Mounting evidence suggests microplastics in our blood are likely capable of crossing our mammalian brain-blood barrier. Now preliminary experiments reveal the potential impact they can have once in there, finding weathered microplastics are far more toxic to human brain cells than fresh ones.

"The implications of microplastics' harmfulness are particularly alarming, as secondary microplastics exposed in natural environments induce a more severe inflammatory response in the brain," says Daegu Gyeongbuk Institute of Science and Technology (DGIST) biologist Sung-Kyun Choi.

We've utterly surrounded ourselves with plastics. We drape ourselves with them, communicate with them, eat from them, drink out of them, live within coats of them, travel in them and on it goes – amounting to 390 million tons of plastic produced in just 2021 alone. Each of these copious sources shed fragments known as microplastics throughout their useful lives, not just after disposal.

Exposed to elements like rain, wind, and sunlight, these tiny fragments change in shape and structure before finding their way back into living bodies. Long before we are even born we absorb a dust of weathered plastic pieces.

While previous research has tested the effects newly minted plastics have on our brain cells, DGIST biologist Hee-Yeon Kim and colleagues challenged them with weathered particles instead. They took a close look at how our brains' immune cells, microglia, respond to weathered polystyrene-derived microplastics compared with similarly-sized 'virgin' ones.

Feeding weathered microplastics to mice for seven days increased levels of inflammatory particles in their blood. They also experienced increased cell death in their brains. So the researchers then compared weathered polystyrene bits in human microglia grown in the lab.

Making up 10 to 15 percent of brain cells, microglia patrol our central nervous system in search of objects that shouldn't be there. Unsurprisingly, previous research undertaken by the team found microparticles accumulating in mouse microglia.

Kim and colleagues found the weathered microplastics affected proteins involved in breaking down sugars into energy, increasing their expression in the microglial cells 10 to 15 times more than in cells belonging to control groups. They also increased the concentrations of proteins involved in brain cell death by a factor of 5.

The team suspects this may be to do with the changes microplastics encounter once exposed to sunlight. Polystyrene absorbs UV waves, causing the plastic to become more brittle and prone to fragmentation. Kim and team found weathered polystyrene had increased surface area and altered chemical bonds; two properties that that affect their reactivity.

This all amounts to an increased inflammatory response by brain cells – far more severe than what was produced by unweathered microplastics tested at equivalent doses.

"We have, for the first time, identified that plastic leaked into the environment undergoes an accelerated weathering process, transforming into secondary microplastics that can serve as neurotoxic substances, leading to increased inflammation and cell death in the brain," explains Choi.

The results have so far only been observed in living mice and human tissue samples under laboratory conditions, but the fact that these pollutants can make such profound changes once they've reached brain tissue strongly suggests they do impact our brain health.

While the experiments relied on small sample sizes and high microplastic concentrations to account for long term microplastic accumulation, the researchers are now planning longer-term studies with more samples and doses that better reflect environmental conditions over time, to verify their findings.

Their results can't come soon enough as fossil fuel companies have poured billions of dollars into ramping up plastic production even more this decade, in the face of potential reductions of fuel use in response to climate change.

What's more, this explosion of plastic production is being backed by government subsidies using our own taxpayer money.

If our health is at stake as research increasingly suggests, the way we produce, use, and dispose of plastics will need more attention as well.

These findings are published in Environmental Research.