Forgotten but not gone: Particulate matter as contaminations of mucosal systems

Abstract

A decade ago, environmental issues, such as air pollution and the contamination of the oceans with microplastic, were prominently communicated in the media. However, these days, political topics, as well as the ongoing COVID-19 pandemic, have clearly taken over. In spite of this shift in focus regarding media representation, researchers have made progress in evaluating the possible health risks associated with particulate contaminations present in water and air. In this review article, we summarize recent efforts that establish a clear link between the increasing occurrence of certain pathological conditions and the exposure of humans (or animals) to airborne or waterborne particulate matter. First, we give an overview of the physiological functions mucus has to fulfill in humans and animals, and we discuss different sources of particulate matter. We then highlight parameters that govern particle toxicity and summarize our current knowledge of how an exposure to particulate matter can be related to dysfunctions of mucosal systems. Last, we outline how biophysical tools and methods can help researchers to obtain a better understanding of how particulate matter may affect human health. As we discuss here, recent research has made it quite clear that the structure and functions of those mucosal systems are sensitive toward particulate contaminations. Yet, our mechanistic understanding of how (and which) nano- and microparticles can compromise human health via interacting with mucosal barriers is far from complete.

I. INTRODUCTION

When passing the North Pacific Gyre in 1997, the oceanographer and boat captain Charles J. Moore came across an enormous patch of floating plastic debris located in this very remote region of the Pacific Ocean. He then coined the phrase “plastic soup,” which is nowadays used to refer to the huge amount of macro- and microplastics polluting all aquatic environments.¹ Over time, such large plastic pieces are broken down into minuscule fragments, so-called microplastic particles.^2,3 Of course, micro- and nanosized pollutants are not only present in water but can also be found in air. In fact, tremendous amounts of nano- and microscopic particles are released into the air via natural processes (e.g., volcanic eruptions) and as a result of human activities (e.g., smoking or the combustion of fossil fuels) all around the globe.^4,5 For instance, in India, the level of air pollution is so high that the country's main national monument, the Taj Mahal, needs to be cleaned regularly to regain its brilliant white color. Still, even though those efforts allow visitors to enjoy the beauty of this majestic building from up close, the Taj Mahal can hardly be seen from the Red Fort of Agra—although the distance between those two landmarks is only ∼3 km.

However, the effects of environmental pollution are often not only unpleasant to view; an exposure of humans and animals to particulate matter can also lead to severe health issues.^6–9 In their 2012 report, the World Health Organization (WHO) linked both long-term and short-term exposure to black carbon particles to a multitude of medical conditions. In this article, we review our current understanding of how water- and airborne particles may impact human health. In detail, we focus on medical conditions that are likely related to alterations in the structural integrity and/or functionality of mucosal barriers. Such mucosal systems play an important role in numerous physiological processes, which is why their composition and biophysical properties are meticulously regulated by the human body. Here, we first summarize the range of physiological functions mucosal systems fulfill in the bodies of humans and animals. We then give an overview of the different sources and origins of particulate matter and discuss pathological alterations and disease patterns that have been linked to an exposure to particulate matter. Finally, we highlight selected mechanistic principles governing particle-induced mucus dysfunction and make suggestions of how biophysical research may help increase our understanding of whether and how different particulate pollutants pose a threat to human health.

II. COMPOSITION AND PHYSIOLOGICAL FUNCTIONS OF MUCOSAL SYSTEMS

Extensive phylogenetic analyses have indicated that the ability to produce and secrete mucus was an early milestone in the evolution of the animal kingdom.¹⁰ Genes encoding mucin-like glycoproteins–the main structural component of mucus–were even found in the genomes of members of the most basal metazoan phyla, such as Porifera (sponges), Cnidaria (e.g., corals and jellyfish), and Ctenophora (comb jellies).^11–13 In fact, mucosal systems are considered to be universal features that are used by all phyla of the animal kingdom. Moreover, the composition of mucus is similar among all vertebrate species, including humans as well as our most distant vertebrate relatives.¹⁴

Mucus is a water-based (∼95%), viscoelastic biomaterial; its biological and physico-chemical properties are mainly brought about by mucin glycoproteins, which are the macromolecular key components of all mucosal systems. The concentration of these mucins varies among different mucus systems. In part, this mucin concentration depends on the function the respective mucus system has to fulfill: for instance, in the tear fluid and in saliva, where the main task mucins are responsible for providing lubrication and protecting the tissue from abrasive wear, the mucin concentration is relatively low (∼0.01% in the tear fluid¹⁵ and 0.02%–0.03 % in saliva^16,17). In contrast, the mucin concentrations are considerably higher in organ systems where the mucins form protective diffusion barriers, e.g., in the stomach or in the lung. In healthy human gastric mucus, mucin levels reach values up to ∼3%; in the lung, corresponding values can be as high as ∼5%. Yet, mucus does not only comprise mucin glycoproteins but is a complex mixture of various secreted proteins, lipids, DNA, salts, cellular debris, as well as intact cells.

In addition to the composition of physiological mucus, the functions of mucosal systems also are similar throughout the animal kingdom. Mucus covers all wet epithelial surfaces, and–in part–this is necessary to keep them well hydrated. For instance, amphibians secrete mucus on their outer body surface to prevent their skin tissue from drying out (as well as for thermoregulation).^18,19 Proper hydration is also important for human mucosal surfaces, even though our tissues are not as much in danger of dehydration. In contrast, on the surfaces of our gastrointestinal system, as well as on corneal tissue, lubrication is essential to prevent tissue damage arising from mechanical challenges and reducing damage-induced inflammation.^20–22 Examples for physiological processes that generate such mechanical challenge include speaking/chewing, the transport of chyme through the gastrointestinal tract, eyelid blinking, and copulation. Moreover, certain animals (e.g., snails) employ the lubricating properties of mucus to facilitate locomotion,^23,24 whereas others (e.g., fish) developed mucus-based strategies to evade predation.^25–27

