Radon Gas

The chemical element radon (Rn) is a radioactive gas and the heaviest of the noble gases. It is produced by the radioactive decay of radium, which is itself a natural decay product of the uranium found in various types of rocks. Trace amounts of radon seep from rocks and soil into the atmosphere, which can become a health hazard when trapped in sufficient concentrations in enclosed spaces such as basements.

Discovery and Properties of Radon Gas

The French physicist Antoine Henri Becquerel discovered radioactivity in 1896 when he accidentally detected spontaneous and continuous radiation emitted by uranium. One of his students, Polish scientist Marie Curie, found that thorium was also radioactive and that the energy of radioactivity was about 1 million times greater than the energy of chemical reactions. In 1898, Marie and her husband Pierre Curie discovered the radioactive elements polonium and radium by separating various components of the uranium ore pitchblende. Within a few years, radioactivity was found to consist of three components in increasing order of their ability to penetrate matter: helium nuclei (alpha particles), electrons (beta particles), and electromagnetic radiation (gamma and X-rays).

In 1900, the German chemist Friedrich E. Dorn detected a radioactive gas given off in the decay of radium along with helium. This gas was originally called “radium emanation” but was later officially named radon. It was found to have a half-life of 3.82 days. Even before Dorn’s discovery, British physicists R. B. Owens and Ernest Rutherford had observed in 1899 that some of the radioactivity of thorium compounds could be blown away in the form of a gas that they called thoron, which was found to have a 51.5-second half-life. In 1904, Friedrich Giesel and André-Louis Debierne independently discovered another radioactive gas produced from actinium that they termed “actinon,” which was found to have a 3.92-second half-life. After the development of the isotope concept by Rutherford and Frederick Soddy in 1912, it was eventually found that “thoron” and “actinon” were isotopes of radon, with atomic masses 220 for thoron (radon-220, ²²⁰Rn), 219 for actinon (radon-219, ²¹⁹Rn), and 222 for radon (radon-222, ²²²Rn). Radon is now known to have at least seventeen artificial radioactive isotopes in addition to its three natural isotopes. Overall, radon has thrity-nine known isotopes.

Radon is an odorless, tasteless, colorless gas nearly eight times heavier than air and more than one hundred times heavier than hydrogen. It has an atomic number of 86 (having 86 protons in its atomic nucleus), and its isotopes have atomic mass numbers (protons plus neutrons) ranging from 204 to 224, all radioactive. It has the stable “ noble gas” electronic configuration of eight electrons in its outer shell, accounting for its usual chemical inactivity, although it is not completely inert. The most massive of the noble gases, xenon and radon, can be induced to form chemical compounds with the appropriate ligand materials. In 1962, the compound radon difluoride was produced in the laboratory and is more stable than other noble-gas compounds.

Radon is rare in nature because its isotopes all have short half-lives and its source, radium, is such a scarce element. Radon-222 is the longest-lived of the radon isotopes and is an alpha-decay product of radium-226, which itself results from the decay of uranium-238 (²³⁸U) along with about one dozen other radioactive isotopes. Traces of radon are found in the atmosphere near the ground because of seepage from soil and rocks, most of which contain a small amount of uranium that produces the minute amounts of radium that continually decay into radon.

Health Effects of Radon Gas

Because it is inert and has a relatively short half-life, radon in itself poses little danger. However, in the process of radioactive decay, it produces several daughters that are normally solids rather than gases, some of which emit alpha particles that can be especially dangerous to the delicate tissues of the lungs. Radon-222 poses the greatest danger because it has the longest half-life and occurs most commonly in nature, making up about 40 percent of background radiation. It is a daughter product of the decay of uranium-238, which makes up 99.3 percent of natural uranium and has a half-life of 4.5 billion years, so it is a virtually unending source of radioactivity in the earth. Other radon isotopes pose less danger because of their short half-lives and less common occurrence, with the possible exception of thoron in some local areas where it occurs at higher-than-normal concentrations.

The decay products of radon-222 are solids, two of which emit high-energy alpha particles. These two radon daughters are polonium-218, with a half-life of 3.05 minutes, and polonium-214, with a half-life of 164 microseconds. Because they form with an electric charge, these isotopes readily attach to airborne particles. When the polonium is inhaled, it lodges in the lung and can cause damage to the lining of the lung by alpha radiation. Most of the damage is done in the bronchial tubes, which contain the precursor (stem) cells that are particularly sensitive to the cancer-causing effects of alpha radiation. The primary data relating to lung cancer deaths caused by radon exposure come from studies of underground uranium miners. These studies indicate that from 3 to 8 percent of the miners developed lung cancer above and beyond those cancers attributed to smoking and other causes.

