**2. Particulate matter inhalation: characteristics and associated diseases**

Among air pollutants, the one that has received greater emphasis in current research is fine particulate matter (PM2.5), consisting of pollutants formed from combustion processes [13]. Particulate matter<2·5 μm (PM2·5) is the most commonly implicated constituent that causes a disproportionate number of global deaths and contributes significantly to global disability. The global burden of disease study report indicated that ambient outdoor air pollution, particularly PM2·5, was the fifth leading risk factor for global mortality in 2015 [14].

The source of PM can be explained as either direct emission into the air or as conversion from gaseous precursors (such as sulfur dioxide, oxides of nitrogen, ammonia, and non-methane volatile organic compounds) released from both anthropogenic and natural sources. The chemical constituents in PM are commonly found to include inorganic ions (e.g., sulfates, nitrates, ammonium, sodium, potassium, calcium, magnesium, and chloride) and can be expanded further to include all varieties of constituents such as metals (including cadmium, copper, nickel, vanadium, and zinc), and polycyclic aromatic hydrocarbons (PAH) [15]. The harmful effects of PM2.5 on health depend on the exposure time and its concentration in the environment.

Ambient particles contain many soluble metals, including transition metals that are capable of redox cycling [16]. Transition metals are thought to be very important in PM cellular toxicity. The bioavailability of transition metals in PM and their redox properties, which are considered very important for the toxic effects and the oxidative damage [17]. PM2.5 can invade the respiratory tract and vascular system [18] and thus cause a systemic effect, acting as a potent inducer of ROS production and the release of pro-inflammatory cytokines (TNF- α, IL-6, IL-8) for circulation [19, 20].

Exposure to PM can occur in many forms, as described by Thompson [21]. As with any chemical contaminant, components of PM may enter the human body by three mechanisms: inhalation, dermal absorption, and ingestion. PM can be ingested by the direct consumption of contaminated beverages and food and during the clearance of particles removed from the airways via mucociliary transport. Another possible route of exposure to airborne PM is impaction or deposition to the skin. Deposition velocities are influenced by substrate surface properties (e.g., roughness, wetness, temperature, surface charge) and particle size-dependent. Inhalation is a rather obvious route of exposure, but to better understand the effects, it is crucial to consider the anatomy of the human airway, patterns of particle size-dependent deposition, and the fate of particles that have been inhaled.

Nevertheless, mainly, the size is determinant for the deposition characteristics and effects of the PM. During a pollution episode, each lung acinus could receive on average 30-million particles and each alveolus about 1500 particles (for 24 h exposure), of which 50% are being deposited. Lung airways and alveoli retain mostly PM2.5 rather than PM10 since the last one is frequently stopped in upper airway anatomic

*Environmental Particulate Air Pollution Exposure and the Oxidative Stress Responses: A Brief… DOI: http://dx.doi.org/10.5772/intechopen.101394*

structures. Also, analytical electron microscopy measurements showed that 96% of effectively retained particles in the lung parenchyma were PM2.5, and only 5% were ultrafine particles (0.1 μm), which means translocation to the pulmonary and systemic circulation, affecting other organs [17]. Thus, the size of PM and their retention play an essential role in the PM cytotoxic effects, including the site of deposition (upper or lower airways), bio-persistence solubility, and its composition [14].

In the USA, studies involving residual oil fly ash (ROFA), one of the components of PM2.5, have helped develop and refine this theory. ROFA, or residual oil fly ash, is the term used to refer to the primarily inorganic residues that remain after the incomplete oxidation of carbon compounds. ROFA contains about 10% by weight of water-soluble Fe, Ni, and V, and its intratracheal instillation in rats leads to aldehyde generation [16, 22].

ROFA particles are generally smaller than 2.5 μm in size and are chemically considered complex compared to other air pollution particles, as they contain sulfates, silicates, carbon, and nitrogen. In addition to the elements mentioned, it also has a large number of metals that are naturally present in fuels (petroleum, paraffin, and diesel oil) and remain when the volatile fraction is distilled [23]. This pollutant has been used in experimental studies as a surrogate particle to investigate the mechanisms of responses to PM inhalation in animals [24, 25]; as it is mainly made up of different metals, it promotes Fenton reactions, producing reactive oxygen species [22].

The harmful effects of PM2.5 on human health depend on the time of exposure and particles concentration in the environment. As expected, repetitive and long-duration exposure to higher doses of PM2.5 induces cumulative and persistent effects. However, subchronic exposure to low doses (5 μg/day, intranasally) of PM2.5 in animal models is sufficient to potentiate metabolic dysfunction in high-fat diet-fed mice, promoting glucose intolerance and increasing fasting glycemia and triglyceride levels [26].

