Chapter 5. Possible Pathways/ Modes of Action 

The mechanisms underlying pulmonary effects of inhaled PM have been well-studied in the 

laboratory and there is general agreement regarding the key roles played by cellular injury and 

inflammation. These pathways are initiated following the deposition of inhaled particles on 

respiratory tract surfaces. Since most of these studies were conducted at concentrations of PM much 

higher than ambient levels, there is some question regarding the relevance of these responses and 

mechanisms to ambient exposures. 

Interestingly, inhaled PM may also affect the cardiovascular, hematopoietic and other systems. 

Mechanisms underlying these extra-pulmonary effects are incompletely understood. However, 

pulmonary inflammation can lead to systemic inflammation and pulmonary reflexes can activate the 

autonomic nervous system (ANS). These latter responses may mediate cardiovascular and other 

systemic effects, as will be discussed below. In addition, it has been proposed that PM or soluble 

components of PM reach the circulation by translocating across the epithelial and endothelial 

barriers of the respiratory tract. In this way, PM or its components may interact directly with cells in 

the vasculature and blood and be transported to the heart and other organs. At this time, evidence 

clearly supports the translocation of small solutes following inhalation exposures and the 

translocation of soluble components of PM following some high dose exposures involving 

intratracheal (IT) instillation. However, there is insufficient evidence to support translocation of 

appreciable amounts of intact particles following inhalation exposures at lower concentrations 

(Section 4.3.3.1). Future studies will be required to resolve these issues. 

The following sections discuss biological pathways which comprise proposed modes of action 

for the pulmonary and extra-pulmonary effects of inhaled PM. Overall themes are emphasized and 

supportive evidence from new in vitro and in vivo animal studies is cited. The characterization of 

evidence here is for PM in general, since most of the potential pathways or modes of action do not 

appear to be specific to a particular size class of PM. However, characteristics of ultrafine particles 

(UFPs) may allow for unique modes of action or effects disproportionate to their mass, as will be 

described below. Recent studies suggest an enhanced potential of this size class of PM to cause 

adverse effects; however evidence supporting this hypothesis is limited. Finally, a compilation of 

results from new inhalation studies which are relevant to ambient PM exposures and which confirm 

and extend these proposed mechanisms is found at the end of this chapter. Detailed descriptions of 

these key new studies are found in Chapters 6 and 7. 

5.1. Pulmonary Effects 

5.1.1. Reactive Oxygen Species 

A great deal of research interest has focused on the role of reactive oxygen species (ROS) in 

the initiation of pulmonary injury and inflammation following exposure to PM. Numerous studies 

have demonstrated PM oxidative potential in in vitro and in vivo assay systems (Ayres et al., 2008, 

155666; Cho et al., 2005, 087937; Shi et al., 2003, 088248; Tao et al., 2003, 156111). Both redox 

active surface components, such as metals and organic species, and the surface characteristics of 

crystal structures have been shown to contribute to oxidative potential (Jiang et al., 2008, 156609; 

Tao et al., 2003, 156111; Warheit et al., 2007, 090482). In this way, PM may be a direct source of 

ROS in the respiratory tract (Figure 5-1). 

 

 Note: Hyperlinks to the reference citations throughout this document will take you to the NCEA HERO database (Health and 

Environmental Research Online) at http://epa.gov/hero. HERO is a database of scientific literature used by U.S. EPA in the process of 

developing science assessments such as the Integrated Science Assessments (ISA) and the Integrated Risk Information System (IRIS). 

December 2009 5-1 
Figure 5-1. PM oxidative potential. 

PM may also act as an indirect source of ROS in the respiratory tract by stimulating cells to 

produce ROS (Ayres et al., 2008, 155666; Tao et al., 2003, 156111) (Figure 5-2). This may explain 

the observation that the oxidative potential of isolated PM does not always correlate with cellular or 

tissue oxidative stress induced by PM exposure. Exposure to PM increases intracellular production 

of ROS by a variety of mechanisms. For example, PM interaction with cell surfaces results in 

stimulation of NADPH oxidase in macrophages (i.e., the respiratory burst) (Dostert et al., 2008, 

155753) and in epithelial cells (Amara et al., 2007, 156212; Becher et al., 2007, 097125; Tamaoki et 

al., 2004, 157040). Absorption of PM soluble components (e.g., PAH, transition metals) by 

respiratory tract cells can occur (Penn et al., 2005, 088257) and be followed by microsomal 

transformation of PAHs to quinones or by redox cycling of transition metals with production of 

intracellular ROS (Molinelli et al., 2002, 035347; Xia et al., 2004, 087486). Disruption of 

intracellular iron homeostasis with the subsequent generation of ROS has also been demonstrated 

following PM exposure (Ghio and Cohen, 2005, 088272). In some cases, mitochondria serve as the 

source of ROS in response to PM (Huang et al., 2003, 156573; Risom and Loft, 2005, 089070; 

Soberanes et al., 2006, 156991; Soberanes et al., 2009, 190483). Furthermore, PM interaction with 

cells can lead to the induction of nitric oxide synthase (Becher et al., 2007, 097125; Lindbom et al., 

2007, 155934; Xiao et al., 2005, 156164; Zhao et al., 2006, 100996) and the production of nitric 

oxide and other reactive nitrogen species (RNS). 

December 2009 5-2 
Figure 5-2 PM stimulates pulmonary cells to produce ROS/RNS. 

Although all size fractions of PM may contribute to oxidative and nitrosative stress, UFPs may 

contribute disproportionately to their mass due to their large surface/volume ratio. The relative 

enrichment of redox active surface components, such as metals and organics, per unit mass may 

translate to a relatively greater oxidative potential of UFPs compared with larger particles with 

similar surface components. In addition, the greater surface per unit volume could potentially deliver 

relatively more adsorbed soluble components to cells. These components may undergo intracellular 

redox cycling following cellular uptake. Furthermore, per unit mass, UFPs may have more 

opportunity to interact with cell surfaces due to their greater surface area and their greater particle 

number compared with larger PM. These interactions with cell surfaces can lead to ROS generation, 

as described above. Recent studies have also demonstrated that UFPs have the capacity to cross 

cellular membranes by non-endocytotic mechanisms involving adhesive interactions and diffusion 

(Geiser et al., 2005, 087362), as described in Chapter 4. This may allow UFPs to interact with or 

penetrate intracellular organelles. 

