At least that is clear from the evidence presented in a study conducted on brain tissue samples.
Researchers at the University of Lancaster, England, discovered that small metal particles that are released from combustion exhaust gases can enter the nose and travel to the human brain.
Once there, the scientists suggest, they can cause damage to the brain and contribute, for example, to Alzheimer's disease.
The finding, the researchers say, raises a new set of questions about the health risks of environmental pollution.
Several studies in the past have focused on the impact of polluted air on the lungs and heart.
But this is the first time that research has focused on the effect on the brain.
For the study, published in the Proceedings of the National Academy of Science (PNAS), the scientists analyzed brain tissue samples from 37 people: 29 of them, ages 3 to 85, had lived and died in Mexico City, a notoriously polluted area.
The other eight people had lived in Manchester, England, were between the ages of 62 and 92, and some had died from neurodegenerative diseases of varying degrees of severity.
It was already known that iron nanoparticles can be present in the brain, but it is generally assumed that they come from the mineral that is found naturally in our body and is derived from food.
But what the researchers now found are particles of another type of mineral, magnetite.
Professor Barbara Maher, lead author of the study, had already identified magnetite particles in air samples collected along a busy street in Lancaster and in front of a power plant.
He suspected that these same particles could be found in the cerberus samples. And that was what he discovered.
"It was very shocking," the scientist told the BBC.
"When we studied the tissue we saw the particles distributed among the cells and when we did an extraction of the magnetite there were millions of particles, millions in a single gram of brain tissue."
"Those are millions of opportunities to cause harm," he says.
To verify that the nanoparticles came from the combustion exhaust gases, the researchers analyzed the shape of the magnetite.
This mineral can also be present in the brain naturally, but in very small amounts, and it has a distinctively jagged shape.
The nanoparticles found in the study, however, were not only more numerous, but also smooth and round.
According to the researchers, these are characteristics that can only be created in the high temperatures of a vehicle's engine or brake systems.
"They are spherical shapes and have small crystallites around their surface, they appear together with other metals, such as platinum, that arise from catalytic converters," explains Professor Maher.
"It is the first time that we see these particles of contamination inside the human brain. It is a finding that raises a whole new area of investigation to understand whether these magnetite particles are causing or accelerating neurodegenerative diseases," he adds.
The study did not show conclusive results in this regard.
The brains of Manchester donors, especially those who had died of neurodegenerative disorders, had elevated levels of magnetite.
Similar or higher levels were found in the victims of Mexico City.
The highest level of magnetite was discovered in a 32-year-old Mexican man who died in a traffic accident.
Large particles discarded by pollution, such as soot, can get trapped inside the nose. Smaller ones can enter the lungs, and smaller ones can reach the bloodstream.
But the nanoparticles of magnetite are thought to be so tiny that they can pass from the nose and the olfactory bulb into the nervous system and into the frontal cortex of the brain.
Some experts believe that this could be a "significant risk" for developing neurodegenerative diseases, such as Alzheimer's, but for now, they stress, there is no proven link.
"This study provides compelling evidence that magnetite from environmental pollution can enter the brain, but it does not tell us what effect this has on the health of our brain or disorders such as Alzheimer's disease," says Dr. Clare Walton, of the Alzheimer's Society organization.
"The causes of dementia are complex and so far there have not been enough studies to show whether living in cities and polluted areas increases the risk of dementia."
"More research is needed in this regard," says the expert.
Professor Barbara Maher - who led the Lancaster study - says her find has forced her to make lifestyle changes to avoid contamination as much as possible.
"Because magnetite is so toxic to the brain, it has made me see the atmosphere that I breathe differently," the researcher told New Scientist magazine.
"If I walk on a busy street I get as far away from the edge of the platform as I can."
"If I walk a sloping street, I cross to the side where the traffic is going down." Vehicles going uphill generate more particulate matter.
"If I am driving, I never stop right behind a car. In heavy traffic the best option is to have an air conditioning in recirculating mode. And I always choose my route to travel through alternative streets," says the researcher.