Microplastics, fragments of plastic debris smaller than 5 millimeters that pollute the environment, have garnered increased media attention in recent months following evidence of the particles in both human blood and in all parts of the human placenta. What is known about how these pollutants could affect the brain?

Scientists have demonstrated neurotoxic effects of microplastics and nanoplastics on marine animals and mice. Studies of microplastics in vitro (in tissue cultures studied in a laboratory) demonstrate that they are toxic to human cells as well.

The magnitude of this toxicity to the human brain depends on whether microplastics can cross the blood-brain barrier, the system of microvessels in the brain with special properties to regulate the flow of solutes from the bloodstream into the central nervous system.

Humans are thought to be exposed to microplastics through daily contact with mucous membranes via inhalation and ingestion. Recent research has determined that not only do orally-administered nanoplastics and microplastic given at doses analogous to human exposure cross the blood-brain barrier, resulting in an inflammatory response, but also impaired learning and memory in rodents.

So far, there is no evidence that microplastics cross the human blood-brain barrier, although many compounds that do cross the mouse blood brain barrier can enter the human brain. As the difference between the human and mouse blood brain barrier continues to be better characterized, scientists researchers will have a better idea of the threats microplastics pose to the human brain.

Longer-term studies of non-human models are needed to determine the effects of lifetime exposure to microplastics. These and more questions about their effects will likely come to light over the next several years as the public becomes more aware of microplastics.

In 2015, the United States banned microbeads in rinse-off cosmetics and non-prescription drugs, and they were officially phased out by 2019. More similar laws may come to pass as the harms of microplastics gain increased public and scientific recognition.