Radon

• 1. Introduction

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  Radon is an invisible gas, and in 1984 it was the hidden cause of a puzzle at the Limerick Generating Station in Montgomery County, Pennsylvania. Stringent radiation testing at the new plant found that a construction engineer named Stanley Watras was contaminated with radioactive substances. The timing made no sense. The reactor had never been fueled. Watras was decontaminated every evening, yet he came back the next day carrying radioactivity again. Whatever was contaminating him was not coming from the plant's fuel. The culprit was an element with atomic number 86 and the symbol Rn. It is a radioactive noble gas that no human sense can detect. How can a gas that decays within days still exist on a planet billions of years old? Why did scientists argue for decades over what to call it? And how did a hazard first recognized among miners end up inside ordinary family homes?

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• 2. Emanation Names

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  In 1899, at McGill University in Montreal, Ernest Rutherford and Robert B. Owens noticed odd variations while measuring radiation from thorium oxide. Rutherford realized that thorium compounds kept giving off a gas that stayed radioactive for several minutes. He called it emanation, from the Latin emanare, to flow out. That discovery made radon the fifth radioactive element found, after uranium, thorium, radium and polonium. Earlier that same year, Pierre and Marie Curie had observed that gas coming from radium stayed radioactive for a month.

  In 1900, Friedrich Ernst Dorn reported that radium compounds gave off a radioactive gas, which he named radium emanation. In 1901, Rutherford and Harriet Brooks demonstrated that the emanations were radioactive, but they credited the Curies with discovering the element. In 1903, André-Louis Debierne saw similar emanations from actinium. Each gas was treated as its own substance. They would later be known as radon, thoron and actinon.

  The naming was messy. In 1904, someone proposed the shortened names exradio, exthorio and exactinio. Radon, thoron and akton arrived in 1918, followed by radeon, thoreon and actineon in 1919. By 1920 the trio had settled as radon, thoron and actinon. The name radon has no connection to the Austrian mathematician Johann Radon.

  In 1904, Sir William Ramsay noticed that the spectra of the three gases resembled those of argon, krypton and xenon, and that the gases were chemically inert. He suggested that the emanations might hold a new member of the noble-gas family. In 1909, Ramsay and Robert Whytlaw-Gray isolated radon. In 1910, they measured its density, which showed it was the heaviest known gas, and they fixed its place in the periodic table. They also complained, in French, that "L'expression l'émanation du radium est fort incommode," meaning the expression "radium emanation" is very awkward.

  Their proposed replacement was niton, from the Latin nitens, meaning shining, chosen to celebrate the glow the gas produces. In 1912, the International Commission for Atomic Weights accepted it. Then, in 1923, the International Committee for Chemical Elements and the International Union of Pure and Applied Chemistry picked the name of the most stable isotope, radon, for the whole element. Thoron and actinon were later recast as isotopes. That left a lingering confusion, because Dorn had discovered radon the isotope but was not the first to find radon the element. As late as the 1960s, some writers still called the element simply emanation.

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• 3. Heavy and Aloof

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  Radon weighs 9.73 kilograms per cubic meter at standard temperature and pressure. That is about 8 times the density of air at sea level, which is 1.217 kilograms per cubic meter. Among the noble gases it is the densest, and only a few gases beat it at room temperature. It dissolves sparingly in water, though better than the lighter noble gases. It dissolves much more readily in organic liquids.

  Chilled below its boiling point of 211.5 kelvin, liquid radon glows with radioluminescence of varying color. Solid radon, cooled past its freezing point of 202 kelvin, shines blue, then yellow, then red. Because of the hazards, almost nobody ever sees it. Ramsay and Whytlaw-Gray measured a melting temperature in 1909, and it remains the only experimental value. Friedrich Paneth and E. Rabinowitsch questioned it in 1925 because it broke expected periodic trends. Simulations run in 2018 agreed almost exactly with the original result.

  In 1962, chemists obtained the first compound of radon, a fluoride. Radon is a poor subject for experiments, because it is costly and radioactive. As a result, the few compounds reported are all fluorides or oxides. Eight electrons fill its outer shell, which makes it inert to most common reactions, combustion included. Yet radon sits below xenon in the periodic table and has lower electronegativity, so it is more reactive than its lighter neighbor.

  Powerful oxidizing agents such as fluorine turn radon into radon difluoride. That compound breaks back down into its elements above 523 kelvin. Its short half-life and the radioactivity of its compounds have blocked any detailed study. Theory predicts a bond distance of 2.08 ångströms between radon and fluorine. Of the oxides, only the trioxide has been confirmed, and the higher fluorides have been claimed and calculated to be stable but never verified.

