Air pollution could be a risk factor for the development of multiple sclerosis (MS), a new study conducted in Italy has found.
The research, presented today at the European Academy of Neurology (EAN) Virtual Congress, detected a reduced risk for MS in individuals residing in rural areas that have lower levels of air pollutants known as particulate matter (PM).
It showed that the MS risk, adjusted for urbanisation and deprivation, was 29% higher among those residing in more urbanised areas.
The study sample included over 900 MS patients within the region, and MS rates were found to have risen 10-fold in the past 50 years, from 16 cases per 100,000 inhabitants in 1974 to almost 170 cases per 100,000 people today.
Whilst the huge increase can partly be explained by increased survival for MS patients, this sharp increase could also be explained by greater exposure to risk factors.
The analysis was conducted in the winter, given that this is the season with the highest pollutant concentrations, in the north-western Italian region of Lombardy, home to over 547,000 people.
Commenting on the findings at the EAN Virtual Congress, lead researcher Professor Roberto Bergamaschi explained, “It is well recognised that immune diseases such as MS are associated with multiple factors, both genetic and environmental.
Some environmental factors, such as vitamin D levels and smoking habits, have been extensively studied, yet few studies have focused on air pollutants.
We believe that air pollution interacts through several mechanisms in the development of MS and the results of this study strengthen that hypothesis.”
Particulate matter (PM) is used to describe a mixture of solid particles and droplets in the air and is divided into two categories. PM10 includes particles with a diameter of 10 micrometres of smaller and PM2.5 which have a diameter of 2.5 micrometres or smaller.
Both PM10 and PM2.5 are major pollutants and are known to be linked to various health conditions, including heart and lung disease, cancer and respiratory issues. According to the World Health Organisation, 4.2 million deaths occur every year because of exposure to ambient (outdoor) air pollution.
Three different areas were compared within the study region based on their levels of urbanisation, of which two areas were found to be above the European Commission threshold of air pollution.
“In the higher risk areas, we are now carrying out specific analytical studies to examine multiple environmental factors possibly related to the heterogeneous distribution of MS risk”, added Professor Bergamaschi.
The number of people living with MS around the world is growing, with more than 700,000 sufferers across Europe.
The vast majority (85%) of patients present with relapsing remitting MS, characterised by unpredictable, self-limited episodes of the central nervous system. Whilst MS can be diagnosed at any age, it frequently occurs between the ages of 20-40 and is more frequent in women. Symptoms can change in severity daily and include fatigue, walking difficulty, numbness, pain and muscle spasms.
Air pollution collectively describes the presence of a diverse and complex mixture of chemicals, particulate matter (PM), or of biological material in the ambient air which can cause harm or discomfort to humans or other living organisms.
The sources of air pollution can either be natural (e.g., volcanic eruptions) or manmade (e.g., industrial activities), and air pollution emerges as a serious health problem especially in rapidly growing countries.
Millions of people worldwide are chronically exposed to airborne pollutants in concentrations that are well above legal safety standards [1]. Therefore, morbidity and mortality attributable to air pollution continue to be a growing public health concern worldwide. Air pollution ranks eighth among the leading risk factors for mortality and accounts for 2.5% of all deaths in developed countries [2].
The World Health Organization (WHO) estimates that air pollution is responsible for over 3 million premature deaths each year [3]. Epidemiological and observational studies identified a strong link between the exposure to contaminants in the ambient air and adverse health outcomes, such as hospitalization and mortality [4].
Exposure to air pollutants has been associated with marked increases in cardiovascular disease morbidity and deaths resulting from myocardial ischemia, arrhythmia, heart failure, and respiratory diseases such as lung cancer and asthma [3, 4].
About a decade ago, the central nervous system (CNS) has also been proposed to be a target organ for the detrimental effects of airborne pollutants [5]. Indeed, emerging evidence from recent epidemiological, observational, clinical, and experimental studies suggest that certain neurological diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and stroke, may be strongly associated with ambient air pollution.
Mechanistically, air pollution may affect the nervous system through a variety of cellular, molecular, and inflammatory pathways that either directly damage brain structures or lead to a predisposition to neurological diseases.
