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Your heart and lungs aren't the only organs affected by air pollution. Recent studies have revealed that tiny particulate matter may harm the brain as well, and scientists believe they have now discovered how.

It appears that airborne ultra-fine particles can penetrate the lungs, seep into the circulation, and ultimately infiltrate the brain in mouse models.

It is far more difficult for the immune system to remove the poisons once they are found in neurological tissue. In fact, the scientists discovered that the brain of mice absorbed airborne particles for a longer period of time than any other organ.

The findings imply that if particles are tiny enough, they can bypass the blood brain barrier, a checkpoint that typically prevents toxic solutes and other components of blood from accessing the central nervous system. It is not yet known if the similar routes exist in people.

The new study is one of the first to demonstrate that air pollutants bypass the brain's border guard, even though a leaky blood brain barrier has previously been related to cognitive harm.

Before, experts believed that tiny particles couldn't cross the blood–brain barrier. Instead, it was believed that the particles entered the body by the nose or the gut's nerve cells, which are directly linked to the central nervous system, and then entered the brain.

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

The cardiovascular system and central nervous system are the two systems that are affected by air pollution more so than the former.

Recent studies, however, have found a link between neuroinflammation and cognitive loss, even in young adults, and chronic exposure to air pollution in big cities. The damage has some uncanny similarities to Alzheimer's disease, which is also associated with a leaky blood brain barrier.

Researchers examined the cerebrospinal fluid of 25 persons who had been exposed to chronic air pollution, and they discovered that dangerous air pollutants including iron, calcium, malayaite, and anatase titanium dioxide had been present in the CSF fluid of nearly a third of the participants.

The results imply that the fluid that surrounds our brains is somehow being contaminated by hazardous air.

The researchers used mice to further explore this hypothesis.

The scientists discovered that the poisons entered the mouse brain via circulating blood when black carbon and titanium dioxide particles were delivered into the lungs rather than the nose.

"Strikingly," the scientists write, "the [blood brain barrier] structure was damaged,"  allowing around 20% more leakage. Toxic particles were discovered inside and outside blood vessels close to the blood brain barrier in multiple mouse brain tissue slices, which lends more evidence to the security breach.

The authors also demonstrated direct ultra-fine particle passage across blood brain barrier cells in a petri dish.

The poisons did not appear in the brain tissue of mice that were not exposed to air pollution.

After around a day in infected mice, the scientists observed an exponential reduction in air pollutants in all of the mouse body's organs, although the brain took longer to eliminate the poisons.

"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 researchers conclude.

"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 (link not yet live at time of publication).