In general, high levels of intracellular ROS/RNS can lead to irreversible protein modifications, 

loss of cellular membrane integrity, DNA damage and cellular toxicity. Lower levels of ROS/RNS 

may cause reversible protein modifications that trigger intracellular signaling pathways and/or 

adaptive responses. Thus PM-dependent generation of ROS may be responsible for a continuum of 

responses from cell signaling to cellular injury. 

5.1.2. Activation of Cell Signaling Pathways 

Activation of cell signaling pathways by ROS/RNS has received increasing attention by 

numerous investigators over the years. An early example was provided by Kaul and Forman (1996, 

155892) who demonstrated that respiratory burst-derived H2O2 activates the transcription factor 

December 2009 5-3 
nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Numerous studies since then 

have demonstrated that PM, which serves as both a direct and indirect source of ROS/RNS, activates 

cell signaling pathways by this mechanism. 

PM also has the potential to activate cell signaling by mechanisms that are independent of 

ROS/RNS. For example, PM delivers water-soluble components, such as endotoxin and zinc (Zn), to 

cell surfaces. Endotoxin binds to toll-like receptors on alveolar macrophages and other cells, 

resulting in the upregulation of cytokines (Becker et al., 2002, 052419). Zn, a transition metal which 

does not redox cycle, inhibits protein tyrosine phophatases in airway epithelial cells resulting in a 

cascade of cell signaling events (Tal et al., 2006, 108588). Similarly, PM-mediated delivery of lipid 

soluble components such as PAH results in binding and activation of the arylhydrocarbon receptor 

(AhR). AhR is a transcription factor responsible for the upregulation of CYP1A1, a cytochrome 

oxidase involved in PAH biotransformation to metabolites capable of forming DNA adducts or 

eliciting oxidative stress responses (Rouse et al., 2008, 156930). In addition, interaction of PM with 

cell surfaces may activate cell signaling by perturbation of the cytoskeleton, adherence, 

internalization, or receptor-mediated pathways. 

Recent studies involving PM exposures have focused on intracellular pathways involving 

protein kinases, such as mitogen-activated protein kinase (MAPK) (Bayram et al., 2006, 088439; 

Lee et al., 2005, 156682; Roberts et al., 2003, 156051; Soberanes et al., 2009, 190483); AKT (Ahsan 

et al., 2005, 156200); src (Cao et al., 2007, 156322) and epidermal growth factor receptor (Blanchet 

et al., 2004, 087982; Cao et al., 2007, 156322; Tamaoki et al., 2004, 157040), as well as ras 

(Tamaoki et al., 2004, 157040), toll-like receptors (Becker et al., 2002, 052419; Becker et al., 2005, 

088590), protein tyrosine phosphatases (Tal et al., 2006, 108588), phospholipases A2 (Lee et al., 

2003, 156678), calcium (Agopyan et al., 2003, 155649; Brown et al., 2004, 155705; Brown et al., 

2004, 088663; Geng et al., 2005, 096689; 2006, 097026; Sakamoto et al., 2007, 096282), caspases 

(Soberanes et al., 2006, 156991; Zhang et al., 2007, 156179), poly (ADP-ribose) polymerase family 

member 1 (PARP-1) (Zhang et al., 2007, 156179) and histone acetylation (Gilmour et al., 2003, 

096959). The transcription factors regulated by these pathways, including NF-κB (Bayram et al., 

2006, 088439; Lee et al., 2005, 156682; Takizawa et al., 2003, 157039), activator protein 1 (AP-1) 

(Donaldson et al., 2003, 156408), signal transducers and activators of transcription protein (STAT) 

(Cao et al., 2007, 156322), antioxidant response element (ARE) (Li and Nel, 2006, 156694), and 

AhR (Rouse et al., 2008, 156930) have also been studied following PM exposures. Activation of 

these intracellular pathways and transcription factors leads to the upregulation of genes responsible 

for inflammatory, immune and acute phase responses as well as genes responsible for antioxidant 

defense and xenobiotic metabolism. 

5.1.3. Pulmonary Inflammation 

Following PM exposure, transcription factor activation in macrophages and epithelial cells 

stimulates the synthesis and release of soluble mediators involved in inflammatory and immune 

responses including cytokines, chemokines, proteases and eicosanoids (Figure 5-3). These 

substances play a role in recruiting inflammatory cells such as neutrophils, monocytes, mast cells 

and eosinophils to the lung. Interactions between macrophages and epithelial cells enhance these 

responses (Tao and Kobzik, 2002, 157044). 

December 2009 5-4 
Figure 5-3. PM activates cell signaling pathways leading to pulmonary inflammation. 

Inflammatory cells can serve as a source of extracellular ROS which, along with soluble 

mediators derived from the inflammatory cells, amplify the inflammatory response. Unchecked 

inflammation may cause cellular and tissue injury through the generation of excess amounts of ROS 

and soluble mediators. In some cases the oxidative potential of PM is well-correlated with the degree 

of inflammation (Dick et al., 2003, 036605), suggesting that the inflammation is a direct 

consequence of PM-generated ROS. However, in other cases the oxidative potential of PM is not 

well-correlated with the degree of inflammation (Beck-Speier et al., 2005, 156262), suggesting that 

the inflammation is a consequence of the ROS-independent mechanisms by which PM activates 

intracellular signaling pathways. Particle surface area has been identified as a key determinant of the 

extent of inflammation in the case of low-toxicity, low-solubility particles (Donaldson et al., 2008, 

December 2009 5-5 
190217). Moreover, UFPs may cause inflammation disproportionately to their mass compared with 

PM of larger sizes given their large surface/volume ratio compared with other PM fractions. 

