Metal dental fillings leak mercury if they are exposed to a new powerful type of medical scan, scientists have found.
They studied the effect of ultra-high-strength MRI - the latest in scanning technology being introduced to several research hospitals in the UK.
The study, in the journal Radiology, shows 20 minutes' exposure is enough to release the toxin from extracted teeth filled with silver-coloured amalgam.
The team say more studies are needed to tell what real-life risk it might pose.
These cutting-edge scans are not yet widely used but are helping with medical research.
Although metal fillings are now being superseded by white, ceramic, composite fillings, they are still the most common type offered by NHS dentists.
They are often used on the back teeth because they are hard-wearing, while visible front teeth are usually given white fillings.
The British Dental Association says dental amalgam is safe. It's been in use and extensively studied for 150 years as a restorative material. Its safety and durability are well established. And it remains the most appropriate material for a range of clinical situations.
It says there is no justification for removing these fillings as a precaution, except in those patients properly diagnosed as having allergic reactions to amalgam. This is a rare situation.
But the BDA agrees that more studies are needed to better assess this new potential risk linked with powerful medical scans.
What the study found
Dr Selmi Yilmaz and colleagues at Akdeniz University, Turkey, measured the amount of mercury released by 60 amalgam-filled extracted teeth placed into 60 separate pots of artificial saliva.
Forty of the potted teeth were scanned using either conventional MRI, which is relatively low strength, or the newer high-powered 7 Tesla MRI. A control group of 20 teeth were placed in artificial saliva only.
The mercury content was four times higher in the pots that had been scanned with the high-powered MRI compared with the controls and the pots scanned with conventional MRI.
Whether this high level of released mercury - 0.67 parts per million - might harm patients is not known.
"It is not clear how much of this released mercury is absorbed by the body," Dr Yilmaz said.
Mercury is poisonous to humans and can cause toxic effects in high enough doses.
The European Parliament has voted in favour of a gradual reduction in the use of dental amalgam to protect the environment, rather than for direct health reasons.
When dental amalgam gets into the environment, the mercury it contains can be converted into methylmercury by aquatic microbes. This can then accumulate in the food chain, meaning people who eat contaminated fish and seafood will ingest it.
British Dental Association's scientific adviser Prof Damien Walmsley said: "The study indicates that people who have amalgam fillings should not be concerned if they need to have a conventional MRI scan.
"This will be a decreasing problem in time [as amalgam fillings are phased out] but the development of new ultra-high-strength MRI scanners, which were only approved by the US Food and Drug Administration last year, needs to be reviewed closely.
"The researchers acknowledge that further studies are needed on any potential risk presented by the new generation of MRI scanners."
The NHS advises that routine MRI scans are painless and safe procedures, although there are some patients who they won't be suitable for because the strong magnets used during the scan can affect metal implants or fragments in the body.
Newer powerful MRI scans use stronger magnetic fields to get even more detailed images. They have been approved for use by medical regulators, who take safety into consideration.