Experts have discovered the presence of the tiny particles, previously associated with the development of neurodegenerative diseases.

Earlier this year the World Health Organisation warned that air pollution led to up to three million premature deaths every year.

The shape of the particles suggest they are less likely to have formed naturally in the body – but rather come from an outside source.

Researchers in the UK and Mexico say they cannot say for sure whether there is a causal link between these nanoparticles and Alzheimer’s.

But they have described the findings as “dreadfully shocking”.

Dr David Reynolds, chief scientific officer at Alzheimer’s Research UK, said: “Little is known about the role of magnetite nanoparticles in the brain and whether their magnetic properties influence brain function.

“It’s interesting to see further research investigating the presence of this mineral in the brain, but it’s too early to conclude that it may have a causal role in Alzheimer’s disease or any other brain disease.

“We know that air pollution can have a negative impact on certain aspects of human health, but we can’t conclude from this study that magnetite nanoparticles carried in air pollution are harmful to brain health.”

He highlighted the fact that age and genetic risk factors also play a role in influencing a person’s risk of Alzheimer’s.

Alzheimer’s disease is caused by a build-up of proteins in the brain.

The build-up leads to the loss of brain tissue, the death of nerve cells and a shortage of important brain chemicals.

It is a common cause of dementia and affects more than 520,000 people in the UK, the Alzheimer’s Society website says.

The findings are published in the journal Proceedings of the National Academy of Sciences.