Although ischemic stroke (chronic exposure to ambient air pollution), multiple sclerosis (MS, exposure to second-hand smoking), and PD (manganese content in the ambient air) are currently the only neurological disorders for which a strong link to ambient air pollution has been established, it is not unlikely that other CNS disorders are also attributable to air pollution [6–8].
It has been suggested from epidemiological and observational studies that exposure to airborne pollutants can contribute to neurodegenerative disease processes already from early childhood on, especially if the individuals are chronically exposed to the contaminants [1, 9–11].
Air pollutants affect the CNS either directly by transport of nanosized particles into the CNS or secondarily through systemic inflammations. Either of the effects can be caused by the physical characteristics of the particle itself or by toxic compounds that adsorb on the particles [12, 13].
Although the exact mechanisms underlying brain pathology induced by air pollution are not fully understood, several lines of current evidence point out that neuroinflammation, oxidative stress, glial activation, and cerebrovascular damage might be the primary pathways [1, 14].
In this paper, we provide an overview of the different classes of air pollutants and their potential ways to entry by which they could get into contact with the CNS. We summarize findings of epidemiological, observational, clinical, and experimental studies which describe a link between air pollution and neurological diseases or neurodevelopmental disturbances. Finally, we summarize the current understanding of the adverse effects of air pollutants on the nervous system and mental health on a cellular and molecular level.
Components of Air Pollution
Air pollution represents a diverse mixture of substances including PM, gases (e.g., ground-level ozone, carbon monoxide, sulfur oxides, and nitrogen oxides), organic compounds (e.g., polycyclic aromatic hydrocarbons and bacterial endotoxins), and toxic metals (e.g., vanadium, lead, nickel, copper, and manganese) that can be found in outdoor and indoor air [1, 15].
Among these, PM and ground-level ozone, which are formed primarily from nitrogen oxides and volatile organic compounds, appear to be the most widespread and harmful components. Of those, PM is especially relevant for nervous system damage and can be found as a mixture of solid particles and liquid droplets, that are suspended in the air [1]. Most individual components of atmospheric PM are not especially dangerous and some major constituents, such as sodium chloride, are harmless [16].
PM is characterized by its size and aerodynamic property which is directly related to its biological effects. For instance, only particles less than 10 μm in diameter can be inhaled deep into the lungs, whereas larger particles usually get trapped in the upper airways. Generally, coarse particles with an aerodynamic diameter of 2.5 to 10 μm (PM10), fine particles of less than 2.5 μm (PM2.5), and ultrafine (UFPs), or nano-sized (NP) particles of less than 0.1 μm can be classified [15, 17].
Road and agricultural dust, tire wear emissions, products of wood combustion, construction and demolition works, and mining operations are the primary sources of PM10. PM2.5 particles commonly originate from oil refineries, metal processing facilities, tailpipe and brake emissions, residential fuel combustion, power plants, and wild fires [15].
They are formed from gas and condensation of high-temperature vapors that are formed during combustion and industrial activities. PM2.5 can be composed of both organic and inorganic compounds, including sulfates, nitrates, carbon, ammonium, hydrogen ions, lipopolysaccharide (LPS), metals, and water [1]. Diesel exhaust particles (DEPs), however, are the major components found among ambient fine particles.
UFPs are mostly combustion-derived NPs, which can be produced by internal combustion engines, power plants, incinerators, and other sources of thermodegradation. They can carry soluble organic compounds, polycyclic aromatic hydrocarbons, and oxidized transition metals on their surface [18].
UFPs have distinct features that render them more dangerous than other PMs. For instance, they have been shown to inhibit phagocytosis and to stimulate inflammatory responses [16].
Although the effects of UFPs have been studied less extensively than those of PM2.5 and PM10, there is evidence that the size of the particles is negatively correlated with their adverse health effects [19].
Indeed, ambient UFP concentrations are found to be directly correlated with mortality [20]. Current national air quality standards are based on the mass concentration of PM. However, when compared to fine particles at similar mass concentrations in the air, UFPs are much more numerous and have a larger combined surface area, enhanced oxidant capacity, greater inflammatory potential, and higher pulmonary deposition efficiency [16, 17, 21, 22].
A major risk of UFPs arises from the fact that they are not filtered out during their passage through the nose and bronchioles but are able to penetrate deep into the lung where they eventually enter the blood circulation and can get distributed throughout the body.
