Microplastics

As negotiations continue on a convention to control plastic pollution, there is frightening evidence that we may already be facing serious consequences from the plastic pollution presently in our environment and in our bodies. We have created 9.4 billion tons of plastics since the 1950s, with only 9 percent recycled. Weathering reduces particle size, making it easier for plastic pollution to be take up by aquatic organisms, through plant roots, and inhaled by insects, thus entering food chains.

A recent global survey found microplastics throughout the ocean water column, and down to the Mariana Trench with 13,000 particles per cubic metre. The smallest particles are evenly distributed, making up 5 per cent of the carbon at 2000 metres. Buoyant plastic consumed by plankton could reduce carbon sinking to the bottom and thus CO₂ uptake from the atmosphere.

Such plastic pollution is now shown to penetrate the brain and affect behaviour and cognition in a variety of animals. Hermit crabs exposed to polyethylene microplastics can no longer pick the right-sized shell to move into. Mice ingesting microplastics have difficulty navigating a maze, and cannot learn and remember something they have encountered before. They even become more reckless when faced by predators. A fish fed polyethylene could not follow food odour through a maze. Freshwater shrimp become hyperactive. Microplastics hamper memory in honeybees that forget any lessons they have learned, such as where there is food and how to find their way home. The tiny particles were found imbedded in their brains. In mice, microplastics appear in their brains just 2 hours 20 minutes after being eaten, showing that the blood-brain barrier becomes leaky.

While it is not possible to do controlled experiments in people, the evidence shows that our brains are not immune to plastic pollution. Brains of dead people show high levels nanoplastics, mostly polyethylene, with 50 per cent more in those who died in 2024 compared with 2016, amounting to 7 grams of plastic per brain. Microplastics in the brain are consumed by microglia immune cells, which then swell up and can obstruct blood flow, with potential neurological problems, and causing inflammation.

In zebrafish, such inflammation undermines swimming ability and produces depression-like behaviour. In newborn mice, microglia cannot easily prune connections between neurons, affecting social interactions. Animal studies show microplastics in the brain reducing neurotransmitters, including acetylcholine important for memory and learning, and oxytocin making them less social. Microplastics even in the gut affect the brain, making mice asocial and with problems with short-term memory. Human neurons in cell cultures accumulate plastic particles, becoming damaged and misshapen.

There is some evidence of effects on people. Chinese over 60 with higher exposure to microplastics from plastic tableware and drinking water from plastic bottles had a raised risk of mild cognitive impairment, with a 24 per cent greater risk for those using plastic utensils. People diagnosed with dementia have more microplastics in their brains than others. Mice show symptoms resembling Alzheimer's disease and dementia, being unusually restless and careless for their own safety. Humans may be at more risk as we are exposed to more plastics. To reduce risks, we should avoid undue exposure. Probiotics can reinforce the blood-brain barrier. Mice with memory troubles from polystyrene were cured with vitamin E antioxidant, so eat more almonds, spinach and broccoli.

SOURCES: Based on Marta Zaraska, "Mind-bending material", New Scientist, 10 May 2025, pp. 39-41; and
James Dinneen, "Microplastics found at every depth of the ocean", New Scientist, 10 May 2025, p. 16.