In recent years, researchers have found microplastics in nearly every part of the body, as well as in blood vessels and atherosclerosis. But whether microplastics can reach the human brain is a major concern for scientists. The latest study looked at a part of the brain called the olfactory bulb that processes information about smell. Humans have two olfactory bulbs located above each nasal cavity. Connecting the bulb to the nasal cavity is the olfactory nerve, according to NBC
Some researchers worry that the olfactory pathway may be the entry point for microplastics to reach the brain, bypassing the olfactory bulb. “Previous studies in humans and animals have shown that air pollution reaches the brain and that pollutant particles are found in the olfactory bulb. That's why we think the olfactory bulb could be one of the first points for microplastics to reach the brain,” said lead researcher Dr. Thais Mauad, an associate professor of pathology at the São Paulo Medical School in Brazil.
Mauad and his colleagues took olfactory tissue samples from 15 cadavers of people who died between the ages of 33 and 100. Samples from eight of the cadavers contained microplastics, tiny pieces of plastic ranging in size from 5.5 to 26.4 micrometers. In total, the team found 16 plastic fibers and particles in the tissue samples. The smallest was thinner than the diameter of a human red blood cell (about 8 micrometers). The most common plastic they found was polypropylene, followed by polyamide, nylon, and polyethylene vinyl acetate. “Propylene is everywhere, in household items, carpets, clothing,” Mauad said. “The place where we are most exposed to plastic particles is in our homes, because our homes are full of plastic.”
Matthew Campen, a toxicologist at the University of New Mexico, said the presence of microplastics in the olfactory bulb was notable but not entirely surprising. “The nose is a critical point of defense against particles and dust entering the lungs, so finding plastics in the olfactory system was completely expected, especially given their presence everywhere else in the body,” Campen said.
According to Campen, the sample also likely contained nanoplastics, which are 1 to 1,000 nanometers in size (1 cm = 10,000,000 nm). A strand of human DNA is about 2.5 nm thick. The presence of microplastics in the olfactory bulb does not necessarily mean that they are present elsewhere in the brain, such as areas related to cognition. It is not yet clear whether microplastics can reach those areas via the olfactory bulb.
The olfactory system is the pathway between the nose and the brain. It detects odors by processing tiny odor molecules emitted from various objects, such as toast or a bouquet of flowers. The odor molecules stimulate the olfactory nerve, and the signal is processed in the brain. Other particles may travel a similar route. Although rare, amoebas like Naegleria fowleri, which are larger than the microplastics found in the study, could enter the brain through the olfactory nerve. “We think if bacteria can go through this route, so can microplastics,” Mauad said.
According to Campen, nanoplastics are more likely to enter the brain through the bloodstream. The bloodstream collects microplastics from the lungs or digestive tract rather than the olfactory bulb. However, it is extremely difficult for particles, even those in pharmaceuticals, to enter the brain through the bloodstream. That's because the brain is surrounded by a semipermeable membrane called the blood-brain barrier.
Research on microplastics in the body is still new. Much of what scientists know, such as how the 4,000 chemicals used to make plastics break down, is limited to animal studies. A recent study found that exposure to microplastics through drinking water for three weeks caused cognitive changes in the brains of mice.