  Antimony pentafluoride, together with chlorine trifluoride, has been considered for stripping radon out of uranium mines, because those chemicals form radon-fluorine compounds. A radon atom caged inside a fullerene has even been proposed as a tumor drug. Chemists extrapolating down the noble-gas column expect radon chlorides, RnCl2 and RnCl4, which would be the first chemically stable noble gas chlorides. None has yet been found.

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• 4. Chains of Decay

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  Thirty-nine radioactive isotopes of radon have been characterized, with mass numbers ranging from 193 to 231. None of them is stable. Six, numbered 217 to 222, occur naturally. Only radon-222 lasts long enough, with a half-life of about 3.8 days, to escape the soil and rock where it forms. It is the immediate decay product of radium, which itself descends from uranium-238.

  A half-life of days should make radon one of the rarest elements, and it is. Yet it will remain on Earth for several billion more years. Uranium and thorium are abundant and have half-lives of at least several billion years, and radon appears as one step in their decay chains, so it is constantly replenished. The uranium series yields radon in significant quantities. The thorium series yields only minute amounts. Both chains finish at stable isotopes of lead.

  Thoron, the radon-220 isotope, comes from the most stable thorium isotope and has a half-life of 55.6 seconds. Actinon, radon-219, comes from the most stable actinium isotope and lasts only 3.96 seconds. Both emit alpha radiation. Thoron forms at about the same average rate as radon-222, but its brief life means far less of it exists in the environment.

  The daughters of radon-222 begin with polonium-218, which lasts 3.10 minutes. Next comes lead-214 at 26.8 minutes, then bismuth-214 at 19.9 minutes. Then polonium-214 lasts a mere 0.1643 milliseconds. After that the chain slows to lead-210, with a half-life of 22.3 years, then bismuth-210 at 5.013 days and polonium-210 at 138.376 days, before stable lead ends it. Because the first four daughters vanish so quickly, their disintegrations mirror where the radon began.

  The radon equilibrium factor compares the activity of the short-lived daughters, which cause most of radon's biological effects, with the activity they would show in full equilibrium with their parent. In many homes it is typically 40 percent, which is also the value used in epidemiological studies. Daughters carry an electrostatic charge, so they cling to surfaces and dust, while gaseous radon does not. Air circulation and filtration lower the factor. Airborne particles raise it, and cigarette smoke is one of them.

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• 5. Where It Turns Up

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  Every square mile of surface soil, to a depth of 6 inches, contains about 1 gram of radium. That radium comes in the form of radium-226, found in uranium ores, phosphate rock, shales, and igneous and metamorphic rocks such as granite, gneiss and schist. It appears in lesser amounts even in limestone. An estimated 2.4 billion curies of radon escape from soil worldwide each year, which amounts to some 15.3 kilograms. The radon activity of the whole atmosphere traces back to only a few tens of grams, replaced continually by the decay of radium, thorium and uranium.

  Outdoors, radon ranges from 1 to 100 becquerels per cubic meter, and drops as low as 0.1 above the ocean. In caves, ventilated mines and poorly ventilated houses, it climbs to between 20 and 2,000. Radon thins out quickly with distance from its source. Weather also matters, since a meteorological inversion with little wind lets it build up in the air. Soils rich in granite or shale carry more uranium, though not every granitic region produces high emissions.

  The springs at Boulder, Montana, Misasa, Bad Kreuznach in Germany, and across Japan are rich in radium and emit radon. To count as radon mineral water, a spring must exceed 2 nanocuries per liter. The water at Merano reaches 2 megabecquerels per cubic meter, and at Lurisia in Italy it reaches 4. Groundwater holds more radon than surface water, because open water loses its radon to the air. Inside the unventilated Gastein Healing Gallery, the air averages 43 kilobecquerels per cubic meter, with a peak of 160.

  Iowa has the highest average radon concentrations in the United States. Glaciers there ground up granitic rocks from the Canadian Shield and laid them down as the soil of its rich farmland. Many Iowa cities, Iowa City among them, now require radon-resistant construction in new homes. Another high-radon area runs through the Appalachian Mountains of southeastern Pennsylvania. In Ireland, the second highest readings turned up in office buildings in the town of Mallow, County Cork, which prompted local fears of lung cancer.

  Oil refineries can run into radon as well. It has a pressure and temperature curve similar to propane, and refineries separate petrochemicals by boiling point, so piping that carries freshly separated propane can become contaminated. Radon decays into solid radioisotopes that coat the inside of the pipework. The sulfate scale from an oil well can be rich in radium.