PM exposure often results in neutrophilic inflammation in laboratory studies (Tao et al., 2003, 

156111). Neutrophilic inflammation is also associated with acute lung injury in humans as well as 

chronic lung diseases such as COPD and certain forms of asthma (Barnes, 2007, 191139; Cowburn 

et al., 2008, 191142). Circulating neutrophils respond to chemotactic factors in the lung such as 

leukotriene B4 and IL-8 (Barnes, 2007, 191139). They migrate into the lung parenchyma across the 

pulmonary capillary network and into the airways from the bronchial circulation (Cowburn et al., 

2008, 191142). As a consequence of priming by inflammatory mediators or contact with 

extracellular matrix components, neutrophils become hyperresponsive to activating signals and 

insensitive to chemotactic signals (Cowburn et al., 2008, 191142). Activation results in neutrophil 

degranulation, respiratory burst responses and soluble mediator release (Cowburn et al., 2008, 

191142). Neutrophils eventually undergo apoptosis and are phagocytized by inflammatory 

macrophages (Cowburn et al., 2008, 191142). This is accompanied by the release of antiinflammatory mediators such as IL-10 and transforming growth factor-β (TGF- β) (Cowburn et al., 

2008, 191142). These steps are key to the resolution of inflammation and prevent unregulated release 

of toxic neutrophil products such as neutrophil elastase (Cowburn et al., 2008, 191142). Thus, 

circumstances leading to the decreased ingestion of apoptotic neutrophils by macrophages may lead 

to tissue injury. Impairment of macrophage function may serve as an important mechanism by which 

PM contributes to disease. 

5.1.4. Respiratory Tract Barrier Function 

Epithelial injury can lead to an increase in permeability of the airway epithelial and 

alveolar-capillary barriers (Braude et al., 1986, 155701). Enhanced transport of soluble and possibly 

of insoluble PM components into the circulation may occur under these conditions. Increased 

epithelial permeability is also associated with enhanced immune responses to proteins, including 

allergens, on the epithelial surface, presumably due to the greater availability of antigens to 

underlying immune cells (Wan et al., 1999, 191903). Furthermore, endothelial injury can 

compromise the integrity of the alveolar-capillary barrier resulting in transvascular fluid and solute 

flux (Braude et al., 1986, 155701). Soluble mediators derived from inflammatory and lung cells 

(Chang et al., 2005, 097776) and peptides released by some nerve cells (Widdicombe and Lee, 2001, 

019049) can increase the permeability of the alveolar-capillary barrier and result in alveolar edema. 

Compromised barrier function in the airways may lead to airway edema. Edema occurring 

secondarily to nerve cell stimulation is one component of the process termed neurogenic 

inflammation. 

Given the small size of UFPs, modest changes in epithelial permeability may particularly 

affect the disposition of this fraction. Enhanced translocation to interstitial compartments or to the 

circulation may be important sequelae. A recent study described in Section 4.3.4.3 demonstrated 

greater translocation of UFPs compared with PM2.5 into the circulation of rodents treated with 

endotoxin to induce acute lung injury prior to IT instillation of PM (Chen et al., 2006, 147267). 

Furthermore, epithelial injury in another model resulted in greater translocation of UFPs into the 

interstitial compartment (Adamson and Prieditis, 1995, 189982). 

5.1.5. Antioxidant Defenses and Adaptive Responses 

Antioxidant defenses and adaptive responses are important modulators of oxidative stress and 

other cellular stresses resulting from PM exposure. Antioxidants are present in the epithelial lining 

fluid in all regions of the respiratory tract. In addition, they are present in cells of the lung 

parenchyma and inflammatory cells found in airways and alveoli. Some antioxidants act directly 

against oxidant species (e.g., glutathione, ascorbate, superoxide dismutase) while others act 

indirectly (e.g., gamma-glutamylcysteine synthetase [γGCS], glutathione reductase). Furthermore, 

some antioxidants (e.g., Phase 2 enzymes heme oxygenase-1[HO-1], NADPH quinone 

oxidoreductase 1 [NQO1], glutathione-S-transferase [GST]) are inducible via activation of the 

nuclear factor (erythroid-derived 2)- related factor 2 (Nrf2)-ARE pathway, which occurs as an 

adaptive response to stress (Cho et al., 2006, 156345; Li and Nel, 2006, 156694). Antioxidants play 

December 2009 5-6 
an important role in reducing the oxidative potential of those PM species that directly generate ROS. 

They also inhibit responses due to generation of intracellular ROS. 

Recently a three-tier response to oxidative stress was proposed (Li and Nel, 2006, 156694). In 

this scheme, mild oxidative stress enhances antioxidant defenses by upregulating Phase 2 and other 

antioxidant enzymes (Tier 1). Further increase in oxidative stress induces inflammation (Tier 2) and 

cell death (Tier 3). Experimental evidence is supportive of this scheme. Numerous studies have 

demonstrated that enhancement of lung and cellular antioxidant defenses inhibits inflammation, 

cytotoxicity and other responses following exposure to PM (Ahsan et al., 2005, 156200; Bachoual et 

al., 2007, 155667; Bayram et al., 2006, 088439; Chang et al., 2005, 097776; Imrich et al., 2007, 

155859; Koike and Kobayashi, 2005, 088303; Koike et al., 2004, 058555; Li et al., 2007, 155929; 

Ramage and Guy, 2004, 055640; Rhoden et al., 2004, 087969; Steerenberg et al., 2004, 087981; 

Takizawa et al., 2003, 157039; Tao et al., 2003, 156111; Upadhyay et al., 2003, 097370; Wan and 

Diaz-Sanchez, 2006, 097399; Wan and Diaz-Sanchez, 2007, 156145; Yin et al., 2004, 087983). 

Cellular and tissue exposure to xenobiotics carried by PM can lead to induction of Phase 1 and 

Phase 2 detoxifying enzymes following the activation of cell signaling pathways and transcription 

factors AhR and ARE, respectively (Rengasamy et al., 2003, 156907; Rouse et al., 2008, 156930; 

Zhao et al., 2006, 100996). 

Figure 5-4. Potential pathways for the effects of PM on the respiratory system. 

December 2009 5-7 
5.1.6. Pulmonary Function 

PM exposure may alter pulmonary function by a variety of different mechanisms (Figure 5-4). 

In the short-term, airway hyperresponsiveness (AHR) may ensue due to the influence of 

inflammatory mediators. In the long-term, morphological changes may occur, in some cases leading 

to mucus hypersecretion and airway remodeling. Activation of irritant receptors and stimulation of 

the ANS in the respiratory tract is another mechanism by which PM exposure may alter pulmonary 

function (Section 5.4). 

5.1.7. Allergic Disorders 

PM exposure sometimes leads to the development of allergic immune responses (Figure 5-4). 