In the 1970s, concern began to be expressed about radon contamination of indoor air, especially for homes constructed on or with waste rock or tailings associated with the mining and processing of uranium and phosphate ores with significant concentrations of radium-226. This led Congress to pass the Uranium Mill Tailings Act in 1978. Wider concern emerged in 1984 when a nuclear power engineer named Stanley Watras set off monitors at his job as they detected radioactivity that was subsequently traced to exposure to high radon concentration in his home in Boyertown, Pennsylvania. Further studies indicated that radon levels in houses far removed from uranium tailings or phosphate ores were often as high as or higher than in houses near such sites, especially in poorly sealed basements.

By the late 1980s, it was recognized that radon gas seeping into the foundations, basements, or piping of poorly ventilated buildings is a potentially serious health hazard. Radon levels are highest in well-insulated homes built over geological formations that contain uranium mineral deposits. Even though these levels might be significantly lower than those in underground mines, it was feared that long-term exposure to even moderate amounts of radon might greatly increase the risk of developing lung cancer. Radon is now thought to be the largest source of natural radiation exposure and the single most important cause of lung cancer among nonsmokers in the United States. Studies indicate that indoor radon exposure increases considerably in the presence of cigarette smoke, both primary and secondary (passive smoke), since radon daughters bind more effectively with smoke particles in the air.

Concentrations of Radon Gas

In the United States, the concentrations of radon and its decay products are usually expressed in picocuries per liter (pCi/L). A picocurie is one-trillionth (10^-12) of a curie, and 1 curie equals 37 billion becquerels (Bq, or disintegrations per second). In the International System of Units, radon concentrations are expressed as becquerels per cubic meter, so 1 picocurie per liter equals 37 becquerels per cubic meter. Average radon concentrations in outdoor air at ground level are about 0.20 picocurie per liter, ranging from less than 0.1 picocuries per liter to about 30 picocuries per liter. Radon dissolved in groundwater ranges from about 100 to nearly 3 million picocuries per liter. Indoor air averages about 1.5 picocuries per liter, but local conditions can result in levels several orders of magnitude higher than these, especially in some single-family dwellings. The Environmental Protection Agency (EPA) estimates that at least 21,000 fatal lung cancers per year are caused by indoor radon. The World Health Organization (WHO) estimates that radon exposure is responsible for between 3 and 5 percent of all lung cancer cases worldwide.

Various standards for indoor radon have been established by extrapolating down from levels as high as 30,000 picocuries per liter in uranium mines associated with lung cancer. The EPA has set a radon guideline of 4 picocuries per liter for remedial action in buildings, which it estimates could produce between one and five lung cancer deaths for every one hundred individuals. The EPA projects up to seventy-seven fatalities out of one hundred people exposed to levels of 200 picocuries per liter. These estimates assume seventy years in the dwelling, with about 75 percent of time spent indoors. The International Council on Radiation Protection (ICRP) has set an indoor radon level of 8 picocuries per liter as unsafe, about seventeen atoms per minute of radon decaying in every liter of air. The EPA estimates that a level of 10 picocuries per liter has a lung cancer risk similar to smoking one pack of cigarettes per day. Since the 1970s, a radiation safety limit of about 100 picocuries per liter has been set for uranium mining.

The indoor contamination problem in the Boyertown area of southeastern Pennsylvania was found to have radon levels as high as 2,600 picocuries per liter. Boyertown lies on a geological formation called the Reading Prong, which extends east from Reading through three counties of Pennsylvania and into parts of New Jersey, New York, and New England, with bedrock minerals containing elevated levels of uranium and thorium. These conditions led to the monitoring of eighteen thousand homes by the EPA in conjunction with the Pennsylvania Department of Health and local utilities, which found radon levels in excess of the EPA’s 4 picocurie per liter guideline for remedial action in 59 percent of the homes. In a nationwide EPA residential survey, average radon levels ranged from 0.1 picocurie per liter in Hawaii to 8.8 picocuries per liter in Iowa. Researchers estimate that over 6 percent of US homes have radon levels in excess of the ICRP guideline of 8 picocuries per liter.

