In a new study published Monday in the journal JAMA Network Open, Brazilian scientists found microplastics in the brain tissue of cadavers.
Over the past few years, a growing body of research has found that microplastics are present in nearly every organ in the body, in the bloodstream, and in the plaque that clogs arteries. Whether these ubiquitous pollutants can find their way into the human brain is a major concern for scientists.
The latest research has looked at the part of the brain called the olfactory bulb, which processes information about the sense of smell. Humans have two olfactory bulbs, one above each nostril. The olfactory bulbs are connected to the nasal cavities by the olfactory nerve.
Some researchers are concerned that the olfactory pathway could also be a gateway for microplastics to enter the brain beyond the olfactory bulb.
“Previous studies in humans and animals have shown that air pollution reaches the brain and has found particles in the olfactory bulb, so the olfactory bulb is probably one of the first sites where microplastics reach the brain,” said Dr. Thais Mahuad, Associate Professor of Pathology at the School of Medicine at the University of São Paulo in Brazil and lead author of the study.
Moad and her team took samples of olfactory bulb tissue from 15 corpses, who died between the ages of 33 and 100. Samples from eight of the corpses contained microplastics, tiny pieces of plastic measuring 5.5 micrometres to 26.4 micrometres in size.
The researchers found a total of 16 plastic fibers and particles in the tissue, the smallest of which was thinner than the diameter of a human red blood cell, about 8 micrometers. The most common plastic found was polypropylene, followed by polyamide, nylon and polyethylene vinyl acetate.
“Propylene is everywhere: in furniture, rugs, clothing,” Maouad says, “and we know that the place we’re most exposed to particles is indoors because our homes are full of plastic.”
Matthew Campen, a toxicologist at the University of New Mexico who studies microplastics in the brain, said the presence of microplastics in the olfactory bulb is “unusual but not terribly surprising.”
“The nose is a key defense against particles and dust getting into the lungs,” Kampen said in an email, “so finding plastics in the olfactory system is entirely unexpected, especially given the fact that plastics are found everywhere else in the body.”
Though not mentioned in the study, Campen said he thinks the samples likely also contained large numbers of nanoplastics, between 1 and 1,000 nanometers in size. A strand of human DNA is about 2.5 nanometers thick. (A micrometer is 1,000 times larger than a nanometer.)
The presence of microplastics in the olfactory bulb does not necessarily mean that they are also present in other parts of the brain, such as areas involved in cognition, and it is not yet clear whether these particles can actually reach these parts of the brain via the olfactory bulb.
“There is evidence that very small airborne particles can travel to the brain through the olfactory bulb, but we don’t know if this is the main route of transport of materials to the brain,” Campen said.
The olfactory system is the pathway that connects your nose to your brain. It detects smells by processing tiny odor molecules that waft from different objects, like freshly baked bread or a bouquet of flowers. These molecules stimulate the olfactory nerves, and the signal is processed by the brain as a smell. Other particles can follow the same path. Although rare, amoebas such as Naegleria fowleri (which are larger than the microplastics found in the study) can enter the brain through the olfactory nerves.
“We reasoned that if bacteria can get through this pathway, maybe microplastics can too,” Maouad said.
Campen said nanoplastics likely enter the brain through the bloodstream, which picks up plastic particles from the lungs and digestive tract, rather than through the olfactory bulb. But it’s extremely difficult for even particles found in medicines to get into the brain through the blood, because the brain is surrounded by a semipermeable membrane called the blood-brain barrier. Research on microplastics in the body is still new, and whether these tiny plastic particles can get through the human blood-brain barrier remains a big question.
Most of scientists’ understanding of the potential health hazards posed by the 4,000 chemicals used to make plastic, or the plastic fragments themselves, is limited to animal studies. A recent study showed that exposure to microplastics through drinking water for three weeks caused cognitive changes in the brains of mice. The particles can also cross the blood-brain barrier.
Earlier this year, the first study was published linking the presence of microplastics to higher mortality in people with higher concentrations of microplastics in their arterial plaque.
“We know that when cells are exposed to microplastics in the lab, they produce an inflammatory response,” said Dr. Mary Johnson, an environmental health research scientist at the Harvard School of Public Health, who was not involved in the study.
Johnson noted that neurological diseases, including dementia, have been linked to exposure to air pollution.
“The idea is that microplastics could be one part of the puzzle,” she said. “It’s not just the particles themselves that are of concern, but the fact that plastics contain additives, some of which we know are potentially toxic.”