoron) are invisible, inert, and odorless radioactive gases formed in the decay of uranium-238 and thorium-232, respectively. Uranium-238 and thorium-232 are radionuclides that are widely distributed in the earth's crust. The half-life of radon-222 is long enough (3.82 days) to enable appreciable quantities of this element to accumulate in the environment, whereas the half-life of radon-220 is so short (55 seconds) that it does not attain environmental concentrations that produce demonstrable biological effects. Radon-222, seeping out of the soil, is ubiquitous in outdoor air, where its concentration averages about 15 becquerels per cubic meter (5 Bqm^-3 or 0.4 pCi/L). (The becquerel [Bq] and the curie [Ci] are units of radioactivity; 1 Bq = 1 disintegration per second, and 1 Ci = 3.7 × 10¹⁰disintegrations per second. Radon is measured in picocuries per liter of air [pCi/L] or becquerels per cubic meter [Bqm^-3].) In indoor air, the concentration of radon tends to be much higher than in outdoor air, especially in poorly ventilated basements and underground mines, where it may exceed 1,000 Bqm^-3 (20 pCi/L). Indoor levels may be increased substantially by the use of groundwater or well water containing elevated concentrations of radon.

The alpha particles emitted by radon outside the body do not penetrate the skin, and radon itself, like other inert gases, is breathed in and out of the lungs without interacting significantly with the surrounding tissues. Hence the biological effects of radon result from inhalation of its solid, short-lived, alpha-emitting decay products (principally polonium-218 and polonium-214), which deposit on the lining of the bronchial airway. The dose to internal organs from radon that is ingested in drinking water, even at high concentrations, is extremely low.

In humans and laboratory animals, the risk of lung cancer increases with increasing exposure to inhaled radon and its short-lived decay products. In underground miners the risk appears to increase in proportion to the total cumulative dose to cells lining the airway, and to be about two times higher in smokers than in nonsmokers. The risk from exposure to residential indoor radon at a given concentration, although yet to be defined precisely, is generally estimated to be comparable to the corresponding risk in miners. As a result, radon is thought to be the single most important cause of lung cancer in nonsmokers and to cause 10 to 15 percent of all lung cancers, or 15,000 to 20,000 lung cancer deaths each year in the United States. Hence, the U.S. Environmental Protection Agency has recommended that indoor radon concentrations not be allowed to exceed 4 pCi/L, a concentration that might be expected to double the risk of lung cancer if inhaled throughout an average lifespan.

Methods for reducing the concentration of radon and its decay products in indoor air include ventilation; air filtration; sealing of cracks in basement floors and walls; installation of a subslab exhaust system beneath the basement floor; and remediation of heavily contaminated groundwater or well water that is used for drinking, bathing, or showering. Radon can be measured in the home with a number of relatively inexpensive devices, which are available from some state and local governments as well as private firms. Pertinent information can generally be obtained from the local state radiation or the Environmental Protection Agency office.

Arthur C. Upton

Bibliography

Eisenbud, M., and Gesell, T. (1997). Environmental Radioactivity: From Natural, Industrial, and Military Sources, 4th edition. San Diego, CA: Academic Press.

Harley, N. (2000). "Radon and Daughters." In Environmental Toxicants, 2nd edition, ed. M. Lippmann. New York: John Wiley and Sons.

National Academy of Sciences/National Research Council (1998). Health Effects of Exposure to Radon. Washington, DC: National Academy Press.

U.S. Geological Survey. The Geology of Radon. Available at http://energy.ct.us.gov/radonhome.html.