  In 1971, Apollo 15 passed 110 kilometers above the Aristarchus plateau on the Moon and detected a significant rise in alpha particles. They were thought to come from decaying radon-222. Data from the Lunar Prospector alpha particle spectrometer later supported the inference that the gas was present.

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• 6. Miners' Sickness

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  In 1530, Paracelsus described a wasting disease of miners that he called the mala metallorum. Georg Agricola recommended ventilation in mines to avoid what he called the mountain sickness, Bergsucht. In 1879, Harting and Hesse investigated miners from Schneeberg, Germany, and identified the condition as lung cancer. The first major studies linking radon and health came from uranium mining in the Joachimsthal region of Bohemia.

  During the 1940s and 1950s, expensive ventilation was not widely required in mines. Radon exposure was linked to lung cancer among non-smoking miners of uranium and other hard rock in what is now the Czech Republic, and later among miners in the Southwestern United States and South Australia. The danger was known in the early 1950s, yet it stayed poorly managed in many mines until the 1970s. American uranium miners suffered for decades during the early Cold War, and standards did not arrive until 1971. In mines, exposure can reach 1,000,000 becquerels per cubic meter.

  Ventilation has since lowered radon in most mines that still operate. The average annual exposure of uranium miners has fallen to levels similar to what some homes deliver. As the miners' relative risk fell, so did researchers' ability to detect any excess risk in that group. Mines that dig for other materials can be worse. Workers there are not continuously monitored for radiation, and uranium-specific regulations do not apply. A review found the highest radon in non-metal mines, such as phosphorus and salt mines, and old uranium mines without ventilation can still be extremely high.

  Mine waste can also release radon. Uncovered dumps and tailing ponds from uranium processing are rich in radium, and the gas easily reaches neighbors downwind. Covering tailings with soil or clay can reduce the release, though other decay products may then leach into groundwater. The International Agency for Research on Cancer classes radon decay products as carcinogenic to humans.

  Researchers have also theorized a link between radon and leukemia. Support from general-population studies is inconsistent, but a study of uranium miners found a correlation with chronic lymphocytic leukemia. Legal action by miners, millers, transporters and nuclear site workers, and their unions, has produced compensation under programs such as the Radiation Exposure Compensation Act in the United States and a compensation scheme for radiation-linked diseases in the United Kingdom.

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• 7. Cures and Curiosities

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  Early in the 20th century, a small sealed room called a radiotorium was used to expose patients to radon for its supposed medicinal effects. The practice is now described as quackery. Radon's damaging radioactivity can be used to kill cancer cells, but it does not improve the health of healthy ones. Ionizing radiation forms free radicals, which damage cells and raise rates of illness, cancer included.

  Radioactive water baths have been used since 1906 in Jáchymov, in the Czech Republic. Bad Gastein, in Austria, used them even before radon was discovered. Radium-rich springs feed traditional Japanese onsen in Misasa, in Tottori Prefecture. Drinking therapy is offered in Bad Brambach, Germany. Early in the 20th century, water from radon springs was bottled and sold, though by the time it reached buyers it held little or no radon, thanks to the short half-life. Inhalation therapy is carried out at Gasteiner-Heilstollen in Austria and in Poland at Świeradów-Zdrój, Czerniawa-Zdrój, Kowary and Lądek-Zdrój. It also appears at Harghita Băi in Romania and at Boulder, Montana, while radon spas such as Bad Schmiedeberg, Germany, let guests sit for minutes or hours in high-radon air.

  In the United States, entrepreneurs opened former uranium mines to the public and advertised relief from pain, sinus problems, asthma and arthritis. The government banned such advertisements in 1975. Even so, the idea of radiation hormesis, which holds that limited exposure may ease autoimmune diseases such as arthritis, drew visitors to health mines in Basin, Montana, in the late 20th and early 21st centuries. The practice is discouraged because high doses of radiation do well-documented harm.

  Radon also served in real cancer treatment. Gold or glass seeds filled with radon were implanted in brachytherapy. For the gold version, a long tube was filled with radon pumped from a radium source, then crimped and cut into short sections. The gold keeps the radon inside and filters out alpha and beta radiation, while gamma rays escape to kill the diseased tissue. Each seed carried between 0.05 and 5 millicuries. After 11 half-lives, or 42 days, the radon is down to 1/2,048 of its original activity, and lead-210 dominates what remains. In the early 20th century in the United States, gold contaminated with lead-210 entered the jewelry industry, because seeds had been melted down after their radon decayed. Radon seeds were largely replaced by radionuclides made in particle accelerators and nuclear reactors.

