Scientists race to study microplastic pollution in the atmosphere

In 2017, biogeochemist Janice Brahney was collecting dust deposited in remote wilderness areas in the western US. She wanted to study how phosphorus that was transported through the air to these wild places might disrupt their ecosystems.

But Brahney’s samples contained more than the soil particles she was expecting. Under the microscope were “enormous amounts of plastic,” the Utah State University researcher says. There were fibers, spheres, and chunks of the stuff, in all different colors.

Plastic is ubiquitous wherever people go, but Brahney’s study areas—including remote parts of the High Uintas Wilderness in Utah and Rocky Mountain National Park—don’t get a lot of human traffic. She was determined to figure out how microplastics could have gotten there. She didn’t have funding to work on the problem, so she investigated this mystery in her spare time, spending years of evenings and weekends cataloging the plastic bits.

Her detailed investigation led Brahney to conclude that the microplastics were being transported in the atmosphere and then falling to the ground with snow, rain, or dusty winds (Science 2020, DOI: 10.1126/science.aaz5819). And she wasn’t the only scientist arriving at the conclusion that microplastics could travel long distances through the air. Two papers published in 2019 suggested that tiny plastic particles and fibers found in remote parts of the French Pyrenees and in snow from the Fram Strait (which lies between Greenland and Svalbard) had been transported there from urban areas via the atmosphere.

This field of atmospheric microplastics is only a few years old, but work so far suggests that the pollution has been traveling around the world in the air for decades. And scientists think that these particles could have grave consequences for human and environmental health and the climate.

But currently the field has more questions than answers. Researchers are racing to understand the problem, but analytical techniques used to study atmospheric microplastics are laborious and time consuming. Given the potential implications of this pollution, scientists are determined to improve these methods and make better measurements of how much plastic is in the atmosphere, what it’s made of, and how it moves around the globe.

Something in the air

Fragments of plastic 5 mm and smaller have been found just about everywhere scientists look for them. Some of this pollution starts as small particles, such as exfoliating beads in cosmetics or fibers expelled from clothes dryers’ exhaust vents. Some of these particles are formed when larger pieces of plastic trash age, fragmenting into ever-smaller pieces. During the past decade, environmental scientists have focused on plastic pollution in water, measuring plastic fragments found in the ocean, rivers, and lakes, as well as the stomachs of fish and other creatures.

When Brahney began her studies in the western US, there was some evidence that microplastics were in the air in densely populated urban areas such as Paris. But there were no publications demonstrating that this pollution could travel the planet’s atmosphere.

To determine if the plastic she saw in her samples traveled through the atmosphere, Brahney needed to do more research. She contacted Cornell University atmospheric scientist Natalie Mahowald and asked her if she had any interest in modeling the transport of atmospheric microplastics. “I was like, ‘What?’ ” Mahowald recalls. She’d never considered that plastics might travel in the atmosphere and was surprised by this idea, but she was excited to have the chance to work on something totally new.

Mahowald has expertise in modeling how dust, aerosols, and gases move around the atmosphere. These models take what scientists know about the way air moves in the atmosphere and the weather at any given time to predict how a particle of a particular shape, density, and size will move around Earth. With enough information, researchers can calculate the path taken by a plastic microparticle that landed on a particular mountain on a particular day during a rainstorm—for example, determining if it came from the ocean or from an urban area.

But Mahowald quickly found that microplastics present unique modeling challenges. “They’re a really funny shape,” Mahowald says. The physics of atmospheric modeling are much easier when chemists assume everything is a sphere—which works pretty well when studying, for example, sulfate droplets. “Plastic microfibers can be a micrometer thick and 200 µm long—that is so not a sphere,” Mahowald says. “It’s bent in a funny shape and has hairs coming out of it.” Mahowald and her group did their best and assumed an oblong shape for the particles.

The model suggested that about 84% of the plastic Brahney found in her initial study came from roadways. As vehicles drive, their rolling tires fling plastic dust up into the air, where it can ride air currents. The wear of tires and brake pads also generates new airborne microplastics. The other main sources were the oceans and agricultural soil dust (Proc. Natl. Acad. Sci. U.S.A. 2021, DOI: 10.1073/pnas.2020719118).

Other researchers have shown that the aerosols generated by breaking waves can contain microplastics. While the oceans have often been seen as a final destination for plastic, Brahney and Mahowald’s modeling work suggests that most of the world’s oceans are actually net sources for atmospheric plastics.

Scientists started finding evidence for the atmospheric transport of microplastics just in the past 3 years, but some studies suggest that this pollution has been circling the globe for decades. In one study, researchers looked at ombrotrophic peat—a plant that is “cloud fed,” meaning it gets nutrients and water from the air. These plants are extremely isolated and do not receive nutrients or pollution from runoff, erosion, the subsurface, or other sources. So any pollution deposited in this peat would have to come from the atmosphere. The team analyzed the peat, which grows in layers over time, forming a sedimentary record of atmospheric pollution over time. They found microplastic pollution dating back to the 1960s (Environ. Sci. Technol. Lett. 2021, DOI: 10.1021/acs.estlett.1c00697).

