The belief that low dose radiation is harmless to humans unless they receive an amount above a "threshold" dose is being debunked by new research, writes Aisling Irwin.

The findings point to a new mechanism by which alpha radiation could cause damage to cells, which could throw light on whether radon or polonium (from car exhausts) are a cause of leukaemia.

At present it is believed that radon, much of which comes from natural sources, will cause lung cancer, but that the doses to the rest of the body are so low that it cannot cause any other cancer.

Eric Wright, head of experimental haematology in the Medical Research Council Radiobiology Unit, now has "very firm" evidence to support his early hypothesis that it only takes one alpha particle to trigger the changes in a cell that can lead to cancer.

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The likelihood of the particle hitting home and causing this instability is extremely low - but it is possible.

The conventional belief is that when radiation damages a cell it is either repaired by the body or it remains damaged, in which case its daughters all have the same abnormality. For cancer to occur, a whole string of abnormalities, or mutations, needs to occur. It is therefore believed that a string of exposures to cancer-causing sources, such as radiation or chemicals, is necessary.

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"We're saying there's a very real possibility that a single hit on an appropriate cell with an appropriate genotype can cause all the potential changes you need for disease to develop," Professor Wright told a meeting to highlight the Rontgen Centenary Congress in Birmingham this week.

This initial "hit" causes "genomic instability", a general instability in the genes which is passed down through the generations of cells so that, perhaps several generations later, a new abnormality appears in a daughter cell.

For the relevant damage to occur, radiation must hit stem cells in the bone marrow, which are a repository from which new bone is made.