RADON IS a colourless, odourless, naturally occurring gas which just happens to be radioactive. It is produced from the decay of a number of chemicals - uranium-238 (which breaks down to produce radium-226, which then decays into radon-222, with a half-life of four days) and thorium, for example.

The trouble with radon is that it can actually make some houses dangerous to live in. Uranium occurs in rocks such as granite, meaning that in places such as Cornwall there is a constant seepage from the ground of the gas. With the trend for modern homes to be snug and draughtproof, that can actually lead to the gas filtering through the foundations and collecting inside - which, without ventilation, can pose a small but real health risk. (You can get rid of radon either by installing a sheet of material under the house to stop the radon getting through, or by installing fans to blow it away. )

If you live in Cornwall, the statistical risk of dying from radon exposure is about 1 in 3,200, though actual cases are difficult to pin down. Certain parts of the county are more hazardous than others, which is why the National Radiological Protection Board (NRPB) has been conducting a wide-ranging survey, spanning the UK, to see whether there is a raised risk from radon exposure.

Why is radon dangerous? When radon-222 decays, it emits an alpha-particle (a high-energy proton), and continues to decay through a series of solid radioactive nuclides until it reaches the stable, non-radioactive lead- 206. Though alpha particles cannot penetrate skin, if emitted inside the body they can eventually cause cancer by damaging DNA. The risk posed by radon is that an atom will lodge in your lung and emit an alpha-particle.

Is that it? Not quite. Earlier this year Bernard Cohen, a radiation physicist at the University of Pittsburgh, who has completed the biggest- ever study into radon - drawing on data from almost half a million homes in the US - suggested that below a certain level of exposure the risk of contracting lung cancer from radon is zero. Others disagree. But if Cohen is correct, it would be a radical finding since it would go against our thinking on how alpha particles, DNA and cancer are intertwined. It might even help property prices in Cornwall.