Yet, the functionality of mucus is not limited to dealing with mechanical challenges. For instance, selected variants of corals and marine mollusks employ the sticky properties of mucus to catch prey.^28,29 Furthermore, mucus also acts as a protective barrier against potentially harmful objects, such as inorganic particles, bacteria, or viruses. This is based on the ability of mucus to filter and trap objects based on size (i.e., diffusing objects larger than the mucus mesh size are trapped) or surface chemistry.³⁰ Since mucosal systems are continuously renewed, trapped objects are cleared from the body together with the shed mucus. At the same time, mucus accommodates and maintains a healthy microbiota.^31–33 Here, mucins modulate microbial behavior and they can suppress the expression of virulence genes in pathogenic bacteria. At the same time, mucins can support beneficial microbes by serving as their nutrient source and mucus regulates the spatial organization of microbial populations. In addition, mucus barriers also regulate cellular uptake of molecules, such as nutrients and vitamins from the gastrointestinal tract.³⁴ Furthermore, several studies have indicated the role of mucins in cell signaling and their suitability as biomarkers for certain forms of cancer.^35,36

Despite its clear importance for many physiological processes, little is known how environmental influences may affect the functionality of mucosal systems. Mucus is constantly exposed to various types of nano- and microscopic particulate matter, and these pollutants can easily accumulate in mucus where they might interfere with its important function. In the next chapter of this article, we will discuss the different types of organic/inorganic air- and waterborne particulate matter to which mucosal systems are regularly exposed.

III. PARTICULATE CONTAMINANTS IN WATER AND AIR

In the context of environmental pollutants that can come into contact with mucosal systems, two main forms of particulate matter can be distinguished: waterborne particulates and airborne particulates. Such particulate pollutants can either originate from natural sources (e.g., pollen, volcanic ash, and marine salt spray) or they emerge as an unwanted by-product of industrial/anthropogenic processes (e.g., brake wear, soot originating from the combustion of biomass, or the mechanical abrasion of plastics) (Fig. 1). In most cases, such as those exemplarily mentioned above, mucus is unintentionally challenged with such microscopic objects, i.e., as a consequence of environmental pollution (Fig. 1). When such exposure persists over extended (or recurring) time periods, or if the dose of particulates challenging a mucosal system is very high, severe medical conditions can be triggered, that require medical treatment (Fig. 2).^37,38

FIG. 1.

Sources of particulate matter and entrance gates into the human body. Humans are challenged with particulate matter with every breath they take and whenever they ingest food or beverages. These particulate pollutants can either be of natural origin or the by-products of anthropogenic activities. All surfaces of the human body are covered by mucus barriers that can trap and accumulate these particles. Under certain circumstances, this can induce adverse medical effects.

FIG. 2.

Parameters contributing to the pathogenicity of particulate matter. The pathogenic potential of particles for human health increases with the duration of exposure to these particles as well as the dose received during this exposure. Yet, also intrinsic, physical properties contribute to particle toxicity—in particular, size, aspect ratio, and shape.

Sometimes, however, the human body is intentionally confronted with micro- or nanoparticles. For instance, shower gels and facile scrubs often contain abrasive microparticles; typically, their function is to exfoliate the skin by mechanically removing dead epidermal cells. In other cases, micro- and nanoparticles are supplied to the human body with the aim to be taken up by tissue cells. For instance, drug-loaded particles are often orally administered to deliver pharmaceuticals. In such applications, the exposure of mucosal barriers to particulate matter is desired–more precisely, the particles are required to pass the mucus layer to reach their target. Yet, if such carrier particles are composed of non-biological, synthetic polymers that cannot be readily decomposed by the human body, they can evade clearance and thus, accumulate in tissues.

Whether particulate pollutants have harmful or even toxic effects on the mucosal system they accumulate depends on a combination of several factors. One aspect is the size of the particles to which mucosal tissue is exposed (Fig. 2).

Indeed, airborne particulate matter is commonly sorted into different categories according to the average particle size: one can distinguish coarse particulate matter (PM₁₀; particles with diameters on the order of ∼10 μm), fine particulate matter (PM_2.5; diameters on the order of ∼2.5 μm), and ultrafine particulate matter (PM_0.1; diameters <0.1 μm). Typically, smaller particles are considered more hazardous than larger particles. On the one hand, this is based on the ability of small particles to reach the deep parts of the lung more easily;^39,40 here, they can pass through the cell membranes of the lung tissue and accumulate in the cytosol. Geiser et al. could detect TiO₂ nanoparticles in all parts of the lungs of mice (i.e., in epithelial and endothelial cells, fibroblasts, connective tissue, and blood capillaries) that have been ventilated with contaminated air for 1 h.⁴¹ On the other hand, inhalable particulate matter was associated with the generation of significantly larger amounts of reactive oxygen species (ROS) than larger particles.^42,43

Independent of the route via which any of such particles may enter the human body, they almost always encounter certain mucosal barriers. However, if there is a lesion in the mucosal tissue, particulate matter will not encounter a barrier and thus, might be able to directly enter the tissue. The concentrations of airborne particles to which mucosal barriers are exposed can vary drastically among individuals. In part, this is due to differences in lifestyle, but also differences in the geographical locations where the individuals live are relevant.^44,45 Although globally present in the atmosphere, the levels of airborne particles vary greatly among different countries. On average, the concentration of black carbon as well as PM₁₀ is considerably higher in African and Southeastern Asian countries than in European and North American countries. Air quality measurement stations all around the world continuously detect the local air pollution in real-time, and the recorded data are made publicly accessible by the nonprofit World Air Quality Index project.⁴⁶ Singular events, such as the outbreak of volcanoes, can–for limited time periods–lead to strongly increased levels of air pollution.^47–49 Also, recurring events, such as festivities on New Year's Eve or during Diwali when large fireworks release tremendous amounts of particulate matter (especially black carbon in the PM_2.5 range) into the air, result in locally very high particle concentrations, especially in densely populated areas.^50–52

A person-specific lifestyle choice relevant in the context of nanoparticle uptake via air is smoking. Smokers (and, to a lower extent, secondhand smokers, i.e., people that do not actively smoke but are regularly exposed to tobacco smoke) inhale significantly larger amounts of particulate matter than nonsmokers.⁵³ Moreover, recent research indicated that also the vapor emitted by electronic cigarettes contains particulate matter—albeit in smaller amounts than the smoke produced by conventional cigarettes.^54,55 Such an intentional and voluntary exposure to increased levels of air pollutants can not only initiate the onset of respiratory diseases but can also cause an unfavorable progression of preexisting conditions (Fig. 2). One of the most prominent (and highly prevalent) examples of a medical condition whose primary cause is the regular inhalation of tobacco smoke or exhaust gas is the chronic obstructive pulmonary disease (COPD). This condition alone is responsible for an estimated three million deaths each year; thus, it is the third leading cause of death worldwide.^56,57