### **3. The role of oxidative stress induced by PM inhalation in cells and tissues**

Oxidative stress is described as a biochemical imbalance, which occurs when the production of pro-oxidants such as free radicals or reactive oxygen species exceeds the body's natural antioxidant capacity, resulting in oxidative damage [27, 28] and consequently leading to the development of a broad spectrum of human diseases.

The main pro-oxidant agents are the reactive oxygen species (ROS), such as radicals, superoxide (O2− ) and hydroxyl (OH− ) radicals, and also some non-radical species derived from O2, such as the hydrogen peroxide (H2O2) [27]. Antioxidants can be classified as exogenous, obtained mainly through the diet, or endogenous, being produced by our body to avoid oxidative stress and consequent tissue destruction. Antioxidant defense systems can inhibit the oxidation of other molecules in the organism, transferring electrons from a substance to an oxidizing agent (ROS), acting in two lines: enzymatic antioxidant defenses [e.g., superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)] and non-enzymatic antioxidant defenses (e.g., vitamins A, C, and E; glutathione, α-lipoic acid; carotenoids, ubiquinone or coenzyme Q10).

ROS production is inherent in all aerobic species, primarily as a product of mitochondrial electron transport. At physiological levels, ROS are essential for the regulation of critical signaling pathways involved in cell growth, proliferation, differentiation, and survival, but excess ROS, mainly resulting from imbalanced antioxidant defense and detoxification, can lead to harmful (i.e., pathological) oxidative stress [29].

The most important pathophysiological mechanism that has been proposed to explain the association of PM exposure and the occurrence of respiratory infections, lung cancer, and chronic cardiopulmonary diseases is oxidative stress through the generation of ROS [17]. Recent studies using different methods have consistently demonstrated that peroxides and ROS are the critical mediators of particle toxicity [29].

The molecular events by which pulmonary oxidative stress occurs in response to particle inhalation involve the inflammatory process. ROS (e.g., superoxide, hydroxyl radical, nitric oxide, and peroxynitrite) generated in air pollution exposure induces oxidative tissue damage in target organs, with contributions of non-immune and immune cells in the inflammatory response. The role of protective proteins (e.g., surfactant, proteins, and antioxidants) in this process is highly complex and may differ depending on experimental models, especially in concomitant disease states [30].

The pieces of evidence, to date, suggest that ROS generation in response to PM2·5 could either involve disruption of cellular redox signaling and upregulation of endogenous ROS production resulting in exaggerated responses, as described in details in some reviews [14, 30].

Wang et al. [31] evaluated two classical mechanisms of oxidative stress and intracellular calcium overload to explore their roles in PM-induced endothelial cell apoptosis from the perspective of subcellular levels in endothelial cells. They showed that internalization of particles induces oxidative stress, followed by the disorder of subcellular structures, including endoplasmic reticulum (ER) stress, mitochondrial dysfunction, activated caspase pathways, which cause endothelial cell apoptosis. They also highlighted that antioxidants and calcium inhibitors confer protective effects. Also, ROS are generated during phagocytosis of the particles, leading to enhancement of oxidative stress and triggering the inflammatory response. Consequently, the activation of inflammatory signaling pathways results in the release of cytokines and other mediators that can further induce ROS production by activating endogenous enzymes, leading to a positive feedback loop, which can aggravate the effects triggered by PM exposure [32].

In summary, it is well known that in the lungs, exposure to PM triggers inflammation, endothelial activation, and oxidative stress, caused by the deposition of PM into the alveolar space in the lung, inducing the release of cytokines from alveolar macrophages. The probable sequence of events for PM-induced lung inflammation involves the following: injury to epithelial cells by ROS, possibly enhanced in the presence of metals via Haber-Weiss and Fenton chemistry; and activation of vascular endothelium and circulating leukocytes, circulating leukocytes; emigration of inflammatory cells from blood to tissue sites, promoting pro-inflammatory condition [32]. Moreover, even at low doses, it is also able to demonstrate early changes in the elastic and viscoelastic pulmonary mechanical components, such as worsening impedance, alveolar collapse, and histological changes, in addition to oxidative stress and inflammation [33].

An epidemiological study conducted by Hu et al. [34] exposed 768 participants to environmental levels of PM2.5, and assessed urinary levels of PAH metabolites and metals, and evaluated urinary 8-OHdG, a biomarker of endogenous oxidative damage to DNA, 8-iso-PGF2α a biomarker of excessive chemical lipid peroxidation in humans, and MDA for determination of lipid peroxidation levels. They discuss that particulate matter alone, its bound polycyclic aromatic hydrocarbons, and heavy metals induce increased oxidative stress on DNA and lipid [34].