Particle problems

Now that scientists have recognized that there’s plastic in the atmosphere, one overarching question is how it affects the environment, climate, and human health.

Mahowald says one reason to be concerned about atmospheric microplastics is their potential to influence cloud formation. Tiny plastic particles might seed the formation of ice clouds, which have a strong although not fully understood influence on the weather and climate. Cirrus clouds—long, wispy formations also called mare’s tails—are some of the most common and are thought to have a warming effect. If increasing levels of atmospheric microplastics are causing the creation of more ice clouds, they could be contributing to global warming. To better understand this risk, Mahowald says, scientists need to study microplastics’ ability to act as cloud-condensation nuclei in the lab.

Tiny plastic particles could also directly affect how much of the sun’s radiation gets trapped or reflected by Earth’s atmosphere. Laura Revell, an environmental physicist at the University of Canterbury, has modeled this effect and found that atmospheric microplastics could be slightly warming or cooling the planet, depending on how high up in the atmosphere they are (Nature 2021, DOI: 10.1038/s41586-021-03864-x). The temperature of the atmosphere varies at different elevations, and this variation affects microplastics’ optical properties and how much radiation the particles absorb or reflect. Revell’s study concludes that microplastics could have a small cooling effect if they stay below the boundary layer, the level of the troposphere that’s most affected by activity on Earth’s surface. But a significant presence of microplastics above the boundary layer may add to the greenhouse effect. Overall, though, the study suggests that the planetary warming or cooling effect of atmospheric microplastics would be very small.

Revell points out that the field still needs more data on atmospheric microplastics to better understand their potential role in global warming. Her group had to make many assumptions about the quantity and composition of microplastics in the atmosphere. The researchers also assumed all the microplastics were clear, which they know isn’t the case, because they lacked data about the color distribution of these particles.

Researchers say their biggest concerns are microplastics’ potential impacts on ecosystems and human health. So far, scientists haven’t done much study of the health effects of inhaling particulates made of plastic. People can inhale particulates in the micrometer size range, and those particles can then enter the bloodstream and travel to other organs in the body. Epidemiological studies have shown that exposure to such particulate matter is associated with a range of health effects, including acute and chronic respiratory problems, as well as premature death. But most of these studies have not looked at the effects of different particle composition, meaning it’s not clear if plastic particles pose any higher or lower health risks than other particulate matter.

Autopsies have revealed plastic fragments in lung tissue. One study showed that 13 out of 20 human lung tissue samples obtained during autopsies contained microplastics, including polyethylene and polypropylene particles and fibers (J. Hazard. Mater. 2021, DOI: 10.1016/j.jhazmat.2021.126124). In lab studies, human lung cells exposed to polystyrene particles 1 and 10 µm in diameter didn’t die. But they did proliferate more slowly, their metabolic activity decreased, and their cell skeletons had a marked change in shape—all signs of potential toxicity (Chem. Res. Toxicol. 2021, DOI: 10.1021/acs.chemrestox.0c00486).

Further, there’s evidence from studies of aquatic microplastics that these particles, which are oily, tend to adsorb persistent organic pollutants, which may pose additional health risks. Researchers are quick to note that this effect has not been studied yet in airborne microplastics.

“We need to be really careful not to say microplastics are doing something we can’t prove,” says Deonie Allen, an environmental engineer at the University of Strathclyde. But, Allen adds, in the meantime, we should all apply the precautionary principle, and take potential harms seriously.

“Until we have a really good picture of the atmospheric distribution and burden of microplastics, it’s hard to inform risks to health, environment, and climate,” Revell says.

Starting from scratch

To fill in that picture, researchers need a lot more data about the amount of microplastics in the atmosphere and what they’re made of. But researchers have only just started looking for atmospheric microplastics, and the particles are fiendishly difficult to analyze. “One day we will look back at the early days of microplastics research and think, ‘They had it so hard,’ ” Revell says.

“There’s no standardized sampling protocol. Everyone’s just having to write the book as they go along,” Revell says.

Researchers can sample airborne plastic by using a vacuum to suck it through filters. Some researchers are interested in how much plastic gets deposited at a particular spot, so they set up collection equipment that they periodically check. This equipment can be as simple as a funnel over a jar or a set of filters stacked over a container. To see what’s in a sample, researchers typically use methods that separate materials by their density. High-power microscopes can provide a more detailed view. Most use spectroscopy techniques, like infrared and Raman, to characterize each particle.

Steve Allen, Deonie’s husband and frequent collaborator, says one problem researchers deal with is how sampling methods can damage plastic particles, leading to either undercounting or mischaracterization. He’s currently designing a study to capture plastic from different heights in the atmosphere. The Dalhousie University scientist says his study, set to launch in July, will outfit slow-moving uncrewed aerial vehicles with filters to capture airborne microplastics. If the vehicle flies too fast, airborne plastics might get smashed as they hit the filter, evading capture or changing shape—for example, from a long fiber into an square particle.

