LONDON • Tiny particles of air pollution detected in human brains could be a clue to the cause of Alzheimer's disease.

New research suggests 'plaques' - the lumps of protein that clog the brain in Alzheimer's sufferers - contain microscopic metal nanoparticles of magnetite which can be derived from pollution. The research provides the first signs that magnetite can reach the brain from outside sources, rather than from within the body as previously thought.

The research led by scientists at Britain's Lancaster University is published in the Proceedings of the National Academy of Sciences.

They admit that they are still a long way from proving a link between Alzheimer's and pollution. The team examined brain tissue from 37 people aged between three and 92 in Manchester in the UK and in Mexico City.

The BBC said the lead author, Professor Barbara Maher, had previously identified magnetite particles in air samples gathered beside a busy road in the UK and from outside a power station. She suspected that similar particles may be found in the brain samples.

"It's dreadfully shocking. When you study the tissue you see the particles distributed between the cells and when you do a magnetic extraction there are millions of particles, millions in a single gram of brain tissue - that's a million opportunities to do damage," the BBC quoted her as saying.

Magnetite can occur naturally in the brain in tiny quantities but those particles are jagged in shape. The particles in the brains studied by the scientists were more numerous but also smooth and rounded - characteristics that can only be created in the high temperatures of a vehicle engine or braking systems. Their route to the brain is unclear, but may be via the olfactory nerve, the main nerve in the nose.

Prof Maher said for every one natural magnetite particle identified, the researchers found about 100 of the pollution-derived ones.

"This is a discovery finding, and now what should start is a whole new examination of this as a potentially very important environmental risk factor for Alzheimer's disease," Prof Maher told the Guardian.

The particles are less than 200 nanometres in diameter, compared with a human hair's thickness of 50,000 nanometres or more.

Prof David Allsop, a co-author of the study, told the BBC it is already known that oxidative damage contributes to brain damage in Alzheimer's patients. "So if you've got iron in the brain, it's very likely to do some damage. It can't be benign."

Other experts are more cautious.

Dr Clare Walton, research manager at the UK's Alzheimer's Society, said there was no strong evidence that magnetite causes Alzheimer's or makes it worse, despite evidence that it can get into the brain from air pollution.