Entry of Air Pollutants into the Central Nervous System
Sustained exposure to significant levels of airborne UFPs, PM, and LPS may result in the direct translocation of these pollutants to the CNS where they can result in neuropathology through a variety of pathways and mechanisms (Figure 1).
Alternatively, air pollutants might not enter the CNS directly, but could exert adverse effect on the CNS by triggering the release of soluble inflammatory mediators from primary entry organs or secondary deposition sites. The release of inflammatory agents could then lead to or alter the susceptibility for neuroinflammation and neurodegeneration in the CNS.
Once taken up by the body, fine PM or NPs could rapidly enter the circulatory system with the potential to directly affect the vascular system. For instance, NPs could be inhaled and cross the alveolar-capillary barrier in the lungs.
The ability of NPs to cross this barrier is influenced by a number of factors that include the size of the particles, their charge, their chemical composition as well as their propensity to form aggregates. Even though the translocation of inhaled or instilled NPs across the alveolar-capillary barrier has been clearly demonstrated in animal studies for a range of NPs [23, 24], it has been more difficult to directly demonstrate this mechanism in humans [3].
Regardless of the route of entry, NPs that reach the circulation could directly affect vascular endothelium cells by creating local oxidative stress or by causing proinflammatory effects similar to those seen in lung tissue. Inflammatory mediators that are produced in the respiratory tract as a consequence of chronic pollutant-induced epithelial and endothelial injury can lead to systemic inflammation [25].
The systemic inflammation is accompanied by the production of proinflammatory cytokines such as tumor necrosis factor alpha (TNFα), interleukin-6 (IL-6), and interleukin-1beta (IL-1β), for which blood vessels in the brain exhibit constitutive and induced expression of receptors [1, 26].
The cytokines could thus activate cerebral endothelial cells, disrupt the blood-brain barrier (BBB) integrity, or trigger signaling cascades that lead to the activation of mitogen-activated protein (MAP) kinase, and nuclear factor kappa B (NFκB) transcription factor-mediated pathways.
Disruption of the BBB could then be followed by trafficking of mast cells and inflammatory cells expressing CD163, CD68, and HLA-DR to the damaged site [10]. In addition, circulating cytokines that are released by inflamed peripheral organs or endothelial cells could stimulate peripheral innate immune cells, activate peripheral neuronal afferents, or enter the brain by diffusion and active transport thereby worsening the condition synergistically [27, 28].
Accordingly, brain tissue samples from individuals residing in highly polluted areas show an increase in the number of infiltrating monocytes or activated microglia, increased expression of IL-1β, BBB damage, endothelial cell activation, and brain lesions in the prefrontal lobe [10, 11].
Airborne LPSs may induce neuroinflammatory responses directly by activating the brain’s innate immune system. The effect of LPS on neuroinflammation is well studied in a bacterial endotoxin/LPS-based experimental model of PD that constitutes an important tool to delineate the mechanisms of neuroinflammation-mediated loss of dopaminergic neurons [29].
This system could also be exploited in combination with exposure to other environmental toxins and air pollutants. Brain uptake of circulating LPSs is usually low, and most effects of peripherally administered LPS are likely to be mediated through LPS receptors located outside the BBB [30].
Thus, LPSs might stimulate afferent nerves, act at circumventricular organs, or alter the permeability of the BBB. Circumventricular organs are specialized brain structures located around the third and fourth ventricle. They are highly vascularised and lack a BBB; therefore, they allow for a direct uptake of chemicals circulating in the blood stream by neuronal cells [31].
The very small UFPs on the other hand easily penetrate cell membranes because of their large surface-to-volume ratio, which also enables them to traverse the classical barriers in the lung and the brain.
Their ability to cross cell membranes easily explains why PM can be found inside neurons or erythrocytes [1, 32]. It has also been proposed that the close contact between endothelial cells and erythrocytes could represent a route for the exchange of PM between activated endothelial cells and UFP-loaded erythrocytes [1, 33, 34].
Another important and more direct route for UFPs to enter the nervous system is through the olfactory mucosa, which is a neuronal epithelium that is in direct contact with the environmental air [35–37].