But that’s not the only difficulty. “Sampling is complicated because it’s very easy to get contamination,” Deonie Allen says. Humans are like the Peanuts character Pigpen, trailing clouds of microplastics. Plastic fragments can come from our clothes, electronics, and the car used to drive to the study site. Scientists have to be meticulous to ensure the plastic in their samples really comes from atmospheric deposition, not from the scientists themselves.

“If someone walks into the lab with some plastic clothing on, you can see it on your samples,” Deonie Allen says. Conditions in the lab must be highly controlled. The Allens say their work is done in a locked room—letting someone in to collect the trash bins can ruin a day’s work. The scientists use only “incredibly basic equipment” made of glass and metal that can be heated and cleaned in an oven to remove plastic traces. “Everything is covered in aluminum foil to seal it so plastic doesn’t fall on top of it,” she says. The room is ventilated with high-efficiency particulate air filters to remove airborne plastic particles. Allen says her group is not as strict as others; some microplastics researchers won’t let anyone bring a phone or computer into the sample-preparation room.

All these efforts to avoid contamination add up. Depending on the method, it can take an entire day to analyze a single sample, Revell says. Deonie Allen’s group is sitting on 600 samples that will need days of preparation each.

Even after the trouble of collecting samples, it’s hard to identify microplastics. “It’s a man-made particle, and there is nothing like it in the world,” Allen says. Plastics are chemically diverse and contain thousands of additives. “Every one has its own densities, sizes, charges, shapes, and additives that make it change as it ages,” she says. An atmospheric chemist looking for sand or sulfate aerosols or black carbon can count on a particular chemical signature. Airborne microplastics come in an incredible variation of species, Allen says.

With current sampling and analysis methods, researchers have mostly found fiber-shaped particles 10 µm or larger. Allen says this result might not reflect what’s actually in the environment but instead be an artifact of scientists’ measurement techniques. As plastic particles age, fibers break into ever-smaller pieces that are spiky, square, or other shapes. It’s possible these smaller, non-fiber-shaped particles may make up a significant mass of atmospheric microplastics but are not being measured because commonly used techniques cannot detect particles smaller than 10 µm. That means scientists may be making the wrong assumptions about how much plastic is in the atmosphere and how it’s moving. Smaller, rounder particles move very differently from long fibers, for example.

Dušan Materić, a researcher at Utrecht University, is interested in understanding the prevalence of smaller particles, in particular nanoplastics, which are smaller than 1 µm. He thinks that ignoring this end of the size scale could lead researchers to misunderstand the scope of the atmospheric plastics problem. And quantifying atmospheric nanoplastics could be important for toxicological studies because smaller particles can more readily penetrate tissues in the body, which makes them more toxic than larger ones.

Materić can quantify these nanoplastics using a method he had originally devised for analyzing dissolved organic matter. When he was using this technique to characterize ice core samples, he saw a lot of plastic fibers. “The plastic was covering up all the other signals,” he says.

He convinced his research adviser to change focus and analyze the plastics in his samples. To characterize a sample, Materić slowly heats it and uses mass spectrometry to identify compounds as they vaporize. He developed an algorithm that can use this information to estimate how much plastic is in the sample. This method can detect nanosized pieces of plastic that are too small to be counted and analyzed by other methods. He recently published a paper demonstrating the presence of several species of nanoplastics in ice cores from Greenland and Antarctica (Environ. Res. 2022, DOI: 10.1016/j.envres.2022.112741).

“Wherever we analyze, we see nanoplastics,” Materić says. “This is global pollution that needs to be acknowledged.”

Besides developing methods, microplastics researchers are coordinating to make sure the data they’re collecting from individual field studies can be reliably pooled. Revell, Materić, and the Allens are about to start a joint project using each of their analytical techniques to analyze the same sample to test whether their data match. “We hope that will lead to a standardized protocol,” Revell says.

Microplastics’ potential impacts on the planet and human health makes this research urgent, and it can be slow-going, hard work, these researchers say. Allen describes the emotional arc of a typical day: “You wake up—existential crisis. By lunch—there’s new science! Maybe if we get information out, it will help!”

And concern about atmospheric microplastics’ effects leads to another question: What can we do about the problem? Nano- and microplastics can’t be re-collected from the air or the water. And even if we stopped using plastics altogether, the legacy pollution would continue cycling from ocean to land to air because it takes a long time for plastic to break down in the environment. Atmospheric microplastics researchers say there’s not much any individual can do, besides try to buy less plastic, and fewer items packaged in plastic. Plastic bag bans help; so do approaches like a recent law in France banning single-use plastic packaging for many fruits and vegetables, Deonie Allen says.

“We have to move away from a throwaway mindset,” Revell says.