So far, we have discussed examples of airborne pollutants. However, an exposure to aquatic particulate matter can entail pathological effects as well. The most prevalent species of aquatic pollutants is polymer-based particles, which are commonly categorized into either macroplastics (i.e., objects with average diameters >5 mm) or microplastics (i.e., all contaminants with average diameters <5 mm).⁵⁸ Even though particulate pollutants can be found in marine and freshwater habitats, research on the impact of aquatic microplastic pollution has predominantly focused on life forms inhabiting marine environments. The pollution of freshwater environments (where, e.g., fish for human consumption are cultivated), however, has been widely neglected. Nevertheless, existing research reports indicate that, in both cases, waterborne particles can pose a threat to the species coming in contact with them.^59–61

Similar to airborne particles, the toxicity of small aquatic pollutants also has been suggested to be more serious than that of larger particles.⁶² Particle shape, however, seems to critically affect the toxicity of a waterborne pollutant as well (Fig. 2). For instance, dendritic nickel clusters lead to higher levels of mortality or developmental defects in zebrafish embryos than spherical nickel nanoparticles.⁶³ A similar correlation between particle shape and toxicity could be observed for a number of other marine animal species (including plankton, crustaceans, mollusks, and fishes) when they were exposed to microplastic pollutants of different shapes.^64–68 Here, the toxicity of particles with an elongated geometry is largely based on their ability to reside in body tissues for extended time periods (Fig. 2). Also, irregularly shaped particles and polymer fragments with sharp edges (e.g., as they emerge when polymeric materials are broken down by recurring shear forces in the ocean) were shown to exhibit increased cytotoxicity compared to spherical particles.^69,70 Using a zebrafish model, Qiao et al. could show that the toxicity of particles strongly correlates with their ability to accumulate in organisms and the propensity to accumulate, in turn, correlates with the particle shape.⁷¹ A shape-dependent particle toxicity has also been described for airborne particles. For instance, it was demonstrated that airborne particulates with a high aspect ratio, i.e., particles with an elongated, fibrous shape, exhibit increased toxicity compared to isotropic, round objects of comparable size (Fig. 2). The toxicity of asbestos, for instance, is largely brought about by its needlelike shape, which allows the fibers to reside in the alveoli for extended time periods.^72,73 This, in turn, provokes an inflammatory response of the lung tissue as well as other serious pathological alterations.

Moreover, several studies indicated that the surface charge of particulates can modulate their toxicity.⁷⁴ For instance, anionic polymer nanoparticles were observed to induce a release of larger amounts of inflammatory cytokines than their electrostatically neutral counterparts. Additional factors affecting the toxicity of particulate matter include their chemical composition and their ability to bind (and thus, transport) harmful substances, including polycyclic aromatic hydrocarbons, which have been linked to different types of cancer.^75–77 Finally, waterborne particles can easily be colonized by aquatic microbial communities, which sometimes happen to be opportunistic pathogens.^78,79

IV. CORRELATION OF MUCUS DYSFUNCTION WITH DISEASE PATTERNS

In recent years, particulate matter of all kinds has been shown to be a potential contributor to a plethora of pathological conditions. In particular, the effects of air- and waterborne particles on the health of humans and animals have been studied in much more detail than in the past. In those studies, exposure to particulate matter was linked to the onset and/or aggravation of a multitude of pathological conditions and developmental disorders; examples include respiratory diseases, cardiovascular pathologies, various types of cancer, impaired embryonic development (as manifested in low birth weights of newborns, premature deliveries, or increased levels of embryonic lethality),^80–82 and even fatalities (Table I and Table II).⁸³

Table I.

Systemic pathological conditions in humans that were linked to an exposure to air- and waterborne particulate pollutants.

Medical condition/disorder	Type of particulate matter	Ref.
Respiratory disorders (e.g., asthma, bronchitis, COPD, and malignant lung tumors)	Unspecified/diverse	182
	PM2.5	183–186
	Diesel exhaust & traffic-related air pollution	89–91, 187
	Mineral dust (e.g., asbestos, silica)	72, 73, 188–191
	Smoke (including tobacco smoke and black carbon)	53, 56, 192–197
	Desert dust	198, 199
	Metal dust	200, 201
	(Inhalable) Microplastics	92, 166, 202–204
	Carbon nanotubes	205, 206
Allergies	Pet dander, dust mite allergens	182, 207, 208
	Traffic-related air pollution	91
Disorders of the gastrointestinal tract (e.g., Crohn's disease and ulcerative colitis)	PM2.5	98, 105
	Smoke (including tobacco smoke and black carbon)	99–101, 209
	Microplastics	167
Disorders of the female reproductive system and effects on the fetal development	PM2.5	210, 211
	Smoke (including tobacco smoke and black carbon)	212
Ocular diseases	PM2.5	147, 151, 152, 213–217
	PM10	218–220
	Mineral dust (e.g., TiO2)	221
Cardiovascular disorders (e.g., carditis, and thrombosis)	Unspecified/diverse	222, 223
	PM2.5	161, 162, 224–228
	Diesel exhaust and traffic-related air pollution	229, 230
	Smoke (including tobacco smoke and black carbon)	194, 231, 232
	Desert dust	199
	Microplastics	233
Neurodegenerative disorders (e.g., Alzheimer's disease and Parkinson's disease)	PM2.5	164
	Diesel exhaust and traffic-related air pollution	165, 168
	Microplastics	92, 166, 167

TABLE II.

Systemic pathological conditions in animals that were linked to an exposure to air- and waterborne particulate pollutants.

