Air pollution doesn't just impact the health of your lungs and heart. Recent research has found fine particulate matter can also cause damage to the brain, and scientists think they've finally figured out how.

In mouse models, it appears that ultra-fine particles in the air can enter the lungs, seep into the bloodstream, and ultimately invade the brain.

Once the toxins are present in neurological tissue, they are much harder for the immune system to clear. In fact, the authors found airborne particles were retained in the brain for longer than any other organ in the mouse body.

It's not yet clear if the same pathways exist in humans, but the findings suggest that if particles are small enough, they can slip past the blood brain barrier – a check point that usually stops dangerous solutes and other harmful components in blood from reaching the central nervous system.

A leaky blood brain barrier has been linked to cognitive damage before, but the current study is one of the first to show air pollutants sneaking by the brain's border patrol.

Previously, scientists thought fine particulate matter couldn't make it past the blood brain barrier. Instead, it was thought that the particles made it to the brain via the nose or the nerve cells of the gut, which are directly connected to the central nervous system.

"This work sheds new light on the link between inhaling particles and how they subsequently move around the body," explains environmental nanoscientist Iseult Lynch from the University of Birmingham in the UK.

Today, much more is known about how air pollution impacts the cardiovascular system than the central nervous system.

In recent years, however, chronic exposure to air pollution in major cities has been tied to neuroinflammation and cognitive decline, even in young people. Some of the damage looks eerily similar to Alzheimer's disease, which is also linked to a leaky blood brain barrier.

When analyzing the cerebrospinal fluid of 25 people exposed to chronic air pollution, the researchers found evidence (in about a third of the group) of particles that are hazardous air pollutants, including iron, calcium, malayaite (CaSnSiO5), and anatase titanium dioxide.

The findings suggest toxic air is somehow invading the fluid that bathes our brains.

To test this idea further, the researchers turned to mice.

When black carbon particles and titanium dioxide particles were injected directly into the lungs, bypassing the nose completely, the authors found the toxins invaded the mouse brain via circulating blood.

"Strikingly," the authors note, "the [blood brain barrier] structure was damaged," and this allowed roughly 20 percent greater leakage. In several mouse brain tissue slices, toxic particles were found inside and outside blood vessels near the blood brain barrier, which further supports security breakdown.

In a petri dish, the authors also showed direct translocation of ultra-fine particles through cells that make up the blood brain barrier.

Meanwhile, mice that weren't exposed to air pollutants showed no evidence of the toxins in their brain tissue.

In those that were exposed, the authors noticed an exponential decline in air pollutants from all the organs of the mouse body after about a day, but the brain was slower to excrete the toxins.

"These findings, hence, offer a line of evidence in proving the risks from particulate pollution to the [central nervous system] and in elucidating the exposure route of exogenous particles from inhalation to the brain," the authors write.

"However, more direct proof of the exposure and transport pathways of ambient fine particles from inhalation via the bloodstream and damage of the [blood brain barrier] to the brain is needed, warranting further detailed investigation, including epidemiological studies, in the future."

The study was published in Proceedings of the National Academy of Sciences.