Radon What is the problem?

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Author: GEOFF CAMPLIN
Date: Feb. 2000
From: Catalyst(Vol. 10, Issue 3)
Publisher: Philip Allan Updates
Document Type: Brief article
Length: 1,026 words

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Fifteen years ago, Stan Watras, a construction engineer from Pennsylvania, USA, made a disturbing discovery. He was working on the site of a new nuclear power station. The station was still under construction, but some radioactive material was on site, so there were detectors in place to check the workers before they went home. On several occasions Stan set off the detectors and no-one could understand how he was becoming contaminated. One day he decided to check himself as soon as he arrived at the reactor site. He set off the alarm! Obviously, the contamination was not from the site.

* To his surprise, it was discovered that Stan was becoming contaminated at home by high levels of a radioactive gas called radon. Tests revealed that he and his family were exposed to radon levels thousands of times the average level found in houses in the UK. He and his family were strongly advised to leave their home, otherwise their risk of lung cancer would be equal to smoking 135 packs of cigarettes a day!

This discovery sparked off important investigations. We now have a much better understanding of how radon enters the atmosphere and accumulates in buildings, as well as the harmful effects it may produce.

WHAT IS RADON?

Radon appears in the noble gases column (Group 0) of the periodic table. Like the other noble gases, it is a chemically inert gas which you cannot see or smell. However, it is radioactive. Like some other gases, radon is soluble in water, and it is present in underground water and in small quantities in our tap water.

The three naturally-occurring radioactive isotopes of radon were discovered about 100 years ago (see Table 1). The gas remained a chemical curiosity for decades. Many people believed it was beneficial to human health and there are still spas where you can spend your days in old mine tunnels, breathing in radon gas! In the 1950s and 1960s during the uranium prospecting boom, radon was used as a pathfinder to discover new uranium deposits.

WHERE DOES RADON COME FROM?

Radon, thoron and actinon are produced naturally from the radioactive decay of thorium and uranium. Thoron and actinon decay very rapidly (they have short half-lives), so only radon stays around long enough to be a problem. The uranium from which radon comes is present in the soil and bedrock of the Earth's crust, which was formed millions of years ago.

Figure 2 shows how uranium decays to become thorium, protactinium, radium and so on. Roughly in the middle of the chain is radium, [Ra.sup.226] which decays to form radon, [Rn.sup.222]. The average concentration of radium is a few parts per million throughout the Earth's crust, so radon is being produced almost everywhere. The more radium there is below ground, the more radon is produced.

Because radon is a gas, it spreads out and some eventually reaches the surface of the Earth. How much radon reaches the surface depends on the nature of the rock it has to pass through; if the rock is permeable to water, radon may be carried away by underground water. In this way it can migrate underground to places away from the source. Even the daily and monthly changes in rainfall and air temperature and pressure can affect the amount of radon entering the atmosphere.

The mechanism of the release of radon from rocks is quite different from that of other gases. Those gases which are chemically active, such as sulphur dioxide, are released by chemical reactions. However, radon is chemically inert and it is released simply by physical means: it diffuses gradually through the pores in rocks, or between grains of sand and gravel.

WHAT IS THE PROBLEM WITH RADON?

Evidence suggests that radon increases the risk of lung cancer. It may also be linked to leukaemia in adults and children, and cancer of the prostate gland in men. There is continued research into how these links can be explained. However, one obvious route for radon to enter the body is through the lungs. Radon atoms are present in the atmosphere, moving around, colliding with molecules of oxygen, nitrogen or carbon dioxide. We breathe radon atoms in and our with the air, so we have a radioactive gas inside us! A radon atom decays by emitting an alpha particle. This can cause damage to the DNA of the tissue cells, which then divide abnormally and a tumour (cancer) develops.

The other product of this decay is an atom of polonium, [Po.sup.218], which itself is hazardous. This is because polonium is a radioactive solid and can attach itself to particles in the air and the tissue of the lungs. It continues to decay down the chain, ejecting several more alpha particles in the process (Figure 2). Each can cause further cell damage. This explanation sounds very straightforward, but the actual mechanism is complicated and very difficult to work out. Research is still being carried out by institutes such as the National Radiological Protection Board, the University of Bristol, the Leukaemia Research Fund and The Pennsylvania Academy of Science. The main method of minimising any risk to our health through radon exposure is to ensure that the radon concentration in our environment is low. In the UK, action is recommended above 200 Bq/[m.sup.3], about 10 times the average level of radon in homes. Above this limit, action is taken to reduce radon to the lowest level that is practicably possi ble. Scientists have developed methods for measuring the concentration of radon and its daughters, and of reducing radon levels so that people can be protected against high levels of radon in their homes.

Geoff Camplin teaches physics and has a longstanding research interest in radon and its effects.

Table 1

The three naturally-occurring isotopes of radon


Isotope  Symbol        Discovery          Half-life

Thoron   [Rn.sup.220]  Soddy and          55 seconds
                        Rutherford, 1900
Radon    [Rn.sup.222]  Dorn, 1901         3.8 days
Actinon  [Rn.sup.219]  Geisel, 1902       4 seconds



The name radon was introduced in 1918 by Schmidt.

BOX 1

You can measure the radon level in your home very easily. There is no risk of you becoming radioactive or harmed in anyway by extra radiation; the radioactive gas is already in the air! All you need is a clean yoghurt pot, some blu-tack, clingfilm and a plastic detector, known as TASTRAK. Many other school pupils have successfully measured their radon levels at home; so can you.

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Gale Document Number: GALE|A79381426