Organism	Medical condition/disorder	Type of particulate matter	Ref.
Mice/rats	Respiratory disorders	Unspecified/diverse	95,234
		PM2.5	96,235,236
		Ultrafine carbon particles	237
	Disorders of the gastrointestinal tract	Microplastics	124,126
		PM2.5 and PM10	238,239
	Liver damages	Microplastic	240
	Ocular disorders	PM2.5 and PM10	156,241–243
	Neurotoxicity	Microplastics	244
Rabbits	Ocular disorders	Mineral dust (e.g., TiO2)	245
Horses	Respiratory disorders	Unspecified/diverse	246
Zebrafish	Disorders of the gastrointestinal tract, intestinal damage, and gut microbiome dysbiosis	Microplastics	265,71,118,247
	Liver damages	Microplastics	248
	Impaired larval development	Microplastics	249,250
		Nickel nanoparticles	63
	Neurodegenerative and behavioral disorders	Microplastics	81,251–253
		Carbon nanotubes	254
Other fish species	Disorders of the gastrointestinal tract, intestinal damage, and gut dysbiosis	Microplastics	255–259
	Impaired larval development and reduced larval survival rate
	Neurotoxicity
	Increased mortality rate
Corals	Bleaching	Microplastics	260–263
	Tissue necrosis
	Reduced skeletal growth
Sea urchins	Increased embryotoxicity	Microplastics	264,264
	Impaired larval development
Clams and mussels	Immunotoxicity	Microplastics	266–268
	Neurotoxicity
Zooplankton	Increased mortality	Microplastics	64,66,140,269,270
	Reduced growth
	Reduced reproduction rate
Nematodes	Reduced survival rate	Microplastics	247,271,272
	Reduced growth
	Intestinal damage
	Neurotoxicity
Bees	Degradation of the hindgut	Microplastics	273
	Gut microbiome dysbiosis

In humans, owing to ethical considerations, insights into the contribution of particulate matter to the onset and progression of medical conditions can, of course, only be retrieved from observational studies. These include cross-sectional studies as well as longitudinal studies. In the former, medical data are collected from a group of people at one specific time point; in contrast, in the latter, such data are collected repeatedly over extended periods of time. Moreover, case-control studies can be employed to identify the impact certain particles might have on the health status of an individual. Here, patients with a given condition (i.e., the “cases”) are compared to healthy subjects without that affliction (i.e., the “control group”). One of the main complications, however, that comes with such observational studies is that they are–compared to randomized, controlled trials–prone to be influenced by biases; for instance, the allocation of subjects to the “case group” and the “control group” is not entirely random (but pre-determined by existing medical diagnoses). Similarly, the contribution of certain parameters, which are deemed responsible for a certain medical condition (such as the exposure to specific air/water pollutants), may be overestimated. In other words, such studies retrieve patterns of correlation–and not necessarily modes of causality.

To overcome this intrinsic limitation of observational studies, systematic investigations can shed light onto the question of how different particle parameters (such as concentration, type of material, size, or shape) contribute to certain physiological dysfunctions. Such tests involving a controlled exposure to putatively harmful substances are often performed using animal models (Table II). Of course, also for such animal studies, researchers need to adhere to basic ethical guidelines; yet, the strictness of those regulations can vary greatly around the globe. For instance, the majority of studies addressing the influence of particulate matter on embryonic development were conducted using the Danio rerio zebrafish (Table II). Such phenomenological studies with animal model systems certainly provide valuable insights into how problems in the embryonic development of vertebrates are linked to an exposure to particulate matter; to what extent those insights can be transferred to the development of human embryos is, however, still unclear.

A. Respiratory disorders

For the remainder of this chapter, we will highlight selected different medical disorders and pinpoint how particulate matter is thought to contribute to their initiation and development. The examples we discuss encompass studies conducted with humans and animals. The best-known disorders associated with the exposure to particles belong to the category of respiratory diseases. In general, smoke, fume, and exhaust gas originating from the combustion of organic material can be considered to be the main culprits responsible for pathological alterations of the respiratory tract. For instance, the regular inhalation of tobacco smoke is considered to be the main reason for the development of chronic obstructive pulmonary disease (COPD).⁸⁴ More severely, even the occupational exposure to mineral dust or fibers (e.g., in the mining or the textile industry) has been linked to the onset of COPD.^84,85 COPD is a progressive disorder characterized by a chronic inflammation of the lung tissue; long-term consequences of this condition include a narrowing of the lower airways and an ensuing reduction of airflow.⁸⁶ Moreover, as highlighted by the ongoing COVID-19 pandemic, patients suffering from COPD have a higher risk for developing severe cases of respiratory complications triggered by e.g., pathogens such as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).^57,87 In addition, in those parts of the world's population that are exposed to high levels of combustion smoke and traffic-related air pollution, there is a higher prevalence of respiratory conditions, such as asthma and lung cancer.^88,89 For instance, Rosenlund et al. and Gehring et al. showed that traffic-related air pollution can lead to higher risks of developing asthma or allergies—and this can already be detected at early life stages of humans.^90,91 In addition to particulate matter originating from combustion processes, other air pollutants, such as metal and mineral dusts, desert dust, and microparticles made from polymeric materials (such as brake wear or microplastics), can have adverse effects on the human respiratory tract (Table I). Interestingly, for a long time, microplastics have gained only little attention as an airborne pollutant. Yet, Prata and Vianello et al. estimated that every person inhales up to a few hundred microsized polymer fibers and particles per day.^92,93

Many of such particle-associated disorders share common symptoms such as wheezing, difficulty in breathing, coughing, mucin hyperconcentration, and mucus hypersecretion. Especially, the latter two symptoms can lead to sputum production and an obstruction of mucociliary clearance (i.e., the self-clearing mechanism of the lung).⁹⁴ Similar respiratory conditions as described for humans could also be detected in animals in response to air pollutants. For instance, Saldiva et al. challenged rats with different levels of ambient air pollution. They reported that, indeed, individuals that were exposed to high levels of pollutants suffered from an impairment of mucociliary clearance and showed increased overall mortality.⁹⁵ Similarly, Xu et al. could show that the exposure of laboratory mice to PM_2.5 particles triggered an inflammatory response and mucus hypersecretion.⁹⁶

