Radon: Our Major Source of Radiation Dose Introduction Radon-222 is a natural, radioactive isotope of element number 86, that occurs in the uranium-238 decay chain (see table 1). Its immediate parent is radium-226, and radon-222 itself decays by alpha particle emission through a series of short-lived decay products (mainly isotopes of polonium, lead and bismuth) to lead-210 and on eventually to stable lead-206. Element Radiation Half-Life Uranium-238 alpha 4,460,000,000 years Thorium-234 beta 24.1 days Protactinium-234 beta 1.17 minutes Uranium-234 alpha 247,000 years Thorium-230 alpha 80,000 years Radium-226 alpha 1,602 years Radon-222 alpha 3.82 days Polonium-218 alpha 3.05 minutes Lead-214 beta 27 minutes Bismuth-214 beta 19.7 minutes Polonium-214 alpha 1 microsecond Lead-210 beta 22.3 years Bismuth-210 beta 5.01 days Polonium-210 alpha 138.4 days Lead-206 none stable Table 1. Uranium-238 Decay Chain Two other isotopes of radon occur in nature; radon-220, which occurs in the thorium-232 decay chain (table 2) and radon-219 in the uranium-235 chain (table 3). Both these isotopes have half-lives of under a minute and are less important than radon-222 which is the subject of the rest of this essay.
Element Radiation Half-Life Th-232 alpha 14,000,000,000 years Ra-228 beta 5.76 years Ac-228 beta 6.13 hours Th-228 alpha 1.9 years Ra-224 alpha 3.66 days Rn-220 alpha 55.6 seconds Po-216 alpha 0.145 seconds Pb-212 beta 10.6 hours Bi-212 beta 61 minutes Po-212 alpha 0.3 microseconds Tl-208 beta 3 minutes Pb-208 stable 22.3 years Table 2. Thorium-232 Decay Chain U-235 Alpha 704,000,000 years Th-231 Beta 1 day Pa-231 Alpha 32,500 years Ac-227 Alpha 21.8 years Fr-223 Beta 21.8 minutes Ra-223 Alpha 11.4 days Rn-219 Alpha 4 seconds Po-215 Alpha 1.78 milliseconds Pb-211 Beta 36 minutes Bi-211 Alpha 2.15 minutes Tl-207 Beta 4.77 minutes Pb-207 Stable Table 3. Uranium-235 Decay Chain
Radon is one of the group 0 elements, the noble gases, and is, therefore, chemically virtually inert. However, it has been reported that fluorine reacts with radon, forming a fluoride. On average, one part of radon is present in 1 x 10 21 parts of air. At room temperature radon is a colourless gas; when cooled below its freezing point (-71 C), radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. Liquid radon boils at -61.7 C. It is formed in the environment by the decay of the trace amounts of naturally occurring radium present in all rocks and soils. With a half-life of 3.8 days, it can migrate considerable distances through the ground, and escape into the air. Outside, radon quickly disperses and levels are low - about 4 Bq m -3, but indoors levels are higher - about 20 Bq m -3 on The radioactivity of radon is measured in Becquerels per cubic metre of air (Bq m -3 ). The Becquerel is named after Henri Becquerel (see essay number 1). A Becquerel is one radioactive disintegration per second. average and can reach over 1,000 Bq m -3. When we breathe in air containing radon, and its short-lived decay products, they irradiate our lungs. When radon levels are high this causes a significant increase in the risk of lung cancer. This essay explains the concern about radon, explores the evidence that it causes harm to humans, and provides details of the concerted effort by the authorities in the UK to tackle the existing situation, and to reduce the risk in the future. But first a little bit of radon history. History of Radon Friedrich Dorn, a German scientist, discovered in 1900 that radium was giving off a gas which he called radium emanation. A few years later, in 1908, William Ramsey and R.W. Whytlaw-Gray isolated enough of the gas to study its physical properties. As well as finding it was the densest gas known (9.73
g dm -3 ), they called it niton. In the 1920's, the name radon, symbol Rn, was adopted for all the isotopes of element 86. Friedrich Dorn William Ramsey Though radon was not discovered until 1900, the effects of prolonged exposure to high levels had been noted over 300 years earlier. Two researchers in the first half of the sixteenth century, Georgius Agricola (a German physician and geologist, 1494 to 1555) and Paracelsus (a Swiss physician, alchemist and scientist, 1493 to 1541) studied the diseases of underground miners in Europe. They found that many miners died early because of lung diseases, and concluded that the causes were dust and gases in the mines. Studies in more recent times have shown that high radon levels in mines in many parts of the world are linked to a higher risk of lung cancer. Agricola Paracelsus Concern about Radon Everybody is exposed to radiation from a variety of sources (see essay number 6 for more details). In the UK, 50% of the annual dose to the average person comes from inhalation of radon and its short-lived decay products, so radon is the major source of radiation exposure for almost all the population. In addition, a significant number of people, perhaps a quarter of a million or so, receive annual doses of 10 msv.