These responses are predominately mediated by T helper 2 cells (Th2). Th1 responses, characterized 

by IFN-γ and classical macrophage activation, are inflammatory; in excess they can lead to tissue 

damage. Alternatively, Th2 responses are associated with allergy and asthma and are characterized 

by IL-4, IL-5, IL-13, influx of eosinophils, B–lymphocyte production of IgE, and alternative 

macrophage activation. PM exposure can also lead to the exacerbation of airway allergic responses, 

such as antigen-specific IgE production and AHR. 

Due to soluble mediators and immune cell trafficking, pulmonary exposure may result in 

systemic immune alterations. Not only do macrophages ingest PM, but they are also antigen 

presenting cells whose level of activation dictates costimulation and thus subsequent T cell 

responses. These cells are highly mobile and can transport PM to other sites such as lymph nodes. 

Dendritic cells (DC) also play a key role as antigen presenting cells and in modulating T and B cell 

activity. A cell culture model of the human epithelial airway wall was used to demonstrate that DC 

extended processes between epithelial cells through the tight junctions, collected particles in the 

lumenal space and transported them across the epithelium (Blank et al., 2007, 096521). DC also 

transmigrated through the epithelium to take up particles on the epithelial surface (Blank et al., 2007, 

096521). Furthermore, DC interacted with particle-loaded macrophages on top of the epithelium and 

with other DC within or beneath the epithelium to transfer particles (Blank et al., 2007, 096521). In 

vitro studies also demonstrated that the adjuvant activity of diesel exhaust particles (DEPs) involved 

stimulation of immature monocyte-derived dendritic cells (iMDDC) to undergo maturation in 

response to an altered airway epithelial cell-derived microenvironment (Bleck et al., 2006, 096560). 

Additionally, DEP directly influenced the profile of cytokines secreted by DC and caused a 

predisposition toward Th2-mediated or allergic responses (Chan et al., 2006, 097468). Thus PM can 

negatively affect both innate immunity through effects on macrophage pathogen handling 

(Section 5.1.8) as well as adaptive immunity by altering macrophage or DC antigen presenting 

activity and subsequent T cell responses. 

Moreover, recent studies have demonstrated that ambient PM can act as an adjuvant for 

allergic sensitization with UFPs having a greater effect than fine particles (de Haar et al., 2006, 

144746; Li et al., 2009, 190457). This has been attributed to higher oxidative potential of the UFPs 

compared with the same mass of particles of larger size (Li et al., 2009, 190457), although the larger 

surface area and particle number per unit mass as well as the propensity of UFPs for trans-epithelial 

movement may also contribute to this effect. 

5.1.8. Impaired Lung Defense Mechanisms 

PM exposure may impair lung defense mechanisms and result in frequent or persistent 

infections (Figure 5-4). Potential targets include mucociliary transport, surfactant function and 

pathogen clearance. Pathogen clearance is dependent on the integrity of macrophages and their 

migration, phagocytosis and respiratory burst functions. PM-mediated cytotoxicity of macrophages 

with the concomitant release of lysosomal contents may affect pathogen clearance and cause damage 

to nearby cells and tissues. IT instillation and cell culture experiments have demonstrated 

PM-dependent impairment of lung defense mechanisms (Jaspers et al., 2005, 088115; Kaan and 

Hegele, 2003, 095753; Long et al., 2005, 087454; Moller et al., 2005, 156770; Monn et al., 2003, 

052418; Roberts et al., 2007, 097623; Yin et al., 2004, 087983). 

December 2009 5-8 
5.1.9. Resolution of Inflammation/Progression or Exacerbation of 

Disease 

Resolution of pulmonary inflammation and injury has been demonstrated in many 

experimental models using higher than ambient concentrations of PM. Factors contributing to this 

complex process are likely to include the uptake and clearance of PM by macrophages; the retention 

of PM in parenchymal cells and tissues; the balance of pro/anti-inflammatory soluble mediators, 

oxidants/antioxidants and proteases/anti-proteases; and the presence of pre-existing disease. These 

factors may also influence the resolution of pulmonary responses to ambient PM exposures (Figure 

5-4). The long-term consequences of prolonged inflammation are not likely to be beneficial and may 

lead to remodeling of the respiratory tract and to the progression or exacerbation of disease. 

5.1.9.1. Factors Affecting the Retention of PM 

Clearance of poorly soluble particles from ET, TB and alveolar regions is extensively 

discussed in Section 4.3. While clearance from ET and TB regions generally occurs over hours to 

days, clearance from the alveolar compartment is much slower, occurring over months to years 

depending on the species. Phagocytosis by alveolar macrophages and transport by the mucociliary 

escalator is the primary mechanism of clearance from the alveolar compartment although neutrophil 

phagocytosis also plays a role (Snipes et al., 1997, 156092). Pre-existing disease can alter the extent 

and localization of PM deposition as discussed in Section 4.2.4.5. In addition, mechanisms of 

clearance may be altered in cases of pre-existing disease as discussed in Section 4.3.4.3. While mild 

asthma was associated with enhanced mucociliary clearance, acute lung injury was associated with 

enhanced particle translocation to the circulation and the interstitial compartment. Whether retained 

particles are localized in alveolar macrophages or parenchymal tissue also differs according to 

species (Snipes, 1996, 076041). 

UFPs may have a special propensity for retention given the decreased efficiency of alveolar 

macrophages for phagocytosis of this size particle (Oberdörster, 1988, 006857) and the 

demonstration that UFPs can readily cross cellular membranes (Geiser et al., 2005, 087362). Some 

studies suggest that UFPs are taken up by epithelial cells and move to the interstitum where they are 

cleared by other pathways (Semmler-Behnke et al., 2007, 156080); however clearance mechanisms 

are not entirely understood. 

Enhanced deposition of particles in “hot spots” may influence retention. For example, 

deposition in the centriacinar or proximal alveolar region, where clearance is slow, may result in 

accumulated particle dose in this region and the potential for prolonged inflammation at the site 

leading to the development of pulmonary fibrosis or emphysema (Donaldson et al., 2008, 190217). A 

recent study suggests an important role for retained particles in the progression of disease. 

Complexation of endogenous iron by retained particles resulted in retained particles growing larger 

over time. The authors suggested that redox cycling of complexed iron may be responsible for 

disease progression (Ghio and Cohen, 2005, 088272; Ghio et al., 2004, 155790). 