Detection and Reduction of Indoor Radon Gas

Indoor radon levels are difficult to measure because of such factors as air movement, the effects of cigarette smoke, water tables, barometric pressure, and seasonal conditions, with higher readings in the summer than in the winter. Both active and passive testing devices can be used to test homes. Active devices include continuous radon monitors used by trained testers over several days. Passive radon kits, available at hardware stores, include charcoal canisters and alpha-track detectors. Sealed charcoal-filled canisters are exposed for two or three days and then sent to EPA-approved laboratories to analyze the level of gamma radiation they have captured, which is related to the radon level. Alpha-track detectors contain plastic strips that must be suspended for two or three months before being sent for analysis based on counting microscopic alpha tracks.

Various techniques can reduce radon levels in residential structures. Site selection is essential in avoiding the high radon contamination associated with highly permeable (porous) soils. These can be identified from soil maps prepared by the Soil Conservation Service. High-radium-content surface materials can be covered with soil that has low permeability and radium content. A 3.3-meter fill depth can reduce radon emanation rates by about 80 percent.

The choice of substructure is also crucial in radon reduction, with well-ventilated crawl spaces providing much lower radon levels than basements. Radon control in basements is aided by using good-quality concrete on top of an impermeable plastic barrier and a complete drainage tile system around the perimeter. Radon can be reduced in existing basements by sealing floor and wall cracks, capping sumps, and venting the air from under the basement floor.

Significance

Radon has both positive and negative implications, although there are some claims that the risks from radon have been overstated. Radon-222 is used in the treatment of some cancers. It can be collected by passing air through an aqueous solution of a radium-226 salt or a porous solid containing a radium-226 salt and then pumping off the accumulated radon every few days. It is then purified and compressed in small tubes, which can be inserted into the diseased tissue. The gas produces penetrating gamma radiation from the bismuth-214 decay product of radon and can be used for both radiotherapy and radiography.

Radon is also a useful tracer for groundwater and atmospheric mixing. It is used in studies of groundwater interaction with streams and rivers. A high radon concentration in groundwater that makes its way into a stream or river is a sensitive indicator of such local inputs. Since atmospheric radon concentrations decrease exponentially with altitude and are lower over water than land, radon can serve as an effective tracer in measuring atmospheric mixing.

Critics of the EPA’s 4 picocuries per liter radon guideline have raised questions about extrapolating from statistical data on lung cancer deaths among miners with radon exposures as high as 30,000 picocuries per liter. Studies have indicated no unusual incidence of lung cancer deaths for US uranium miners when exposures are below 12,000 picocuries per liter. A massive study (published in Science magazine on August 22, 1980) of two groups in China, one living in a high-radiation area and the other in a low-radiation area, showed no significant cancer rate difference between the two groups. A 1996 Finnish Center for Radiation and Nuclear Safety study found no increased risk for residents exposed to as much as 2.5 times the EPA’s guideline. Perhaps the nearly half-billion dollars spent by Americans testing for radon and renovating their homes has been an overreaction to the natural background radiation humans have lived with for thousands of years. In the twenty-first century, the EPA has admitted to flawed modeling, which lacked scientific backing and exaggerated the radon threat. Studies in the twenty-first century have suggested low radon exposure does not pose a significant risk to human health. Still, radon remains linked to lung cancer through decades of study. It is still considered the second leading cause of lung cancer after smoking, contributing to approximately 12 percent of lung cancers annually in the United States.

Principal Terms

half-life: the time required for one-half of the nuclei in a sample of a radioactive isotope to spontaneously decay through the process of nuclear fission

isotopes: atoms of an element containing an identical number of protons but different numbers of neutrons in their nuclei

ligand: an atom, ion, or molecule that combines with a central metal atom or ion, without being chemically bonded to it, to form a stable molecular complex

noble gas: any of the elements helium, neon, argon, krypton, xenon, and radon; they are often called inert gases since they are normally chemically inert

picocurie (pCi): a unit of radioactivity corresponding to one-trillionth of that from 1 gram of radium (0.037 disintegration per second or 2.22 disintegrations per minute)

radioactivity: the spontaneous emission from unstable atomic nuclei of high-energy sub-nuclear particles and electromagnetic radiation; radioactive emissions typically include helium nuclei (alpha particles), electrons (beta particles), and electromagnetic waves (gamma and X-rays)

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