In recent months, the Observer has reported on how electronic stimulants, from iPads to Pokémon Go, may pose a higher risk of Alzheimer’s disease.

But what if an airborne phenomena was also to blame?

A study published this week in the Proceedings of the National Academy of Sciences found that the particulate matter in the air around many urban areas doesn’t just harm the environment—it may also adversely affect the brain once people breathe it in.

Dr. Barbara Maher, a professor of environmental and earth science at the University of Lancaster in England whose current research interests include magnetic minerals in the environment, led the study. She focused specifically on magnetite, a mineral which she said is “ubiquitous and abundant” in cities—there are several deposits in the Adirondacks.

The iron compound is formed during coal combustion—when a car engine turns on or a fire breaks out, molten magnetite droplets develop. The spherical particle quickly cools and condenses when released into the air—indeed, magnetite matter is so small that people can breathe it in through the nose without realizing it (even wearing a mask won’t help).

This is particularly concerning because the path from the nose to the brain is not protected by the blood-brain barrier, which normally prevents foreign particles from reaching the cerebral cortex.

Another worrisome factor is that magnetite particles are also found in amyloid, a protein which forms plaque deposits in the brain and contributes to the development of Alzheimer’s disease.

With all this in mind, Maher’s team set to work, taking postmortem brain tissue samples from 37 people in Manchester, England and pollution hotspot Mexico City (where Maher’s colleagues are based). To ensure a truly blind study, the researchers didn’t get diagnostic information (age, cause of death, etc) about their subjects.

The scientists first measured how much magnetite was present in each brain—each sample contained at least trace amounts of the compound. They then chemically analyzed the particles by cutting the brain into thin sections and looking at the pieces under a microscope. Finally, they dissolved the brain tissue and microscopically examined the individual magnetic particles.

The study found that magnetite disrupts normal cellular function, and that it contributes to the creation of unstable “free radical” molecules in the brain, thereby damaging the cerebral cortex.

Dr. Maher could not pinpoint a definitive link between magnetite and Alzheimer’s because not all of the brain samples showed signs of the disease. However, she was able to prove that externally sourced particles can enter the brain, and that magnetite in particular is neurologically toxic.

“The magnetite shouldn’t be there,” Dr. Maher said. “It’s difficult to imagine the particles are just sitting there stably, not doing any damage.”

While she acknowledged that this was a “discovery study” which wasn’t meant to correlate magnetite with increased risk of neurodegenerative disease, Dr. Maher said she is now curious about just that, and wants to examine this possible causal relationship in future studies.

However, she also concluded that there are some things people can do now to prevent exposure to or ingestion of particulates, including walking as far away from the curbside as possible, and avoiding the uphill side of the street, where the vehicles emit more particulates—magnetite tends to congregate in these areas.

“Go through the quiet places and the parks rather than the main roads,” Dr. Maher advised.