5.1.9.2. Factors Affecting the Balance of Pro/Anti-Inflammatory Mediators, 

Oxidants/Anti-Oxidants and Proteases/Anti-Proteases 

Inflammation can be enhanced by pro-inflammatory mediators or dampened by antiinflammatory mediators. Production of anti-inflammatory mediators normally occurs at several steps 

of the inflammation pathway, such as the release of IL-10 and TGF-β during phagocytosis of 

apoptotic neutrophils by macrophages (Cowburn et al., 2008, 191142). Dsyregulation of the 

inflammatory process may prevent the resolution of inflammation. PM exposure may result in the 

production of pro-inflammatory mediators as well as decrease the production of anti-inflammatory 

mediators by impairing macrophage function. 

An unfavorable balance of oxidants to antioxidants in the lung is associated with inflammatory 

lung diseases including asthma and COPD (Rahman et al., 2006, 191165). PM is likely to contribute 

to an unfavorable balance through its oxidative potential and capacity to promote cellular production 

of ROS. Exacerbations of asthma and COPD resulting from bacterial and viral infections are also 

associated with increased oxidative stress (Barnes, 2007, 191139). Conversely, antioxidants may 

reduce neutrophilic inflammation associated with oxidative stress (Barnes, 2007, 191139). 

December 2009 5-9 
Protease/anti-protease balance has long been tied to the pathogenesis of emphysema and other 

forms of COPD (Owen, 2008, 191162). Key steps include the release of proteinases by inflammatory 

cells which degrade the extracellular matrix components of alveolar walls. Destruction of alveolar 

walls and airspace enlargement ensues. Endogenous anti-protease defenses in the lung modulate this 

response but may be insufficient to prevent it during prolonged inflammation. Proteases also play a 

role in pathologies which lead to small airway fibrosis (Owen, 2008, 191162). Oxidative stress has 

been linked to both activation of proteases and inactivation of anti-proteases (Owen, 2008, 191162). 

PM may contribute to an unfavorable protease/anti-protease balance through the generation of ROS. 

Although there are numerous inflammatory cell-derived proteases and lung anti-proteases, 

many recent studies have focused on matrix metalloproteinases (MMPs). MMPs are a family of Zncontaining enzymes normally found in an inactive pro-enzyme form. Activation involves proteolytic 

cleavage or oxidation of the “cysteine switch” (Pardo and Selman, 2005, 191163). Inhibitors include 

tissue inhibitors of metalloproteinases (TIMPs) (Pardo and Selman, 2005, 191163). In particular, 

MMP-1 is well-studied and found to play an important role in physiological processes such as 

development and wound repair as well as in diseases such as pulmonary emphysema, fibrosis, 

asthma and bronchial carcinoma (Li et al., 2009, 190424; Pardo and Selman, 2005, 191163). In 

addition to its activity in degrading collagenase, MMP-1 also acts on non-matrix substrates and cell 

surface molecules suggesting that it may influence cell signaling (Pardo and Selman, 2005, 191163). 

MMP-2 and MMP-9 are thought to be involved in the pathogenesis of disease through their 

gelatinase activity. Interestingly, recent in vitro studies have demonstrated upregulation of MMP-1 

by hydrogen peroxide, cigarette smoke and DEPs (Amara et al., 2007, 156212; Li et al., 2009, 

190424; Mercer et al., 2004, 191180), and up-regulation of MMP-12 following instillation of PM 

collected from the Paris subway (Bachoual et al., 2007, 155667). These considerations suggest that 

particulate air pollution may act via MMP to mediate progression or exacerbation of lung disease. 

5.1.9.3. Pre-Existing Disease 

In addition to its effects on deposition, retention and clearance of PM described above, preexisting disease may also alter the balance of the aforementioned factors. For example, acute 

exacerbations of COPD are characterized by a rapid influx of neutrophils into the airways (Owen, 

2008, 191162). However, clearance of apoptotic neutrophils by macrophages is impaired in COPD 

leading to greater release of neutrophil-derived inflammatory mediators, oxidants and proteases 

(Owen, 2008, 191162). Thus, exacerbation of disease may occur as a result of unchecked 

inflammation. 

5.1.10. Pulmonary DNA Damage 

Pulmonary DNA damage can occur primarily or secondarily to PM exposure. Primary effects 

include oxidative DNA injury or DNA adduct formation due directly to PM while secondary effects 

occur due to PM-mediated inflammation (De Kok et al., 2005, 088656; Gabelová et al., 2007, 

156457; Gallagher et al., 2003, 140171; Schins and Knaapen, 2007, 156074). These responses may 

lead to chromosomal aberrations or DNA strand breaks. PM effects on cell cycle arrest, proliferation, 

apoptosis, and DNA repair mechanisms may also influence the genotoxic, mutagenic or carcinogenic 

potential of DNA damage as reviewed by Schins et al. (2007, 156074). 

5.1.11. Epigenetic Changes 

Epigenetic mechanisms regulate the transcription of genes without altering the nucleotide 

sequence of DNA. These mechanisms generally involve DNA methylation and histone 

modifications, leading to alterations which may have long-term consequences or are heritable (Jones 

and Baylin, 2007, 191153; Keverne and Curley, 2008, 191154). DNA methylation and histone 

modifications, which include methylation, acetylation, phosphorylation, ubiquitylation and 

sumoylation, are known to be linked (Hitchler and Domann, 2007, 191151; Jones and Baylin, 2007, 

191153). Numerous studies have identified epigenetic processes in the control of cancer (Foley et al., 

2009, 191144; Gopalakrishnan et al., 2008, 191147; Jones and Baylin, 2007, 191153; Valinluck et 

al., 2004, 191170), embryonic development (Foley et al., 2009, 191144; Gopalakrishnan et al., 2008, 

December 2009 5-10 
191147; Keverne and Curley, 2008, 191154) and inflammation and other immune system functions 

(Adcock et al., 2007, 191178). 

Epigenetic modifications resulting in decreased expression of tumor suppressor genes and 

increased expression of transforming genes have been observed in human tumors (Valinluck et al., 

2004, 191170). In general, transcription repression is associated with DNA methylation in promoter 

regions of genes. Cytosine methylation in CpG dinucleotides has emerged as an important, heritable 

epigenetic modification which can result in chromatin remodeling and decreased gene expression 

(Valinluck et al., 2004, 191170). Global changes in DNA methylation are also seen in cancer and 

hypomethylation is associated with genomic instability (Gopalakrishnan et al., 2008, 191147). 

