An Ill Wind? Growing Recognition of Airborne Nano- and Microplastic Exposures
Publication: Environmental Health Perspectives
Volume 131, Issue 4
CID: 042001
From the top of Mount Everest1 to the bottom of the deepest ocean trenches,2 from remote wilderness areas3 to the open sea,4 microplastics (MPs) are, quite literally, everywhere. MPs, generally considered to be plastic particles smaller than in diameter5—with even smaller nanoplastics (NPs)—have been found in human lung6 and placental7 tissues, blood,8 feces,9 meconium,10 and breastmilk.10 Researchers have quantified—and raised concerns about—MP ingestion through salt,11 honey,12 bottled and tap water,11 and seafood.13 However, recognition is growing that inhaling airborne MPs may present additional health risks.14–16 “We are drinking, eating, and breathing plastic particles,” says Messika Revel, an associate professor of environmental engineering at UniLaSalle in Rennes, France, who has studied MPs in the marine environment.
A 2019 review of the literature concluded that factoring in inhalation increased estimates of the number of MPs consumed by the U.S. population overall to 74,000–121,000 particles daily, up from 39,000–52,000 when only food sources were included.17 But as more and varied sources of exposure are measured—and as measurement techniques improve—those figures will certainly change.
“You’d actually get more microplastics from the air into the human body” than from table salt and water, says Norah Muisa-Zikali, an environmental science doctoral student at the Chinese University of Hong Kong. She pointed to a 2020 study11 that assessed the human body burden from the three sources. The study’s finding was surprising at the time it came out, because food and water were thought to be the major sources of exposure, she says. That perception has rapidly changed. “Microplastics in the air get into your food, they get into your water, and you can also inhale those plastics,” Muisa-Zikali says, “so we found that air [exposure] is actually worrisome.”
Maxine Yee, an assistant professor of engineering at the University of Nottingham Malaysia who investigates the cellular toxicity of MPs and NPs, agrees. “The inhalation route is most concerning to me,” she says. With ingestion, most MPs are excreted,18 Yee explains, and people with healthy skin have a natural barrier to the entry of MPs—the third major route of exposure. “But in the case of inhalation,” she says, “MPs and NPs have a tendency to deposit along the surfaces of the airways, with limited chances of removal.”
MPs and NPs take a variety of forms: jagged fragments, shreds of film like that used for plastic bags, fibers derived from synthetic textiles, and manufactured spheres or pellets.19 Particles are further categorized as primary, or initially produced at a small size, or secondary, resulting from degradation of larger items.20
Plastic polymers, generally considered chemically inert, can exert biological effects through their physical presence—similar to the case of asbestos, which has shown that particle size and shape alone can trigger biological reactions.21 In addition, MPs may contain or be contaminated with chemicals that are active in biological processes. Plastics may be combined with phthalates, bisphenols, and flame retardants (such as polybrominated diphenyl ethers) during manufacturing; such chemicals show endocrine activity and may have other health impacts.22,23 Post manufacturing, MPs may adsorb microbes,24–28 toxic metals,29–31 polycyclic aromatic hydrocarbons,32 and other chemicals,33 thanks to physical characteristics such as hydrophobicity and their large surface area relative to volume.
“Understanding the health impacts of microplastics is challenging because they can have wildly different sizes, shapes, and chemical formulations—all of which can impact their toxicity,26” says Joel Rindelaub, a chemistry research fellow at the University of Auckland who has studied MPs in New Zealand. There is also evidence that weathered particles behave differently in the body than manufactured ones.34 Add to that the myriad substances that MPs and NPs adsorb, and the importance and the complexity of researching human health effects become clear.
