Editor's Note: The headline and lede of this story were updated on March 13, 2024 at 1:40 p.m. E.S.T. to remove reference to the total amount of plastic inhaled by humans every week. Researchers originally estimated that humans ingest a credit cards' worth, but this was a miscalculation; it's actually much lower than that.

Humans may be inhaling a staggering amount of toxic microplastic every week, and for the first time scientists have worked out where it ends up in your body.

In 2019, a team of scientists estimated that up to 16.2 bits of microplastic enter our airways every hour. Now, researchers have built on these findings to figure out how the plastic moves around our respiratory systems.

Microplastics are tiny chunks of plastic debris measuring less than 0.2 inch (5 millimeters) long, according to the National Oceanic and Atmospheric Administration (NOAA). These broken-down remnants of industrial waste and consumer goods are impossible to avoid; they can be found across the ocean and the atmosphere, inside bottled water and even in human poop.

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There have been few studies into how toxic microplastics impact human health, especially respiratory health. However, recent studies suggest these tiny particles could pose serious health problems. Now, researchers have used a computer model to find the regions of our airways most impacted by breathing in microplastics. The scientists published their findings June 13 in the journal Physics of Fluids.

"For the first time, in 2022, studies found microplastics deep in human airways, which raises the concern of serious respiratory health hazards,” first author Mohammad S. Islam, a senior research fellow at the University of New South Wales, Sydney, said in a statement.

The scientists built a computer model to analyze where the tiny chunks tend to travel inside our airways, and where they get deposited.

By analyzing this circulation under slow- and fast-breathing conditions with three possible plastic shapes (spherical, tetrahedral, and cylindrical), the researchers found that the biggest chunks of microplastic — those measuring about 5.56 microns (one-seventieth the breadth of a human hair) — were the ones most likely to get lodged. The places these larger chunks tended to go were in the upper airways, such as in the nasal cavity and the back of the throat.

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The full health impacts of microplastics on the human body are still unknown. However, microplastics have been shown to kill human cells, and cause bowel inflammation and reductions to fertility in mice.

Microplastics can also carry viruses, bacteria and other hazardous chemicals, which hitchhike on the plastic’s microscopic surfaces.

The researchers say their next steps will be to investigate how the plastics are deposited inside human lungs, taking into account factors such as humidity and temperature. They noted that microplastics are becoming increasingly ubiquitous.

"Millions of tons of these microplastic particles have been found in water, air and soil. Global microplastic production is surging, and the density of microplastics in the air is increasing significantly," Islam said.