B. Disorders of the gastrointestinal tract

Different from the large number of research studies addressing the impact of particulate matter on the respiratory system, similar studies investigating the gastrointestinal tract are less frequent. Nevertheless, there are strong indications that particulate matter can lead to a range of pathological conditions. Overall, such micro- and nanoparticles reach the gastrointestinal mucosa in two different ways: On the one hand, most particles that have entered the human body via the respiratory system are eventually cleared from the respiratory tract and then typically end up in the gastrointestinal tract as well (only very few particles are cleared from the body by coughing).⁹⁷ Beamish et al. have reviewed and summarized the impact of airborne particles on the health state of the bowel: as they already stated 10 years ago, the inhalation of air pollutants is associated with different cancer types of the gastrointestinal tract as well as with ulcerative colitis, inflammatory bowel disease, and Crohn's disease.⁹⁸ Other studies, in particular, linked active smoking (as well as the passive exposure to tobacco smoke) to the onset and aggravation of these diseases.^99–101 On the other hand, humans and animals alike take up significant amounts of particles via food and water consumption.^102–104 It is, however, difficult to determine whether the amounts of particulate matter humans ingest on a daily basis as dietary habits vary greatly among individuals. Processed food, for example, often contains dietary additives such as TiO₂ or SiO₂ nanoparticles, which are used as whitening components or anticaking agents.^103,105,106 Also, natural and organic food, such as honey, can contain copious amounts of non-natural particulate matter including soot, textile fibers, as well as polymer fragments.^107,108 Several studies described the trophic transfer of particulate matter in aquatic food webs, which can result in an accumulation of these contaminants in animals that are part of the human diet,^109–111 e.g., fish and sea food.^112–114 In addition, microplastic particles were also detected in bottled beverages (disposable/reusable plastic bottles and glass bottles) as well as in tap water.^104,115,116 Even infants are already challenged by microplastic fragments during their very first days: Li et al. found that minuscule polypropylene fragments can be released from feeding bottles.¹¹⁷

Similar to airborne pollutants, the pathological effects ingested particles may have on the gastrointestinal system have been mostly demonstrated in animal models (mostly using mice and zebrafish). For instance, several research groups working with zebrafish showed that polymer microfibers and microspheres can cause intestinal injuries, gut inflammation, disorders of the metabolome, and dysbiosis of the gut microbiota.^{65,71,118,119} Similar pathological effects were detected in other fish species and marine invertebrates.^120–122 Furthermore, in laboratory mice, alterations in the composition of the gut microbiota were observed when the animals were fed microparticles.^123,124 This “microplastics diet” further led to intestinal inflammations and dysfunctional mucus barriers.^123–126

C. Impact on embryonic development

In addition to the respiratory system and the gastrointestinal tract, particulate pollutants can also provoke disorders in other mucosal tissues. Ragusa et al. were the first to describe the presence of microplastic particles in placentae, and such particles were found on the maternal and the fetal side of this tissue.¹²⁷ Bové et al. could demonstrate an accumulation of black carbon nanoparticles on the fetal side of placentae, and the particle load in placentae is correlated with maternal exposure to polluted air.¹²⁸ It was suggested that after inhalation/ingestion, these particles might have translocated from either the respiratory system or the gastrointestinal tract into the bloodstream; from there, they could be distributed throughout the body and accumulate in the placenta. In a sensitive environment such as the placenta, such nanoparticles might have adverse effects on the health state of the mother and the unborn child. However, systematic research on the impact of particulate pollutants on the female reproduction system and the development of embryos is, to date, scarce. Li et al. evaluated and summarized previous studies that linked maternal exposure to particulate matter to adverse birth outcomes and abnormalities in embryonic development.¹²⁹ Indeed, several retrospective cohort studies identified a positive correlation between maternal exposure to ambient air pollution during pregnancy and premature deliveries, low birth weights of newborns, and stillbirths (Table I).^130–133 Moreover, reduced fertility of women has been put forward as a potential adverse effect of airborne particulate matter.¹³⁴

A causal relation between particulate pollutants and embryonic development has been identified using model animals, with the most commonly used being zebrafish (Table II). Indeed, exposure of zebrafish embryos to different types of particulate matter (such as microplastic spheres and fibers as well as carbon nanotubes) had severe consequences on their development.¹³⁵ For instance, zebrafish embryos that have been challenged with microparticles hatched earlier, and the overall hatching rate was lower than in a control group.^82,136 Duan et al. found that nanosized polymer particles inhibited embryo hatching more strongly than microsized particles.¹³⁷ Moreover, exposure to particle pollutants during embryonic development can induce a number of morphological abnormalities, such as vascular defects or deformities of the jaws, fins, or tail.^136,138 Adverse effects of microplastic pollutants on the reproduction process (including fertility and embryonic development) were also described for other aquatic animal species, such as sea urchins and zooplankton.^139,140 Although studied to a smaller extent, a negative impact of air pollutants on the reproductive system was also observed for terrestrial model animals.¹⁴¹ For instance, Veras et al. found that mice exposed to non-filtered, urban air showed decreased fertility compared to a control group of mice that received filtered air. In addition, maternal exposure to air pollutants led to decreased fetal weights, low birth weights, and preterm birth.^142,143 By administering PM_2.5 particles to laboratory mice via intratracheal instillation, Liao et al. could show that exposure to airborne pollutants resulted in increased levels of apoptotic oocytes;¹⁴⁴ this finding, in turn, might explain the compromised fertility of mice observed by Veras et al.¹⁴⁴

D. Ocular disorders

An increasing number of studies have also linked the occurrence of ocular disorders to an exposure of the eye tissue to particulate air pollutants.¹⁴⁵ Such disorders include conjunctivitis, eye redness, dry eyes, and others. For instance, Mimura et al. found a positive correlation between the number of patients suffering from acute conjunctivitis and air levels of PM_2.5 particles.¹⁴⁶ However, although high concentrations of air pollutants seem to be one of the main drivers of allergic conjunctivitis during the non-pollen season,^146,147 the causal connection between airborne particles and allergic conjunctivitis is still under debate.¹⁴⁸ In contrast, a direct linkage between ocular disorders and ambient air pollution has been identified for the development of the dry eye disease and eye irritation.^149,150 Moreover, Gutiérrez et al. demonstrated that individuals exposed to high levels of ambient airborne particulate matter exhibited stronger ocular surface alterations than those challenged with comparatively low concentrations of particulate matter.¹⁵¹ Interestingly, Torricelli et al. found that an exposure to ambient levels of air pollutants resulted in an enhanced density of mucin secreting goblet cells in the conjunctival epithelial layer of humans. Such an increase in goblet cells resulted in higher expression levels of ocular mucins, and the authors speculated that this might be an adaptive response to long-term exposure to particulate air pollutants.¹⁵² Yet, this topic is still under debate as Uchino et al. found that a regular exposure to cigarette smoke resulted in a lower conjunctival density of goblet cells and, thus, decreased levels of secreted mucins.¹⁵³