Plastic Rain in Urban Environments
MPs are a relatively new discovery in ambient air, with the first studies on atmospheric fallout emerging in the mid-2010s.35–38 In a 2015 study, scientists from the University of Paris quantified MPs in atmospheric fallout deposited on one urban and one suburban roof in Paris over a 3-month period. Ninety percent of the MPs measured were fibers, and the rest were fragments. The authors found daily concentrations of (the average was 118), with fluxes they hypothesize were influenced by rainfall. The Paris study noted that MP concentrations were higher on the urban vs. the suburban rooftop.35 Another study reported indoor air levels substantially higher than levels measured outdoors, with concentrations between 1.0 and indoors and 0.3 and outdoors.38
Since then, researchers have measured MPs in the air or atmospheric fallout in several cities in Europe, Asia, Australia, and Iran.39 Although a study in China found higher MP levels in urban vs. rural environments,40 the particles have been detected in places as remote as the Tibetan plateau, the French Pyrenées, and wilderness areas in the United States.3,39
In a 2021 study, a team of scientists estimated that people in five Chinese megacities inhale 1–2 million MPs annually, based on their air samples of suspended particulate matter, including MPs.41 Within the five cities, the researchers measured , of which 95% were less than in diameter. In contrast with the Paris study, nearly 90% of MPs measured were fragments; the rest were fibers. Films and spheres were not quantified due to difficulties in distinguishing them from the other particulate matter collected.
The concentration of MPs was significantly higher in the northern cities of Beijing and Tianjin—part of a single urban conglomeration that is known for high levels of particulates—compared with the southern Shanghai–Hangzhou–Nanjing urban conglomeration. However, previous studies have reported three orders of magnitude more MPs in Shanghai than in Beijing.42,43 The authors explained the variance between the studies as likely due to different sampling methods and possibly seasonal variations.
But drawing conclusions from these comparisons is less than straightforward, as Muisa-Zikali points out. “The way airborne microplastics are being analyzed is so different from one study to another,” she says. “We need to standardize the way we sample and where we sample airborne microplastics. If we’re looking at human health, which height is really critical? Should we sample from the rooftop? And if it is on the rooftop, then what height [of building]?”
NPs present additional challenges. “It is very challenging to measure nanoplastics in the environment,” Yee says. “Electron microscopy can detect and differentiate between [NPs and MPs], but it is not able to quantify a specific particle amount per sample volume. Computational models using microplastic data are an alternative way to estimate nanoplastic concentrations in the environment.”
A few researchers have attempted to take such measurements. The authors of one 2022 study measured NPs in ice samples from Greenland and Antarctica using a relatively new method called thermal desorption–proton transfer reaction–mass spectrometry.44 In a different 2022 study, investigators developed another new technique based on the physics of aerosols to measure airborne NPs.45 But as Yee says, “There is much room for further research.”
Mobility Complicates Source Tracking
In the past, technological limitations hampered researchers’ ability to quantify the smallest airborne MPs.46 In a 2022 study,46 Rindelaub and his team sought to overcome those limitations by combining fluorescence microscopy using Nile Red dye—which selectively stains plastic particles47—with the more sophisticated technique of pyrolysis−gas chromatography−mass spectrometry; this approach enabled detection of MPs as small as . The researchers also found exponentially more individual particles as sizes became smaller. Deposition rates from their two sampling locations averaged —equivalent to 74 metric tons of MPs raining down every year48 and an order of magnitude higher than rates found in previous studies of urban areas.35,49,50
Rindelaub and his colleagues observed that atmospheric conditions appeared to influence the abundance and types of particles deposited on any given day. “Heavier winds originating from the ocean resulted in higher numbers of small microplastics,” he explains. “When winds originated from the city, larger microplastics were observed, suggesting that these microplastics had undergone less environmental aging and were closer to their source compared to those coming from the marine environment.”
Recent modeling suggests these tiny plastic particles appear to follow atmospheric cycling processes, with terrestrial, atmospheric, and marine components.33,51,52 In 2022 Laura Revell, an associate professor of environmental physics at the University of Canterbury in New Zealand, published what is thought to be the first observation of MPs in Antarctic snow.53 “We did atmospheric modeling that showed MPs in the Antarctic could have come out of the ocean and been transported over a couple of thousand kilometers, or they could have come from local sources,” she says.
Revell further notes that the wind can transport microplastics from soil and water across continents in a matter of days to weeks.51 Relative to oceanic transport, she says, “The atmosphere is a very efficient way for moving microplastics around, which should not be a surprise.”
