By Dr David Whitehouse
BBC News Online science editor
Exposure to low-level magnetic fields could be harmful, say US scientists.
Exposed brain cells release damaged DNA
Rats exposed to magnetic fields similar to those humans encounter developed damage to the DNA in their brain cells.
Professor Henry Lai, of Washington University, said that people should be prudent in their use of electrical devices held close to the head.
The peer-reviewed study is published in Environmental Health Perspectives, a journal of the US National Institute of Environmental Sciences.
Duration can be damaging
The researchers discovered that rats exposed to a weak magnetic field oscillating 60 times per second for 24 hours showed DNA damage. Rats exposed for 48 hours showed even more damage.
They also say that the exposure resulted in an increase in brain cell apoptosis or "cell suicide" - a process in which the cell self-destructs because it cannot repair itself.
But what are the implications for people and the magnetic fields most of us encounter in our daily lives?
Speaking to BBC News Online, Professor Lai said that, in his opinion, prolonged exposure to low-level magnetic fields, such as those emitted by hair dryers, electric blankets and razors could damage human brain cells.
Unexposed rats brain cells do not
"We do not use hair dryers or electric razors for more than a few minutes each day. However, the exposure to magnetic fields from these devices, held close to the head, is quite high," he added.
"Our important result is that in rats the harmful effect accumulates over time. The big question is, if we use a hair dryer for five minutes a day, will the harmful effect accumulate in humans? We do not know.
"But our results raise the possibility that it might."
Professor Lai said that people should be cautious and limit their exposure as much as possible.
In the summary of the research paper, he and co-worker Narendra Singh said their work could "...have an important implication on the possible health effects associated with exposure to extremely-low frequency magnetic fields in the public and occupational environments".
A spokesman for the UK's National Radiological Protection Board (NRPB) described Professor Lai's study as "interesting. Work of this sort is noteworthy".
"The research will need to be replicated to see if it is a significant effect. If it led to a real health effect we would have expected to have seen it in behavioural studies of rats exposed to stronger magnetic fields for longer periods. So far we have not.
"What Professor Lai may have seen is a biological effect," the spokesman said. "It remains to be seen if that will be translated into a real health effect."
The NRPB advisory group has already noted a weak association between the incidence of childhood leukaemia and exposure to elevated magnetic fields.
The association is not observed in the UK, but has been seen in the USA and Sweden.
One possible reason for this difference is because the USA uses a 110-volt electricity system, which confers a higher exposure to residual magnetic fields than the 240-volt system used in the UK and on mainland Europe.