Also here, to systematically investigate causal relations between the exposure of ocular surfaces to airborne particles and medical conditions, model animals are typically employed.^154–158 For instance, recent research on mice indicated that exposure to PM_2.5 particles can delay wound healing of the corneal surface.¹⁵⁹ In addition, several studies showed that the tear film breakup time and the secretion of tear fluid were significantly reduced in the mice that received a dose of airborne particulate matter.^154,155,160

E. Other disorders

Whereas it seems intuitive that medical disorders can be triggered in such organ systems that come into direct contact with particulate contaminants, many studies have also linked the inhalation or ingestion of particulate matter to a broad range of disorders of the cardiovascular system (Table I). Since humans have a closed cardiovascular system, it is not clear if and how particulate matter can enter the bloodstream. Thus, identifying the origin of these pathological alterations is not trivial. To date, human exposure to different types of particulate matter (such as PM_2.5, smoke, or microplastics) was shown to result in both a stiffening (i.e., atherosclerosis) and a narrowing of blood vessels. Those alterations are brought about by the formation of plaque deposits and an inflammation of endothelial cells, and this can lead to heart attacks, peripheral artery disease, and other medical conditions of the circulatory system.^161–163

In addition to cardiovascular alterations, some studies also describe a positive correlation between exposure to airborne particles and high risks of developing neurodegenerative disorders (such as Alzheimer's disease and Parkinson's disease) as well as other conditions of the nervous system (Table I).^164–169 In certain cases, e.g., when analyzing the occurrence of strokes, a narrowing of blood vessels as mentioned above is to blame; blood vessels affected in such a way cannot ensure a sufficient cerebral blood supply.^170,171 In other scenarios, it has been suggested that particulate matter, especially nanosized particles, may have the potential to reach the neural tissue. Indeed, a translocation of different particle species to the brain has already been documented in rodents and humans. For instance, Oberdörster et al. described an enrichment of radio-labeled, ultrafine graphite particles in the brain tissue of laboratory rats.¹⁷² Maher et al. even showed that magnetite particles with diameters <0.2 μm could enter the human brain via the olfactory bulb.¹⁷³ Once they arrive in the brain tissue, the toxic effects such particles may induce can wreak havoc. For instance, particulate matter was implicated to be involved in the production of ROS, which are responsible for the onset of certain neurodegenerative diseases.^174,175 Moreover, particulate matter can be involved in inflammatory processes, interfere with the functions of cellular organelles, or disturb protein homeostasis.^176–179 All of these dysfunctions can eventually lead to damage in or even loss of neurons.^180,181

V. MECHANISTIC ASPECTS OF PARTICLE-INDUCED ALTERATIONS OF MUCOSAL FUNCTIONS

In the previous section, we have discussed several medical conditions that can be induced or aggravated by an exposure to particulate matter. So far, research has mostly focused on describing the pathological effects particulate matter can induce in the bodies of humans or animals by identifying correlations between particle exposure and the risk of developing a certain disorder. Studies shedding light on the mechanistic origins of such pathological alterations are, however, still scarce. In particular, it appears that the impact particulate matter can have on mucosal barriers has been largely neglected. This is astonishing considering that one of the key functions of mucus is preventing the translocation of harmful objects by trapping and immobilizing them (thereby protecting the underlying tissues from putative pathogens).^34,274 Since mucosal barriers cover virtually all potential “entrance gates” into the bodies of humans and animals, this scarcity of studies addressing the influence an exposure to particulate matter may have on the integrity and functionality of mucus, represents a knowledge gap that future research definitely should fill. Thus, in Sec. VI of this review article, we will discuss how biophysical tools and techniques could help to increase the prevalence of studies to systematically investigate the impact of particulate matter on mucosal systems.

First, however, we summarize our current understanding of how particulate matter may influence mucosal barriers and how these effects could contribute to the occurrence of certain disease patterns we discussed above (Fig. 3). In this context, two different general modes of action can be distinguished by which particulate matter may alter the physiological properties of mucus. On the one hand, the expression patterns of mucin genes can change in response to particulate exposure, and the ensuing alterations in mucin expression can result in a manifestation or aggravation of certain pathological conditions. On the other hand, particulate matter can directly interfere with the physiological function of mucus, e.g., by altering the selective permeability properties of mucosal barriers or by disrupting mucus lubricity.

FIG. 3.

Mechanistic aspects of particle-induced alterations of mucosal functions. Exposure of mucosal systems to particulate matter can disrupt mucus structure and functionality in different ways. First, particles can interfere with mucin homeostasis (a); the expression of mucins can either be upregulated or downregulated. Likewise, particulate matter can either trigger hyposecretion or hypersecretion of mucus. Second, particles can engage in multiple binding interactions with several mucin molecules at the same time, thereby introducing additional cross-links (b). Moreover, particulate pollutants can induce major alterations of the mucus microstructure, thus affecting the permeability properties of mucus barriers (c). Finally, particles are often colonized by pathogenic species (d). On the one hand, these pathogens can provoke dysregulation of the mucosal microbiome. On the other hand, they can have direct toxic effects on the mucosal tissues.

In several disorders—mostly those occurring in the respiratory tract—a disturbance of mucus homeostasis has been described as a key symptom (Fig. 3). For instance, hypersecretion of mucus is a hallmark in asthma, cystic fibrosis, and COPD.²⁷⁵ In more detail, cystic fibrosis is characterized by the production of mucosal secretions with abnormally high viscosity, and a mutation in the gene of the CFTR protein is responsible for this.²⁷⁶ Similarly, mucus secretion can also be enhanced in individuals who do not suffer from acute or chronic respiratory diseases but are either regular smokers or occupationally exposed to airborne particulate matter.²⁷⁷ Independent of how mucus hypersecretion is triggered, this phenomenon entails (in many cases) undesired medical consequences. Using a tissue model simulating the air–liquid interface of human airways, Cao et al. found that, even in vitro, exposure to cigarette smoke can result in an increased density of mucin secreting goblet cells.²⁷⁸ Accordingly, in this study, both mucin gene expression and mucus secretion were found to be upregulated. Whereas an acute upregulation of mucus secretion is considered a physiological and protective response mechanism of the human body, a long-term, chronic hypersecretion of airway mucus has been put forward as one of the main contributors to several severe respiratory conditions. In such pathological scenarios, the excessive production of mucus hampers mucociliary clearance from the airway tissue and thus, increases the risks of infection by pathogens.^{275,279–282} Interestingly, in addition to tampering with the composition and secretion rates of airway mucus, particulate matter can also more directly contribute to the failure of mucociliary clearance: both cigarette smoke and environmental pollutants have been demonstrated to be responsible for structural and motile dysfunctions of lung cilia: those pollutants can not only reduce the beating frequency and disrupt the concerted beating action of the cilia, they can also lead to a reduced average cilia length.^283,284