Thus, fine and UFPs may reach the brain through olfactory receptor neurons or the trigeminal nerve. Olfactory receptor neurons are bipolar sensory neurons that mediate the sense of smell by conveying sensory information from the nose to the CNS.
The olfactory epithelium is covered by a layer of sustentacular cells, but olfactory sensory neurons extend their dendrites into the mucous layer covering the olfactory epithelium where they directly interact with odorants inhaled with the air. Nasally inhaled pollutants that reach the olfactory mucosa could enter the cilia of olfactory receptor neurons by pinocytosis, simple diffusion, or receptor-mediated endocytosis.
Once incorporated into sensory neurons, they could be transported by slow axonal transport along the axons to the olfactory bulb [38]. From there, pollutants could be transported further into the CNS along mitral cell axons that project from the olfactory bulb to multiple brain regions, including the olfactory cortex, the anterior olfactory nucleus, the piriform cortex, the amygdale, and the hypothalamus.
Accordingly, UFPs have been observed in human olfactory bulb periglomerular neurons and trigeminal ganglia capillaries [10]. Similarly, a decreasing gradient of metal (vanadium and nickel) deposition and accompanying tissue damage from the nose to the brain has been reported in the canine nervous system, confirming the importance of the nasal route for the entry of air pollutants into the CNS [39].
Controlled exposures of rats to UFPs and metals also demonstrated their accumulation in the olfactory bulb [40–42]. Taken together, these findings suggest that NPs can be taken up directly by the olfactory mucosa and enter the CNS or the cerebrospinal fluid by bypassing the circulatory system [12].
Uptake through the nose might even be enhanced by additional pollutant-induced systemic inflammation by deteriorating the olfactory mucosal barrier, which would result in increased neuropathology.
Additional direct neuronal entry routes for NPs have been described that involve the retrograde and anterograde transport in axons and dendrites such as the transport of inhaled NPs to the CNS via sensory nerve fibers that innervate the airway epithelia [12]. Ground-level ozone exposure activates the CNS through the vagal nerves without the involvement of the thoracic spinal nerves [43]. PM-related LPS is likely to play an important role in these pathways, as shown by vagal upregulation of CD14 [44].
Even though the translocation rate of NPs from their site of entry to secondary organs might be rather low, continuous or chronic exposure to NPs may result in their accumulations in the brain as a secondary target organ in significant amounts [12].
Thus, it is also important to obtain data on the retention characteristics of NPs in both primary and secondary target organs, including associated elimination and clearance pathways [12]. With regard to the CNS, no data on NP elimination are available yet. It is conceivable, however, that CSF circulation provides an excretory pathway for NPs that enter via neuronal uptake.
Usually, the CSF serves as a fluid cushion for the brain, but also distributes substances to all brain regions and acts as an elimination route for metabolic waste products [45]. NPs could be eliminated from the CSF through the same mechanisms: uptake of CSF by the blood circulatory system through arachnoid vili or via the nasal lymphatic system. The exact details of NP clearance from the brain, however, await future investigation [12].
Air Pollution and Neurological Disease
Results about the direct effects of air pollutants and airborne particles on neuronal cells have been reported from experimental studies in vitro, using cell culture systems and in vivo, using inhalation and instillation paradigms in rodents as well as from epidemiological and controlled clinical studies in humans.
Experimental Studies
In Vitro Studies
A variety of in vitro studies assessed the potential toxic effects of air pollutants (Table 1), by measuring changes in cell viability, alterations of apoptosis, the dysfunction of mitochondria, the production of reactive oxygen species (ROS), or the production of pro-inflammatory cytokines as sensitive identifiers [1].
Varying degrees of proinflammatory- and oxidative stress-related cellular responses and decreased cell viability were reported upon stimulation with laboratory-generated or filter-collected ambient air particles in different cell culture systems [42].
Of particular interest are studies utilizing neuronal and microglial cell lines or primary cultures of those cells that were exposed to concentrated ambient air particles (CAPs), diesel exhaust particles (DEPs), toxic gases, such as ozone, bacterial endotoxins, such as LPS, or toxic elements, such as manganese.
All investigated neuronal, glial or cerebral endothelial cell types were shown to be targets of the toxic effects of air pollutants [46–48]. However, the underlying mechanisms could be rather complex, and some insight into the interaction of different cell types was derived from coculture systems.