Embryonic development is characterized by several phases of epigenetic modifications. DNA 

methylation is very dynamic following fertilization, with demethylation and re-methylation of egg 

and sperm genomes occurring immediately (Foley et al., 2009, 191144). Imprinted genes, however, 

retain the methylation profile of the parent of origin (Foley et al., 2009, 191144). Epigenetic changes 

accumulated through a life course may be passed from parent to offspring in the germline (i.e., 

germline transmission of epimutation) if they survive the epigenetic remodeling that occurs during 

gametogenesis and early embryogenesis (Foley et al., 2009, 191144). Early development is 

characterized by the process of cell differentiation, which produces different cell types and involves 

the selective activation of some sets of genes and the silencing of others in a temporal pattern (Foley 

et al., 2009, 191144; Gopalakrishnan et al., 2008, 191147). DNA methylation is postulated to provide 

a basis for cell differentiation (Gopalakrishnan et al., 2008, 191147). 

In the lung, histone acetylation and methylation have been linked to inflammatory gene 

expression, T cell differentiation, and the regulation of macrophage function following pathogen 

challenge (Adcock et al., 2007, 191178). Furthermore, altered patterns of methylation and 

acetylation have been reported in inflammatory diseases (Adcock et al., 2007, 191178). Reduced 

expression and activity of histone deacetylase have been demonstrated in lung and inflammatory 

cells in COPD and asthma (Adcock et al., 2007, 191178; Barnes, 2007, 191139). Consequently, 

histone deacetylase has been identified as a potential therapeutic target for epigenetic therapy 

(Adcock et al., 2007, 191178; Jones and Baylin, 2007, 191153). 

Epigenetic mechanisms have been identified as potential targets for gene-environment 

interactions and recent studies have demonstrated that diet, cigarette smoking, endocrine disrupters, 

heavy metals and bacterial infection can alter the epigenetic profile in animals and humans (Foley et 

al., 2009, 191144). A role for PM in promoting epigenetic changes has been proposed and new 

studies, discussed in later chapters, provide some evidence for this pathway (Baccarelli et al., 2009, 

192155; Liu et al., 2008, 156709; Reed et al., 2008, 156903; Tarantini et al., 2009, 192010; Tarantini 

et al., 2009, 192153; Yauk et al., 2008, 157164). 

Early life exposures may be especially important in this regard since periods of rapid cell 

division and epigenetic remodeling are likely to occur at this time (Foley et al., 2009, 191144; 

Keverne and Curley, 2008, 191154; Wright and Baccarelli, 2007, 191173). This may provide a basis 

for fetal origins of adult disease. 

It has been suggested that DNA methylation is regulated by oxygen gradients and redox status 

(Hitchler and Domann, 2007, 191151). While this is of particular importance during development 

where oxygen gradients and redox status are linked to cellular differentiation, these processes are 

also important for cell signaling during all stages of life. A common metabolic precursor for both 

methylation reactions and glutathione availability (involved in redox status) is methionine (Hitchler 

and Domann, 2007, 191151). Methionine availability regulates the cell’s ability to generate 

S-adenosyl methionine which is directly involved in DNA and histone methylation and the cell’s 

ability to generate homocysteine/cysteine which is involved in glutathione biosynthesis (Hitchler and 

Domann, 2007, 191151). Furthermore, the folate cycle is a key determinant of methionine 

bioavailability (Hitchler and Domann, 2007, 191151). In this way, cellular intermediary metabolism 

is linked to epigenetic processes, with oxidative stress necessitating a metabolic shift resulting in 

decreased DNA methylation and increased glutathione production. 

5.1.12. Lung Development 

Lung development is a multi-step process which begins in embryogenesis and continues to 

adult life (Pinkerton and Joad, 2006, 091237). This allows for a long period of potential vulnerability 

to environmental and other stressors. Furthermore, enzymatic systems responsible for detoxification 

of xenobiotic compounds are not fully developed until the postnatal period (Pinkerton and Joad, 

December 2009 5-11 
2006, 091237). Disruption of cell signaling during development could affect cellular differentiation, 

branching morphogenesis and overall lung growth, possibly leading to life-long consequences. 

Although very little is known about the effects of maternal exposure to PM on the fetus or the effects 

of exposure during childhood, recent animal studies demonstrate respiratory and immune system 

effects of perinatal exposure to sidestream cigarette smoke (Pinkerton and Joad, 2006, 091237; Wang 

and Pinkerton, 2007, 179975). 

5.2. Systemic Inflammation 

Pulmonary inflammation resulting from PM exposure may trigger systemic inflammation 

through the action of cytokines and other soluble mediators which leave the lung and enter the 

circulation (Figure 5-5). Epithelial permeability may exert an important influence on this process 

(Section 5.1.4). Cytokines released by alveolar macrophages can stimulate bone marrow production 

of leukocytes resulting in an increased number of total and immature leukocytes in the circulation 

(Van and Hogg, 2002, 088111; Van Eeden et al., 2001, 019018). They also can activate neutrophils 

and promote their sequestration in microvascular beds (Van Eeden et al., 2001, 019018). The time 

course of these responses varies according to the acute or chronic nature of the PM exposure 

(van Eeden et al., 2005, 157086). 

Systemic inflammation is seen under conditions of mild pulmonary inflammation – and 

sometimes under conditions of no measurable pulmonary inflammation – following PM exposure. 

The time-dependent nature of pulmonary and systemic inflammatory responses may in part explain 

these findings since biomarkers of inflammation are frequently measured only at one time point. 

Furthermore, chronic exposures may lead to adaptive responses. In general, systemic inflammation is 

associated with changes in circulating white blood cells, the acute phase response, pro-coagulation 

effects, endothelial dysfunction and the development of atherosclerosis (Figure 5-5). Adverse effects 

on the cardiovascular and cerebrovascular systems such as thrombosis, plaque rupture, MI and stroke 

may result. Systemic inflammation may affect other organ systems such as the liver or the CNS. 

One recent study demonstrated that alveolar macrophage-derived IL-6 mediated 

pro-coagulation effects in mice exposed by IT instillation to PM10 (Mutlu et al., 2007, 121441). This 

study provides a clear link between lung cytokines and systemic responses in one model system. 

Whether this mechanism or others account for the majority of extra-pulmonary effects following 

inhalation of PM at concentrations relevant to ambient exposures is not yet known. 

December 2009 5-12 
Figure 5-5. Potential pathways for the effects of PM on the cardiovascular system. 