“A lot of people think about microplastics being emitted only once into the environment, and then they fall down and they disappear,” says Natalie Mahowald, who is Irving Porter Church Professor in Engineering at Cornell University and codeveloper of an atmospheric transport model using the most extensive data set to date.52 “But our agricultural dust mechanism and our ocean mechanism [show] resuspension.… MPs go up in the atmosphere, redeposit, and become resuspended multiple times as they move around.” As it does so, a single MP particle may break down into billions of NPs.54
Identifying Sources
Experts have begun identifying the contributions of various sources of MPs, including mismanaged waste, incinerators, litter, road wear of tires, agricultural uses of plastic for coverings and mulch, and consumer products.55,56 Textiles also add to the environmental burden. Clothes dryers may emit up to 40 times more MP fibers into the air57 than washing machines emit into wastewater, although the latter has received more public attention.58–60
Kenneth Leung, a professor at the City University of Hong Kong and the chair of its Chemistry Department, started studying MPs released from dryers after finding fibers in his kitchen. “I was drying my clothes in the tumble dryer. The duct disconnected from my window, and when I returned from shopping, I found … these fibers everywhere,” says Leung. Discussions with colleagues led him to design an experiment61 to measure MPs released from dryers in a dormitory far from the urban environment. “Our results showed that the synthetic microfibers from the dryer increased with increasing mass of cloth in the machine,” he says. In contrast, cotton textiles produced roughly the same number of fibers regardless of how much laundry was in the dryer.
New sources of MPs are being identified as well. For example, one of the most common forms of sewer repair in urban areas around the world, cured in-place pipe (CIPP) repair, is a newly recognized—and as yet unmeasured—potential source of NPs. The process releases steam, which, when condensed and analyzed by researchers in a recent laboratory study,62 was found to contain particles that analyses suggested were a mixture of multiple polymers, based on characteristics such as size, shape, viscosity, organic components, and spectral features. The team estimated that each CIPP project may emit more than (500 lb) of aerosolized NPs; yet according to the authors, studies of emissions have so far focused only on gas-phase and water-soluble chemicals, not airborne plastics.
On a global scale, most models postulate that large quantities of MPs derive from the African continent,63 primarily because many African countries do not have adequate solid waste management.63–65 For example, in most parts of Zimbabwe, official dumpsites are not managed well, says Muisa-Zikali, who is from that country. Plastic litter is prone to degradation, she explains, and weather conditions in much of Africa—hot, windy conditions in the desert and rain in tropical areas—are conducive to weathering the exposed trash.
Many other low- and middle-income countries (LMICs) also face serious environmental pollution due to inadequate management of waste and wastewater, although laws and conditions vary widely from country to country,66 and much more research is needed in these areas. Muisa-Zikali coauthored a 2022 book chapter reviewing the effects of NPs and MPs on human health, emphasizing the paucity of studies on exposures in LMICs.16 For example, there were no data from any African country, she says, noting that more sophisticated equipment would help scientists on the continent ascertain the content of samples.
The Question of Human Health Impacts
One potential health risk of airborne MPs appears to involve inhaling particles that have adsorbed microbes and unreacted monomers, additives, dyes, pigments, and other chemicals.67,68 No published studies have yet addressed transfer of MP pollutants to human tissues, but under experimental conditions, ingestion of MPs with adsorbed hydrophobic organic compounds has been shown to transfer these pollutants to tissues of worms69 and mussels.70 Ingestion has also been associated with presence of these pollutants in the tissues of whales and birds.71 Other animal studies have found evidence that MPs may adversely affect the liver,72 kidney,73 cognition,74 and metabolic health.75
Despite such evidence, it is still unclear how MPs ultimately affect humans, says Yee. Past research suggests particles in the lungs causes irritation and inflammation, with potential for secondary genotoxicity and continuous formation of reactive oxygen species.68,76 Decades ago, studies showed that employees who worked with nylon flock—finely chopped fibers used to create plush textiles—developed a unique form of interstitial lung disease77–79 and a condition with features of allergic alveolitis,80 which investigators attributed to the inhalation of microscopic flock fragments. In studies of asbestos, fiber length and biopersistence in the airways and lungs determined its toxicity.81,82 The authors of a 2018 review noted that “the legacy of asbestos toxicology can in part help predict health effects of fibrous MPs.”68
Other researchers have characterized MPs present in human tissues. One study83 of individuals with lung cancer found MP fibers in 97% of malignant tumor samples and 83% of samples collected from adjacent noncancerous tissue. A more recent study observed a dozen different polymer types in 11 of 13 lung tissue samples from individuals who underwent surgery for either cancer or lung volume reduction.6 MPs were detected in samples from all lung regions.