Annual dose of radiation in UK (NRPB) It is well known from studies of the survivors of the atomic bombs dropped on Japan, some early medical procedures, and events such as the Chernobyl accident that radiation can cause cancers. Using the data from these studies, it is possible to estimate risk factors for the much smaller environmental radiation exposures we all receive. However, these risk factors are so small it is normally not possible to observe them directly for each source of exposure against the natural background cancer rate. However, the exception is radon as the exposures are often more bigger for some sections of the population. The Uses of Radon Radon is still produced for therapeutic use by a few hospitals by pumping it from a radium source and sealing it in minute tubes, called seeds or needles, for application to patient. This practice has been largely discontinued as hospitals can get the seeds directly from suppliers, who make up the seeds with the desired activity for the day of use. There are still places where bathing in radon laden water is thought to be healthy for the body and soul. One such place is the Rudolf-Stollen mine that uses radon inhalation as a healing tool.
A radon monitoring method is employed in the Chuko fault zone in south central Taiwan for earthquake prediction. Soil gas radon is monitored continuously with a solid-state detector and recorded with a data logger. The detector assembly is housed in a PVC pipe to reduce the influence of environmental factors. The fault zone is known to have deep source gases and sensitive to earthquake activities. The quantities of natural gas releases are known to vary with earthquake activities. Data retrieval from the end of October 2000 to the end of February 2001, showed that spike-like radon anomalies, i.e. rapid increases in the amount of radon, occurred before every major earthquake with a magnitude of more than 4.0 on the Richter scale. The strong correlation between spike-like anomalies and major earthquakes suggests that this might become a method of earthquake prediction. The Evidence for the Risk from Radon The first populations to be studied in detail were groups of underground miners. Some 60,000 miners were involved in 12 main epidemiological studies. The miners worked underground for significant periods between the years 1941 and 1990 in 12 different groups of mines in 8 different countries. Many of the mines produced uranium ore, but others were iron, tin and fluorspar mines. Over 2,600 lung cancers were observed in these miners, which is far more than the 750 predicted on the basis of the number of cancers in the appropriate general population. Subsequently, studies have been carried out to look for a direct link between radon in the home and lung cancer. Two of the biggest were in Sweden and in the Southwest of England. The study in Devon and Cornwall involved 982 individuals with lung cancer and 3,185 matched controls. A number of analyses were carried out and most indicated that higher lung rates were found in those exposed to higher levels of radon. However, in most cases the result did not reach statistical significance so the conclusion was that the overall result was compatible with the results of the analyses of the study of the miners. This indicates that 5% of all lung cancers in the UK are caused by radon.