5.2.1. Endothelial Dysfunction and Altered Vasoreactivity 

The lumenal surface of blood vessels is lined by endothelial cells which, in addition to 

providing a barrier function, are key regulators of vascular homeostasis. Endothelial cells synthesize 

and release vasodilators such as nitric oxide (NO) and prostacyclin and vasoconstrictors such as 

endothelin (ET), which act on neighboring smooth muscle cells. ET also stimulates endothelial NO 

synthesis through a feedback mechanism. Inhalation of high concentrations of PM has been reported 

to increase ET levels in the circulation (Thomson et al., 2005, 087554). ET has also been proposed to 

play a role in hypoxia-induced MI (Caligiuri et al., 1999, 157365). However, the role of ET in 

mediating cardiovascular effects following ambient PM exposures is unclear (Section 6.2.4.3). 

December 2009 5-13 
Endothelial dysfunction can lead to or follow endothelial activation under conditions of 

systemic inflammation and/or vascular oxidative stress. Both systemic inflammation and vascular 

oxidative stress have been associated with PM exposure (Nurkiewicz et al., 2006, 088611; 

Van Eeden et al., 2001, 019018). Cytokines activate endothelial cells and upregulate endothelial cell 

adhesion molecules. They also promote the sequestration of neutrophils in microvascular beds. 

Neutrophil sequestration is sometimes associated with the deposition of myeloperoxidase (MPO) on 

endothelial cell surfaces (Nurkiewicz et al., 2006, 088611). ROS-derived from neutrophils, MPO, 

other adhered inflammatory cells and/or other sources can perturb the balance of vasodilator and 

vasoconstrictor species produced by endothelial cells. Oxidative stress can result in decreased 

synthesis of NO due to limitation of the redox-sensitive essential cofactor tetrahydrobiopterin and in 

decreased bioavailability of NO due to reaction with superoxide. Prostacyclin synthesis is also 

decreased by oxidative stress. Importantly, these processes can affect vasoreactivity such that blood 

vessels may be unable to respond to vasoconstrictor stimuli with compensatory vasodilation. 

Loss of NO and prostacyclin synthesis due to PM-dependent vascular oxidative stress may 

have other consequences since both exert negative influences on platelet and neutrophil activation. 

While endothelial surfaces normally are anti-thrombotic, endothelial dysfunction can contribute to 

thrombus formation. Furthermore, inflammation and oxidative stress associated with endothelial 

dysfunction can contribute to the development or progression of atherosclerosis (van Eeden et al., 

2005, 157086). 

5.2.2. Activation of Coagulation and Acute Phase Response 

The primary function of the coagulation cascade is to stop the loss of blood after vascular 

injury by forming a fibrin clot. However in some cases, activation of coagulation can promote 

intravascular thrombosis (Karoly et al., 2007, 155890). It has been proposed that air pollutionassociated PM can activate clotting pathways and enhance the likelihood of an obstructive cardiac 

ischemic event (e.g., MI) or cerebral event (e.g., stroke) (Seaton et al., 1995, 045721). 

Coagulation is regulated by intrinsic and extrinsic pathways. The intrinsic pathway occurs 

following activation of Factor XII and does not require the addition of an exogenous agent 

(Mackman, 2005, 156722). On the other hand, the extrinsic pathway is an inducible signaling 

cascade that can be activated by tissue factor (TF) produced in response to inflammation or 

endothelial injury (Karoly et al., 2007, 155890). 

In general, platelets, red blood cells (RBCs) and endothelial cells are effector cells for 

inducing a pro-coagulant state in the vasculature. Circulating factors may enhance coagulation or 

promote activation of platelets. Cytokines formed during tissue damage and inflammation lead to TF 

induction. TF is the initiating stimulus for coagulation following vascular injury or plaque erosion. 

Complexes of TF:Factor VIIa form on endothelial cell surfaces and play a key role in thrombin 

generation by initiating the extrinsic blood coagulation pathway (Gilmour et al., 2005, 087410). 

Thrombin generates fibrin from fibrinogen and amplifies the intrinsic pathway (Karoly et al., 2007, 

155890). TF and thrombin also have pro-inflammatory actions independent of coagulation functions 

(Chu, 2005, 155730); thus activation of coagulation may lead to or potentiate inflammation. 

Endothelial cell-derived von Willebrand factor also contributes to coagulation. 

The fibrinolytic system opposes these processes by facilitating the removal of a clot. The 

fibrinolytic pathway is regulated by the ratio of tissue plasminogen activator (tPA) and plasminogen 

activator inhibitor (PAI). Furthermore, the endothelial cell surface has anti-thrombotic properties due 

to the expression of tissue factor pathway inhibitor (TFPI) and thrombomodulin (Mackman, 2005, 

156722). 

Inhibition of the fibrinolytic pathway, along with increased plasma viscosity and increased 

concentrations of plasma fibrinogen and Factor VII, contributes to a pro-thrombotic state (Gilmour et 

al., 2005, 087410). In acute lung injury, vascular cells have enhanced pro-coagulant activity and 

impaired fibrinolytic activity (Gilmour et al., 2005, 087410). In arterial atherosclerosis, TF 

expression is increased within plaques. As a result, spontaneous plaque rupture may trigger 

intravascular clotting (Karoly et al., 2007, 155890). 

Acute phase responses also play a role in hemostasis by exerting pro-coagulant effects. 

Cytokines such as IL-6 stimulate the liver to produce acute phase proteins including C-reactive 

protein (CRP), fibrinogen and antiproteases (van Eeden et al., 2005, 157086). To date, there is 

limited evidence supporting a role for ambient PM in stimulating acute phase responses (Ruckerl et 

al., 2007, 156931 and reviewed therein) (also Section 6.2.7 and 6.2.8 in this ISA). 

December 2009 5-14 
5.2.3. Atherosclerosis 

Atherosclerosis is a chronic progressive disease which contributes greatly to cardiovascular 

morbidity (Libby, 2002, 192009). Mainly a disease of the large arteries, it is characterized by the 

accumulation of lipid and fibrous tissue in atheromas, or swellings of the vessel wall (Libby, 2002, 

192009). Although a strong link is known to exist between hypercholesterolemia and atherogenesis, 

there is growing appreciation of the key role played by inflammation in the initiation and progression 

of atherosclerosis (Libby, 2002, 192009). Furthermore, inflammation has the potential to promote 

thrombosis which can complicate this disorder and lead to MI and stroke (Libby, 2002, 192009). As 

discussed above, PM exposure is associated with systemic inflammation, potentially contributing to 

the development of atherosclerosis. 