In vitro experiments using synthetic extracellular lung fluid have shown MPs to be extremely durable, with no changes to surface area over a 180-day study period.84 Moreover, clearance of particles can be particularly limited in individuals with compromised lung function.68
NPs may pose unique concerns. “By virtue of its smaller size, a nanoparticle has the ability to travel further into the airways and enter the smaller capillaries in the lungs,” says Yee. Airborne NPs may enter organ tissues, cross the blood–brain barrier, and even penetrate cells.22 Like other nanomaterials, NPs have different chemical and physical characteristics than those of the same material at larger sizes,76 further complicating health effects research.
Yee reviewed the literature on NPs and human health54 and found that most studies assessed how manufactured polystyrene NPs affected common cell lines, such as human lung carcinoma cells. The studies’ findings suggested that, in vitro, the particles initially caused irritation that led to inflammation, but the reactions were size- and context-dependent.54 “Interestingly, larger nanoparticles caused more inflammatory reactions than smaller ones,” she says. “It is unclear whether the observed inflammation is due to the chemical composition of the particle or its physical presence.” Either way, she says, the introduction of the polystyrene nanoparticles caused serious effects in cell studies, but in vivo research is needed to assess potential health implications.
Another in vitro study85 of alveolar cells and two sizes of NPs— and —found that although cells took up the smaller particles more rapidly and efficiently than the larger size, both sizes affected cell viability, activated transcription of inflammatory genes, and changed cell cycle and cell death protein expression.
Next Steps: Research, Standardization, and Clean-up
To learn more about the effects of MPs and NPs on human health, experts have recommended forming an interdisciplinary global long-term observation network to help standardize monitoring and enhance the sharing of information,51 using existing World Meteorological Organization Global Atmosphere Watch stations to promote standardization and ease launching such a project.
“We need more measurements in more places, and then we need to go into the places where we think there are emissions and figure out if they’re actually being emitted,” says Mahowald. “This needs to be tackled from so many different directions. I’ve seen a lot of interest from all different sides, and that’s what we need—all kinds of people coming together.”
Involving all stakeholders in working toward common goals can help. In February 2021, members of academia, industry, government, nongovernmental organizations, and other groups participated in a series of online workshops to define research priorities on airborne MPs and identify knowledge gaps and objectives for the future. They agreed that human and environmental health were the two highest priorities among five fundamental objectives identified; the others were biodiversity preservation, food chain improvement, and functionality, or the role of the plastic used in a process or product.86
Regulation is another issue that nations must grapple with on an international scale, but some researchers, like Revel, think individuals and companies can take responsibility in the meantime. “Regulation is important, but we cannot wait [for it],” says Revel. “We should stop using plastic for food packaging since this is what we find the most in the environment. Fibers from [synthetic] textiles are a big issue as well.” At the individual level, people can use plastics more thoughtfully and, in many cases, choose reusable options.87
In March 2022, the United Nations Environment Assembly committed to draft an international legally binding treaty by 2024 to combat plastic pollution.88 Although the United States89 and Canada90 have banned microbeads in cosmetics, these appear to be relatively minor constituents of most environmental MPs.56 As one pair of researchers stated,91 “[B]ans on primary microplastics are not the route to significant reductions of microplastics in the environment. A major part of the issue can and must be prevented by proper (macro)plastic waste collection.”
“The environment is filled with plastic particles, and even if there is drastic reduction of plastic production,” says Revel, “we would continue to have a diffusion of plastic waste into the environment for several years.” That truth is a sobering reality.
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San Diego–based writer Wendee Nicole contributes regularly to Environmental Health Perspectives.
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Received: 27 December 2022
Accepted: 7 March 2023
Published online: 28 April 2023
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