Microscopic particles, possibly from air pollution, have been found in human brain tissue, according to a new study which flagged an Alzheimer's risk.

The study authors urged further research into any "possible hazard to human health", even as outside experts cautioned it was premature to draw a definitive link between the particles and neurodegenerative disease.

A team of scientists from Britain, Mexico and the United States conducted magnetic tests on frozen brain tissue obtained from 37 people aged three to 92.

They found nanoparticles of magnetite, a form of iron ore, that looked different from those which are formed naturally by the human brain.

Instead, the particles showed "compelling similarity" to particulate matter formed by fuel combustion, found in urban air pollution -- from car exhausts, factory fumes and indoor cooking fires, said the team.

"Previous work has shown a correlation between the amount of brain magnetite and the incidence of Alzheimer's disease," they wrote in the US-based journal Proceedings of the National Academy of Sciences.

And they said that "exposure to such airborne PM-derived magnetite nanoparticles might need to be examined as a possible hazard to human health."

Experts not involved in the study said it did not provide conclusive proof that the particles came from pollution, or that they had anything to do with causing Alzheimer's disease.

"There needs to be a better study carried out" to compare magnetite in the brains of Alzheimer's patients from areas with high and low levels of pollution, said Jennifer Pocock of University College London's Institute of Neurology.

"I don't think we can say yet if it causes Alzheimer's," added Peter Dobson of King's College London.

"But it is cause for concern more generally, because magnetite particles have been linked to other health problems such as cardiovascular disease and pulmonary diseases," Dobson told the Science Media Center in London.