Frequent dental X-rays may significantly increase the risk of brain tumours, say researchers.
Over a lifetime, having dental X-rays can double or triple the chances of developing the non-malignant meningioma tumours, a study has found. The tumours grow in the outer membrane covering the brain.
In one case, involving X-rays on children, a five-fold increase in risk was seen.
However, the likelihood of developing a brain tumour at all is very small.
Meningiomas, which account for one in five primary brain tumours, affect two or three in every 100,000 people in Britain each year. The tumours are slow growing, often causing no symptoms, and usually benign.
Scientists in the US looked at the dental histories of 1,433 patients diagnosed with meningioma tumours. They were compared with a "control group" of 1,350 matched individuals who were free of the disease.
The tumour patients were consistently more likely to have been exposed to dental X-rays.
One of the most common procedures is the "bitewing" X-ray which uses an X-ray film clenched between the teeth in a tab of plastic or cardboard.
Bitewing X-rays check for decay between the teeth and can also expose bone loss caused by severe gum disease.
Over a lifetime, patients with meningioma were between 1.4 and 1.9 times more likely than controls to have undergone bitewing X-rays on a yearly or more frequent basis.
An increased risk of meningioma was also linked to "panorex", or "panoramic" X-rays which provide a broad view of the jaws, teeth and nasal area. They reveal problems such as impacted teeth.
Researcher Dr Elizabeth Claus, of Yale University, wrote: "Our findings suggest that dental X-rays, particularly when obtained frequently and at a young age, may be associated with an increased risk of intracranial meningioma."
But cancer expert Dr Paul Pharoah said: "As the disease is rare, the increase in absolute risk is tiny -- the lifetime risk increasing from 15 in every 10,000 people to 22 in 10,000.
"People who have had dental X-rays do not need to worry about the health risks of those X-rays. Nevertheless, dental X-rays should only be used when there is a clear need."