Another study was performed in Gansu province, China, of people who move house very rarely and suffer high mean radon levels. Also many dwellings are below ground. Measurements were made with two one-year alpha track detectors. The mean radon concentrations were 230 Bq m -3. These are approximately ten times higher than average UK values. While the general picture was of a clear association between domestic radon exposure and lung cancer, one observation was striking. The association seemed stronger in those living in below ground dwellings (439 cases), rather than above ground houses and apartments (329 cases). The study concluded 'that effects of residential radon may equal or exceed miner-based estimates, which are currently used to evaluate risk'. The Mechanism for Harm from Radon Radon is present in air in very small concentrations and it moves in and out of our lungs with the rest of the air in the normal process of breathing. When radon in the air decays into its daughter products, it forms atoms of solid elements which are negatively charged. These anions attach themselves to the small dust particles in the air to form a radioactive aerosol. When we breathe these particles into our lungs, they stick to the lung-lining and are not exhaled. As they are still radioactive, they irradiate the lung tissue. The polonium-214 and -218 emit highly energetic alpha radiation causing damage to the DNA of cells lining the lungs. Most of the damaged cells are killed. However, some cells are partially damaged and get replicated. These cells can induce lung cancer. The Radon Programme in the UK It was not until the latter half of the twentieth century that it was realised that high radon levels in homes were a matter for concern. In the last thirty of so years, much work has been done in the UK by the National Radiological Protection Board (NRPB) with the full support of successive governments. A strategy to control and reduce excessive exposure to radon has been
developed and maps, based on many measurements in homes, show the areas with the greatest risk of high radon levels. Each local council will have such a map for their area. Radon Map of England and Wales from the NRPB
How to Measure Radon There are many different methods of measuring radon levels. The two most important are activated charcoal adsorption and alpha track detection. For activated charcoal adsorption, an airtight container with activated charcoal is opened in the area to be sampled, and radon in the air adsorbs onto the charcoal granules. At the end of the sampling period, the container is sealed and may be sent to a laboratory for analysis. In alpha track detection, the detector is a small piece of special plastic or film inside a small container. Air being tested diffuses through a filter covering a hole in the container. When alpha particles from radon and its decay products strike the detector, they cause damage tracks. At the end of the test the container is sealed and returned to a laboratory for reading. The detector is treated to enhance the damage tracks and then the tracks over a predetermined area are counted using a microscope or optical reader. The number of tracks per area counted is used to calculate the radon concentration of the site tested. Reducing Radon Levels in Existing Houses One or more of the following methods may be used to reduce the radon level in an existing building. A 'sump' and extract pipe can be installed beneath the floor, from outside. A fan can be fitted to draw out the radon and blow it into the atmosphere above the roof of the house. Gaps between the ground floor and walls and gaps around service pipes can be sealed. Positive pressurisation of the house using a fan in the roof-space prevents gas entering, i.e. making the air pressure inside slightly higher than outside. Also natural or forced ventilation of the void under the ground floor will reduce radon levels.
How to Prevent High Levels in New Buildings New buildings are generally protected by full protection to a suspended concrete ground floor. In this the radon-proof barrier is positioned over the floor structure and linked to cavity trays at the edges. Supplementary protection is also provided by locating under floor vents on two or more sides of the under floor space. If necessary the rate of ventilation and radon dispersion can be increased by fitting an electric fan at a later date. The radon barrier comprises of a cavity tray through the wall linked to a membrane across the floor. This is then sealed to a 300 µm polyethene membrane laid across the beam and block floor. To make it easier to seal the two materials the cavity tray is laid so that it laps about 300mm over the edge of the floor. The membrane over the floor can then be sealed to the cavity tray using a double sided butyl jointing strip just prior to installing the floor topping. Airbricks are installed where possible on all sides of the building at intervals at least as frequent as would be normal for an ordinary suspended timber floor.
Web-links Building Research Establishment http://www.bre.co.uk/radon Landauer Incorporated http://www.landauerinc.com/radtrak Cornwall Radon Gas Centre http://www.cornwallradon.co.uk National Radiological Protection Board (NRPB) http://www.nrpb.org DEFRA http://www.defra.gov.uk/environment/radioactivity/radon Health and Safety Executive (HSE) http://www.hse.gov.uk/radiation/ionising/radon Radon Hotline http://www.bradford.ac.uk/acad/envsci/radon_hotline/index.htm Radon Spa at Bad Kreuznach http://www.showcaves.com/english/explain/misc/speleotherapy.html