Since PM can invade the bloodstream, particles exposure may also impact cardiovascular health. Air pollution increases the risk of myocardial infarction, stroke,

#### *Environmental Particulate Air Pollution Exposure and the Oxidative Stress Responses: A Brief… DOI: http://dx.doi.org/10.5772/intechopen.101394*

and acute heart failure. Since ROS act mainly inducing endothelial dysfunction, monocyte activation, and some proatherogenic changes in lipoproteins, which initiate plaque formation, this context favors thrombus formation because of an increase in coagulation factors and platelet activation [2, 35]. In this way, animal models of acute exposure to higher levels of particles in chambers showed that PM oxidative effects are associated with autonomic nervous system imbalance, which is avoided by the pre-treatment with drugs acting as sympathetic or parasympathetic blockers by antioxidant pre-treatment [36]. In addition, exposure to particles may induce electric alterations on the heart [37], suggesting an explanation of the increased risk of myocardial infarct under inadequate air quality for susceptible subjects [38].

In the lungs and heart, but also in other organs, oxidative stress may induce a cell stress response, characterized by the increased expression and release to the bloodstream of heat shock proteins (HSPs). High plasma levels of these proteins are simultaneously correlated with impaired energy balance, with an alteration in the pro/anti-inflammatory status and with an imbalance in the body's pro/antioxidant systems, and also has been used as a biomarker of cell stress response in diseases as obesity, hypertension, and diabetes [26, 39, 40].

HSPs are highly conserved proteins during species evolution and are found in eukaryotic and prokaryotic organisms. HSPs can be grouped according to their molecular weights into families (HSP110, HSP100, HSP90, HSP70, HSP60, and HSP30). Specifically, proteins from the 70 kDa HSPs family (HSP70) are highly conserved [41] and have a cytoprotective role in cells, as well as present anti-apoptotic and anti-inflammatory proprieties in various stressful conditions [42]. The increase in the synthesis of HSPs may show an increase in stress tolerance, preventing protein damage related to oxidative stress [43].

In this way, a study carried out by Kido et al. [44] suggested that inhalation of air pollution induces an increase in HSP70 in lung macrophages and also a systemic increase in blood HSP70 levels (a.k.a eHSP70 since it is located in extracellular fluids). These stress responses marked by alterations in both intracellular (iHSP70) and extracellular (eHSP70) suggest these proteins as a relevant immunological mediator that contributes to other aggravating factors (vascular dysfunction and cardiovascular events). For example, exposure to ROFA suspension for three consecutive days (750 μg) promoted an increase in plasma levels of eHSP70 associated with plasma oxidative stress, showing that HSP70 represents a potential inflammatory and indirectly an oxidative biomarker [45].

Furthermore, a study conducted by Goettems-Fiorin et al. [26] highlights that exposure to the PM2·5 potentiates metabolic dysfunction in mice treated with HFD, associated with altered cellular stress response, assessed by the ratio [eHSP70]/[iHSP70], called H-index, a biomarker of the low grade chronic inflammatory state, increasing the risk of type 2 diabetes development. The study shows a positive correlation between adiposity, increased body weight and glucose intolerance, and increased glucose and triacylglycerol plasma levels. And when evaluating the H-index in the pancreas, demonstrated that the pancreas exhibited lower iHSP70 expression, accompanied by a 3.7-fold increase in the plasma to pancreas [eHSP72]/[iHSP70] ratio, highlighting that exposure to PM2.5 markedly enhances metabolic dysfunction in HFD-treated mice.

Thus, at least in animal models, HSP70 has been used recently as a biomarker for early assessments of harmful health effects, such as those caused by exposure to air pollution [26, 45, 46]. In humans, the study of Chao et al. [47] with drivers showed an increase in pro-inflammatory activity, with an increase in plasma levels of TNF-α related to the level of pollution exposure. In terms of inflammation, exposure to air pollutants

also promotes responses by cytokine signaling, and PM exposure is associated with the expression of IL1, IL6, and TNF-α [48], and the imbalance pro-inflammatory response in immune cells is also associated with unbalanced eHSP70/iHSP70 ratio [49, 50].

In this way, the intensity of PM2.5 exposure and its association with other risk factors might reduce HSP70 levels [51–53]. Independently, metabolic impaired condition and PM exposure may increase the eHSP70/iHSP70 ratio [26, 50, 53–55]. Since eHSP70 is related to increase pro-inflammatory signaling, while iHSP70 is had antiinflammatory roles by inhibiting NF-kB signaling for inflammatory mediators [42], the unbalance in eHSP70/iHSP70 ratio (increased values in favor of eHSP70) indicates an organism under stress condition without an effective stress response [39, 50, 56].