For instance, it was shown that the neurotoxic effects of DEPs on dopaminergic neurons could be either direct or indirect via the release of inflammatory mediators and ROS from activated microglial cells [46, 49]. Interestingly, pretreatment of neuron-glia cocultures with LPS increased the vulnerability of the cells to the toxic effects of DEP, while DEPs alone were not harmful [49].
An important aspect of in vitro toxicity studies is the establishment of dose-response relationships. For instance, low concentrations (20–40 μg/mL of gas per mL of complete medium) of oxygen-ozone were not toxic to astroglial cells, while higher concentrations (60 μg/mL) severely decreases cell viability [48].
Transcriptomic and proteomic profiling of cultured cells upon exposure to CAPs may provide insights into alterations of gene and protein expression. One such study demonstrated the upregulation of inflammatory and innate immunity pathway components in mouse immortalized BV2 cells when exposed to CAPs [50].
Likewise, the expression profiles of microRNAs, which emerged as crucial mediators of posttranscriptional gene regulation, might change during exposure to air pollutants [51]. Indeed, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a common environmental contaminant and explosive nitroamine that is widely used in military ammunition, has been shown to change brain microRNA expression in exposed mice [52].
The rapidly growing number of engineered nanoparticles (ENPs) and nanomaterials (NMs) might also contribute to air pollution as new nanotechnologies are constantly developed, and NMs are used increasingly in daily life through the advent of new products.
In addition, ENPs are extensively tested for their usefulness in medical diagnostic and therapeutic applications. Although no human ailments have been directly attributed to NMs so far, preliminary experimental studies indicate that NMs could initiate adverse biological responses and that NPs could have toxicological properties [53].
Thus, ENPs constitute a novel neurotoxic risk and several in vitro studies could demonstrate adverse effects of ENPs on CNS cells (not included in Table 1). For instance, titanium dioxide, aluminum oxide, and nanosized silica particles were shown to decrease cell viability and to increase apoptosis in neuronal and endothelial cell cultures [54–58].
These substances also increased the amount of ROS, which resulted in concomitant activation of microglia [54–59]. An important point in in vitro nanoneurotoxicity studies is therefore the necessity to accurately characterize particle size, as particles of different size might exert different effects or similar effects to different degrees.
In addition, a controlled investigation of the physicochemical properties of the NPs over time and their interactions with culture media should also be considered [60, 61]. Although NPs in environmental air samples might be much more heterogeneous, epidemiological and toxicological studies with airborne ultrafine particles can be viewed as the basis for the expanding field of nanotoxicology [42].
In vitro studies bear several distinct advantages for studying neurotoxic effects of air pollutants because the technology is cheap, the cultured cells grow rapidly, and the assays provide reproducible results. However, many times immortalized cell lines are used, which might not correctly reflect the more complex responses of native CNS cells or of neurons in their natural complex environment.
Unfortunately, long-term and large-scale cultures of primary CNS cells are still challenging and thus might not be useful for high-throughput screening of toxicological effects. The emerging field of induced pluripotent stem cells, which can be easily derived from somatic cells such as dermal fibroblasts and keratinocytes, may provide a solution to this problem and induced pluripotent stem cells could soon emerge as a novel experimental paradigm for human neurotoxicity studies [62, 63].
Despite their advantages, in vitro studies have also important limitations, some of which are methodological. The interpretation and cross-comparison of results from different research groups might be hampered because of the use of particles with different chemical compositions or different culture cells.
The duration of exposure and concentrations might differ across laboratories. More importantly, however, responses of cultured cells might not faithfully reflect the responses of the entire body system or target organ. In general, ultraphysiological doses of air pollutants are used in cell cultures studies and the long-term study of the effect of chronic exposure to low doses of potentially toxic material is not feasible.
Organotypic cell cultures and tissue explant cultures might be more useful in this regard since the integrity of tissue of interest is fully or partially preserved. Because systemic effects and biodistribution of air pollutants cannot be investigated in in vitro assays, in vivo studies provide additional and important information on the adverse effects of air pollutants.
Conclusions and Future Prospects
Air pollutants have been, and continue to be, major contributing factors to chronic diseases and mortality, thereby dramatically impacting public health. Air pollution is a global problem and has become one of the major issues of public health as well as climate and environmental protection.