Typically, an upregulation (e.g., as induced by an exposure to particulate matter) of mucin concentrations in mucus results in altered viscoelastic properties, i.e., in abnormally high elastic moduli representing an increased mucus stiffness.^285–287 In healthy individuals, the mucin concentration in airways mucus is precisely regulated such that the mucus has the elastic properties of a stable hydrogel that remains well in place so that it establishes a protective barrier against pathogens. However, at the same time, the viscous properties of mucus have to be low enough that mucus clearance is still possible, e.g., when shear stress is applied during coughing or by the beating motion of the airway cilia.^279,288 Thus, an increase in the viscous properties (as brought about by higher mucin concentrations) hampers the removal of lung mucus.^289–291 Microscopically, this effect can be rationalized by the ability of the long mucin glycoproteins to physically entangle with each other. In addition, mucins can form a broad range of intermolecular bonds with each other (as well as with other mucus components), which creates a polymeric network containing both covalent and transient cross-links.^292–296 Yuan et al. demonstrated that, at oxidizing conditions, the elastic properties of mucus are increased–and this phenomenon was attributed to the formation of additional disulfide bonds between neighboring mucin molecules.²⁹⁷ Importantly, inhalable particulate matter was shown to trigger the formation and release of ROS.^174,298,299 Thus, an exposure of mucus to such particulate matter–triggered ROS can increase the stiffness of mucus by increasing the density of cross-links in the mucus gel, and this can either create pathological effects or exacerbate existing medical conditions.^297,300 Owing to their ability to engage in multiple, simultaneous binding interactions with mucin glycoproteins, particulate pollutants might also be able to alter the rheological properties of mucus by introducing additional cross-links (Fig. 3);^95,301–303 however, so far, the impact of particulate pollutants on the viscoelastic properties of mucosal systems has not been systematically studied.

So far, the best mechanistic understanding of how particulate matter impacts mucin expression and mucus secretion was obtained for airway mucus. However, similar investigations have been conducted for the gastrointestinal tract as well. For instance, Lu et al. and Jin et al. found that—different from how particles affect airway mucus—the transcription levels of gastrointestinal mucin variants and the secretion of gastrointestinal mucus were reduced in laboratory mice that received regular doses of microplastic particles via their drinking water.^124,126 One result of reduced mucus secretion rates is the formation of thinner mucus barriers; yet, a sufficiently high mucus thickness is one of the key factors determining the retention efficiency provided by mucus barriers. By infecting MUC2 deficient mice (i.e., mice unable to produce and secrete any intestinal mucins) with a rodent pathogen, Bergstrom et al. showed that the absence of mucus barriers led to the rapid colonization and infection of intestinal epithelial surfaces.³⁰⁴ Thus, abnormally thin mucus layers are likely to be less suitable in protecting mucosal organ systems from colonization with pathogens. In line with this notion, patients suffering from an infection with the pathogenic bacterium Helicobacter pylori tend to exhibit thinner mucus barriers in the gut.^305–307 At this point of research, however, it is not fully understood if such a reduction in mucus thickness is the cause for Helicobacter infections or a symptom.

Different from those results obtained for gut mucus, in zebrafish, Gu et al. described gut mucus hypersecretion after exposure to microplastic particles.³⁰⁸ This finding demonstrates that, at this point of research, no definite statement can be made if and how a certain nano- or microparticle species affects different mucosal systems. Probably, both the type of mucus challenged with such particles and the properties of the particulate pollutant interacting with the mucosal systems need to be considered in detail to understand and, ultimately, predict the consequences such particle-based contaminations can have on mucus homeostasis. Yet, independent of whether particulate matter triggers hyper- or hyposecretion of mucus, abnormal mucus levels are almost never desirable. In the gastrointestinal tract (as well as in other locations of the body), maintaining intact and balanced mucus barriers is critical for human and animal health. At this specific location, mucus allows for the passage of beneficial molecules (e.g., nutrients) but retains putatively harmful objects, i.e., bacteria, viruses, certain inorganic particles, and others.^274,309 At the same time, mucus accommodates a healthy microbiota by supporting commensal micro-organisms and suppressing the proliferation of pathogenic species.³³ Deviations in mucus composition and secretion can disturb commensal bacterial communities and may enable opportunistic micro-organisms to dominate. Indeed, such a particle-induced dysbiosis of the gut has been described before in animal models.^71,310 Moreover, particulate contaminants might even introduce new pathogens into the body (Fig. 3): it has been demonstrated that microparticles dispersed in aquatic systems are often densely colonized by potentially pathogenic (biofilm forming) bacterial communities. Thus, those particles might act as carriers that transport foreign bacteria into the gut microbiome thus, facilitating the onset of pathological conditions in humans and animals alike.^311,312

In addition to the composition and viscoelastic properties of mucus, researchers have identified another factor that is crucial for the function of many mucosal barriers, the microstructure of mucus hydrogels. This parameter critically contributes to the selective permeability properties of mucus barriers by regulating the penetration and translocation behavior of diffusing objects.^313,314 This is—at least in part—accomplished by mucus due to its ability to exclude objects based on size, i.e., objects larger than the apparent mesh size are limited from entering the mucus hydrogel.^34,313 Thus, an unphysiological rearrangement of the mucin network would intimately affect the permeability properties of mucus: An increase in the average pore size might facilitate the penetration of large objects (e.g., pathogens or harmful particles) into mucus that would be retained otherwise. Such a restructuring of the mucus network can be triggered by microscopic contaminants including particles (Fig. 3). Wang et al. found that adding mucoadhesive, polymeric nanoparticles to native cervicovaginal mucus mobilized muco-inert microparticles that normally would be trapped in the mucus mesh due to size constraints.³¹⁵ There, this mobilization effect was attributed to an increase in the average pore size of the mucus gel. Similar outcomes were obtained for both native mucus and reconstituted mucus models, when they were intentionally enriched with other contaminants, e.g., airborne particles such as diesel exhaust particles or molecules such as oligomeric carbohydrates or peptides.^316–318 Interestingly, it was demonstrated that the microstructure of mucus hydrogels can also be tuned in the other direction (i.e., toward the formation of a mucin mesh with smaller pore sizes), e.g., by adding a nonionic detergent.^319,320 Such treated mucus could efficiently trap nanosized particles that were mobile in untreated mucus.