  In the 1940s and 1950s, radon from a radium source was used for industrial radiography. Other X-ray sources that appeared after World War II replaced it quickly, at lower cost and hazard. Scientists found other uses for the gas. Because radon rises from soil in amounts that vary with uranium content, outdoor levels can track air masses to a limited degree. Hydrologists use it to study how groundwater interacts with streams, since a high radon level in a river hints at local groundwater inputs. Soil radon has mapped buried geological faults and has seen limited use in prospecting for geothermal gradients.

  Some researchers have studied groundwater radon for earthquake prediction. Increases were noted before the 1966 Tashkent and 1994 Mindoro earthquakes. One hypothesis says new underground cracks let more groundwater circulate and flush radon out. In the 1970s and 1980s, measurements near faults found earthquakes with no radon signal, and radon with no earthquake following, so many dismissed it as unreliable. As of 2009, NASA was investigating it as a possible earthquake precursor.

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• 8. The Basement Problem

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  The basement in Stanley Watras's home registered radon levels above 100,000 becquerels per cubic meter, or 2.7 nanocuries per liter. He was told that living there equaled smoking 135 packs of cigarettes a day, and that he and his family had raised their lung cancer risk by 13 or 14 percent. The incident showed that radon in particular dwellings can be orders of magnitude higher than typical. Indoor radon had been documented as early as 1950, and the first indoor survey of decay products was most likely done in Sweden in 1956. After the Pennsylvania case, millions of short-term measurements were taken in American homes. Outside the United States, measurements are typically taken over the long term.

  Typical American homes sit near 50 becquerels per cubic meter, or 1.3 picocuries per liter, and some radon will be found in every building. It mostly enters from the soil through the lowest level touching the ground. Cracks in solid foundations and walls, construction joints, gaps around service pipes, cavities inside walls and the water supply are all entry points. Radon is denser than air, so it collects in basements and crawlspaces. Levels in one place can double or halve within an hour, and one room can differ sharply from the next.

  The United States Environmental Protection Agency, or EPA, calls radon the second most frequent cause of lung cancer after cigarette smoking. It estimates 21,000 lung cancer deaths a year in the United States, about 2,900 of them among people who never smoked. By the EPA's estimates, it is the number one cause among non-smokers. In Europe, 2 percent of all cancers have been attributed to radon. Epidemiologist R. William Field and colleagues found a 50 percent increased lung cancer risk even at the EPA's action level of 4 picocuries per liter. Others dispute low-level effects, and some analyses find elevated risk only above 100 becquerels per cubic meter indoors.

  Smoking changes the picture. In the United States, cigarette smoking is estimated to cause 80 to 90 percent of lung cancers. According to the EPA, about 62 in 1,000 smokers exposed to radon will die of lung cancer, compared with 7 in 1,000 people who never smoked. A 2005 review by Darby and colleagues, covering 13 case studies across Europe, stated: "It is not appropriate to talk simply of a risk from radon in homes. The risk is from smoking, compounded by a synergistic effect of radon for smokers. Without smoking, the effect seems to be so small as to be insignificant." Small-cell lung carcinoma, strongly tied to smoking, carries a higher risk after radon exposure than adenocarcinoma, the type that mainly strikes non-smokers.

  In 2009, the World Health Organization recommended a national reference level of 100 becquerels per cubic meter, and 300 as the highest acceptable. The EPA encourages action at levels as low as 74 becquerels per cubic meter, which is 2 picocuries per liter. The European Union recommends action at 400 for old houses and 200 for new ones. On the 8th of July 2010, the United Kingdom's Health Protection Agency set a Target Level of 100 and kept an Action Level of 200.

  Since nothing can be smelled or seen, only testing reveals the level. A short-term kit includes a collector hung on the lowest habitable floor for two to seven days, then mailed to a laboratory. Long-term kits collect for up to a year or more, and an open-land kit can test ground before construction begins. A first result over 4 picocuries per liter justifies repeating the test. Readings between 4 and 10 call for a long-term test, and readings over 10 call only for another short-term test, so that action is not delayed. The EPA has advised buyers to delay or decline a purchase if the seller has not abated radon to 4 or less.

  The method the EPA says is used most is a vent pipe and fan that pull radon from beneath the house and vent it outdoors. This is called sub-slab depressurization. The EPA does not recommend sealing alone, which has not lowered radon significantly or consistently. Simply exhausting basement air outdoors can backfire by drawing radon into the dwelling. For crawl space homes, a high-density plastic sheet over the earth floor, with a vent pipe and fan beneath it, is the most effective approach. Computer modelling validated against a real building found that depressurization cut indoor radon by 45 to 90 percent relative to a sealed chamber, with smaller buildings gaining the most. Forced-air ventilation can instead create overpressure in the chamber if inlet flow exceeds outlet capacity, which may push radon in and raise indoor levels.

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