The effects of air pollutants are thus at a high level of interest for scientific, governmental, and public communities. An increasing number of people are exposed to a complex mixture of inhalable NPs and toxic chemicals occupationally or as a result of man made and natural disasters, such as war, fires, and volcanic eruptions [210, 211].
Air pollution is increasingly recognized as an important and modifiable determinant of cardiovascular and respiratory diseases in urban communities [3, 16]. Although adverse cardiopulmonary outcomes have been the focus of many studies, air pollution-related damage to the CNS has been widely neglected. However, there is mounting evidence that air pollution also contributes to CNS damage or increased progression of neurodegenerative disorders.
The data discussed as part of this critical update highlight that UFPs rapidly translocate from the lungs into the cells and into the blood circulation. There is good evidence that oxidative stress occurs in other organs, such as the heart and the brain.
The breadth, strength, and consistency of the preclinical and clinical evidence provide a compelling argument that air pollution, especially traffic-derived pollution, causes CNS damage and that there is a clear link between air pollution and neurological diseases. Airborne particles cause neuropathology, which seem to be mediated by direct or indirect proinflammatory and oxidative responses.
Both, the physical characteristics of the particle itself and toxic compounds adsorbed on the particle may be responsible for the damage. The time of exposure has a key role in damage. Minimum doses of pollution can be handled by the organism when this exposure is acute, but the same doses administered chronically lead to an oxidative stress state that can produce neurodegeneration.
Astroglia, cerebral endothelial cells, and microglia in particular respond to components of air pollution with chronic activation, inflammation, and oxidative stress [1]. CNS effects can be chronic, can begin in early childhood, and may accumulate with age [1].
Given the enormous complexity of the CNS and the complex nature of air pollution, the resulting CNS pathology can have many underlying causes and pathways and could be due to synergistic interaction of multiple pathways and mechanisms making it difficult to pinpoint a clear stimulus-response relationship.
While epidemiological data link increased risk for stroke, MS, and PD to the exposure to specific air pollutants, further experimental and mechanistic studies aiming at the association between the components of air pollution and the development of CNS diseases are of pressing importance for mental health [1]. The adverse effects of the complex mixtures of polluted air components are poorly understood.
For instance, the contribution of direct effects of airborne UFPs to CNS injury remains to be worked out in detail, and data on the presence of UFPs in the human CNS are still lacking to date. The biological studies can be strengthened by the use of recent discovery tools and platforms, such as proteomics and genomics, to develop biomarkers for toxicity screening [142].
The main problems that are encountered in testing air pollutants toxicity in humans are dosimetry, the lack of appropriate standardized protocols, and good quantitative descriptions of real-world exposure conditions [60, 142]. Novel detection methods need to be developed for exposure assessment and dosimetry calculation.
Our current knowledge provides a basis for much more extensive epidemiological, forensic, and toxicological studies aimed at identifying the underlying mechanisms of neural damage, and strengthening of the association between chronic exposure to air pollutants, and the risk of developing neurological diseases.
However, epidemiologic and observational data are limited by imprecise measurements of pollution exposure, the potential of environmental, and social factors to confound the apparent associations.
Since genetic susceptibility is likely to play a role in response to air pollution, gene-environment interaction studies can be a tool to explore the mechanisms and the importance of molecular pathways for the association between air pollution and CNS damage [198]. Inconsistencies between studies sometimes prevent us from drawing firm conclusions.
The limited sample size of most studies, difficulty in quantifying exposure, providing a qualitative description of active components from complex environmental air samples, method of ascertainment, time of measurement, and collinearity between pollutants make difficult to use for the study of gene by gene interactions [200].
More studies and more intensive collaborationsare needed to generate larger and more diverse cohorts and standardized data that would allow us to draw stronger conclusions [198]. The roles of gene-air pollution interactions and epigenetic mechanisms need to be considered [200]. Better understanding of the mediators and mechanisms of CNS injury due to air pollution will help to develop preventive and treatment strategies for the protection of individuals at risk. Improving air quality standards, minimizing personal exposures, and the redesign of engine and fuel technologies will also reduce air pollution and its consequences for neurological morbidity and mortality.
Provided by European Academy of Neurology