In summary, mucus is an important and versatile biomaterial that fulfills a broad range of physiological functions. Thus, it is not surprising that the human body controls the composition and architecture of mucosal barriers with high precision. An extended exposure of mucosal barriers to particulate matter can easily disturb the physiological state of mucus. On the one hand, contact with such particulate contaminants can directly alter the properties of mucus—often with undesired consequences. On the other hand, particulate matter can trigger an unphysiological body response, that, in turn, can entail the onset of pathological conditions or aggravate existing disease patterns.

VI. HOW CAN BIOPHYSICAL APPROACHES CONTRIBUTE TO RESEARCH ADDRESSING THE INFLUENCE OF PARTICULATE MATTER ON HUMAN HEALTH?

Our understanding of the mechanistic principles by which particulate pollutants can tamper with the structure and function of mucosal barriers is still very basic. Instead, most research in this area has, so far, rather focused on identifying correlations between the onset of a broad range of disorders and an exposure to particulate matter. At least in part, this can be attributed to the high structural and functional complexity of physiological mucus. Biophysical tools may help to fill this gap by conducting a set of state-of-the-art characterizations of both uncontaminated and particle-enriched mucus samples. Here, it is possible to obtain a clear and quantitative relation between the amount of intentionally added nano- or microparticles and the ensuing changes in mucus structure or function. Moreover, by comparing the effects different types of well-characterized particulate contaminants have on mucosal systems, such research will allow us to find physico-chemical interaction patterns between particles and mucus components. Also, results from those in vitro experiments can help identify possible consequences these interactions may have on physiological processes, where alterations in mucus structure and function are already known to have adverse effects.

For instance, the viscoelastic properties of mucus can be easily assessed by performing macro- and microrheological tests.^287,321 In addition, as mucosal systems have been recognized as excellent bio-lubricants, tribological measurements using material pairings with different properties (including hardness or hydrophobicity/hydrophilicity) and geometries (e.g., ball-on-plate, ball-on-pins, and others) have been introduced as valuable tools to detect alterations in this particular material property.^322–324 To assess the permeability properties of mucus, translocation assays can be performed. Here, the partitioning of molecules or particles into mucus as well as their translocation behavior across the hydrogel is monitored and can be rationalized by diffusion-reaction models.^325,326 Alternatively, the diffusive mobility of particles in mucinous systems can be assessed by following their Brownian motion and calculating diffusion coefficients from the recorded trajectories.³²⁷ The permeability of mucus barriers is strongly dictated by their ability to bind diffusing objects, and the diverse chemistry of mucins gives rise to a broad range of physico-chemical interactions that can trigger binding events that interrupt the Brownian motion of microscopic objects. To quantify these binding phenomena, a broad range of techniques has been applied, including spectroscopical approaches as well as the direct quantification of objects that have been depleted by mucins from a solution using colorimetric assays. Finally, the microstructure of mucus can be visualized using various imaging tools, such as confocal laser scanning microscopy (CLSM), cryo-scanning electron microscopy (SEM), or super-resolution microscopy techniques.³²⁸

When trying to identify mechanistic principles in complex biomaterials, another common approach used in the field of biophysics is to establish a simplified model system (i.e., a mucus mimic), that comprises only those key components deemed necessary to recreate certain physico-chemical, mechanical, or biological properties of the actual biomaterial. These model systems come with the advantage that they allow for assessing alterations in a controlled manner. For instance, here, it is easily possible to quantify how certain additives (e.g., nanoparticles) change selected material properties of the model system. In the case of mucosal barriers, the primary functional and structural component is the mucin glycoprotein. However, this biological material does also contain non-mucin proteins, nucleic acids, and lipids as well as many small molecules and ions. To further complicate matters, the particular composition of mucus can vary strongly—and not only between individuals but also between different mucosal systems within one individual. This makes it quite challenging to identify overarching physico-chemical principles that govern mucus-particle interactions.

Indeed, reconstituted systems of carefully purified mucins have already been put forward as suitable model systems to study the diffusive spreading of pharmaceuticals or drug carriers.³²⁹ By tuning a few parameters, including the mucin concentration, the pH level, and the ion concentrations, two physiologically relevant variants of mucosal systems can be easily recreated: mucin containing solutions (as they naturally occur as tear fluid or saliva) and mucin-based hydrogels (which can be found, e.g., in the gut or the lung). In addition, anchoring mucins onto surfaces allows for creating a model representation of membrane-tethered mucosal systems.³³⁰ In fact, such mucus surrogates are increasingly used in applied research focusing on drug delivery applications and biomedical engineering.^329,331 Moreover, mucus models have also been proven to be valuable tools in basic research, e.g., for studying particle-mucin interactions: such mucus mimics demonstrated the versatile ability of mucins to engage in binding interactions with an extremely broad range of biological or synthetic entities—including bacteria and viruses as well as the toxins they secrete.^325,332,333 In the context of particulate contaminants, the ability of mucus to voraciously trap a plethora of objects seems to be a property that creates issues for our health. But then, our mucosal barriers were never meant to encounter high numbers of particles over extended time periods. This is a problem that mankind has created and that mankind has to solve.

VII. CONCLUSION

The adverse health effects an exposure to particulate matter may have are well documented. The mechanisms by which particulates may affect human health, however, are not understood equally well. In particular, research addressing the impact of particulates on mucosal systems is still scarce; this is alarming as mucus is of critical importance for a multitude of physiological processes—and this includes constituting a barrier against pathogens and other noxious agents. Considering the ongoing and even increasing release of (man-made) particulate matter into our environment, it is evident that the prevalence of studies that systematically investigate the effect of particulates on mucosal systems, needs to increase—and that mankind has to quickly find ways to reduce particular emissions.

The authors have no conflicts to disclose.

Approval of ethics is not required.

DATA AVAILABILITY

Data sharing is not applicable to this article as no new data were created or analyzed in this study.