Atheroma formation in experimental animals fed a high fat diet begins with the accumulation 

of modified lipoprotein particles in the arterial intima, as reviewed by Libby (2002, 192009). The 

modification of lipoprotein particles often involves oxidation. As discussed above, PM exposure is 

associated with oxidative stress, suggesting a potential role for PM in the modification of lipoprotein 

particles. Endothelial dysfunction may also be key to these early events (Halvorsen et al., 2008, 

191149). Recent studies, which are discussed in later chapters, demonstrate PM-dependent 

endothelial dysfunction (Nurkiewicz et al., 2009, 191961). As described by Libby (2002, 192009), 

oxidative stress leads to lipid modification and uptake by endothelial cells and initiates an 

inflammatory response by activating NF-κB. As a result, cell adhesion molecules such as vascular 

cell adhesion molecule-1 (VCAM-1) are upregulated and expressed in endothelial cells. Proinflammatory cytokines associated with systemic inflammation may also contribute to adhesion 

molecule upregulation. Subsequently, monocytes and T cell lymphocytes adhere to the activated 

endothelium, then migrate into the tunica intima directed by chemokines such as monocyte 

chemoattractant protein-1 (MCP-1) and IL-8. Monocytes undergo transformation to tissue 

macrophages and later to foam cells. As a part of this transformation, monocyte/macrophages 

express scavenger receptors and bind to and internalize the modified lipoprotein particles. These 

cells secrete growth factors and cytokines, produce ROS, replicate within the lesion and contribute to 

further lesion progression. Macrophage colony-stimulating factor (M-CSF) and granulocytemacrophage colony-stimulating factor (GM-CSF) are thought to be involved in these latter steps 

which eventually lead to the formation of a fatty streak. T cell lymphocytes also become activated in 

the atheroma and secrete pro- or anti-inflammatory cytokines. Degranulation of mast cells found in 

the atheroma may also contribute to the lesion. 

Atheroma progression involves the proliferation of smooth muscle cells, which is stimulated 

by macrophage-derived growth factors (Libby, 2002, 192009). This results in smooth muscle 

accumulation in the lesion, the elaboration of extracellular matrix material and the formation of a 

more bulky lesion which can occlude the arterial lumen. Matrix proteins contribute to the evolution 

of the lesion from a fatty streak to a fibrous plaque. Mechanisms which trigger plaque disruption, 

including endothelial erosions and plaque rupture, can result in thrombosis as well as in further 

expansion of the lesion. Resident T cells, mast cells and circulating platelets may also play a role in 

destabilizing plaques (Halvorsen et al., 2008, 191149). It is thought that most atheromatous lesions 

progress in a discontinuous manner due to cycles of disruption, expansion and repair. 

A major factor regulating plaque disruption is the thickness of the fibrous cap, with more 

stable plaques characterized by a thick fibrous cap (Libby, 2002, 192009). Collagen in the fibrous 

cap can be degraded by proteases, especially MMPs. Inflammation in the intima reduces collagen 

production by smooth muscle cells and promotes the expression and activation of MMPs. ROS may 

mediate MMP upregulation (Lund et al., 2009, 180257). Both macrophages and smooth muscle cells 

produce MMPs in the lesion areas (Halvorsen et al., 2008, 191149); fully differentiated macrophages 

selectively upregulate certain MMPs with more destructive potential (Newby, 2008, 191161). 

Rupture of the fibrous cap allows pro-thrombotic factors within the plaque (e.g., TF) to come into 

contact with coagulation factors in the blood possibly resulting in the formation of an occlusive 

blood clot. However in some cases, blood fibrinolytic mechanisms minimize clot formation and 

repair processes ensue. The result is a more fibrous plaque and/or an expanded lesion. The proposed 

role of PM in activating coagulation pathways, which was discussed above, may influence the 

outcome of plaque disruption. 

In this manner oxidative stress, inflammation and pro-coagulant activity in the blood are 

involved in the initiation and progression of atheromatous lesions as well as plaque disruption and 

occlusive blood clot formation. PM exposure may contribute to these pathways and in fact, studies 

December 2009 5-15 
described in later chapters demonstrate PM-dependent effects on atherosclerosis progression (Araujo 

et al., 2008, 156222; Chen and Nadziejko, 2005, 087219; Sun et al., 2005, 087952; Ying et al., 2009, 

190111). 

5.3. Activation of the Autonomic Nervous System by 

Pulmonary Reflexes 

Chemosensitive receptors, including rapidly adapting receptors (RARs) and sensory C-fiber 

receptors, are found at all levels of the respiratory tract and are sensitive to irritant particles as well 

as to irritant gases (Alarie, 1973, 070967; Coleridge and Coleridge, 1994, 156362; Widdicombe, 

2006, 155519). Activation of trigeminal afferents in the nose causes CNS reflexes resulting in 

decreases in respiratory rate through a lengthened expiratory phase, closure of the glottis, closure of 

the nares with increased nasal airflow resistance and effects on the cardiovascular system such as 

bradycardia, peripheral vasoconstriction and a rise in systolic arterial blood pressure (Alarie, 1973, 

070967). Sneezing, rhinorrhea and vasodilation with subsequent nasal vascular congestion are also 

nasal reflex responses involving the trigeminal nerve (Sarin et al., 2006, 191166). Activation of vagal 

afferents in the tracheobronchial and alveolar regions of the respiratory tract causes CNS reflexes 

resulting in bronchoconstriction, mucus secretion, mucosal vasodilation, cough, apnea followed by 

rapid shallow breathing and effects on the cardiovascular system such as bradycardia and 

hypotension or hypertension (Coleridge and Coleridge, 1994, 156362; Widdicombe, 2003, 157145; 

2006, 155519; Widdicombe and Lee, 2001, 019049). Some evidence suggests that cardiovascular 

responses may be mediated primarily by the C-fiber receptors (Coleridge and Coleridge, 1994, 

156362) and that irritants in the lower respiratory tract cause more pronounced cardiovascular 

responses than irritants in the upper respiratory tract (Widdicombe and Lee, 2001, 019049). 

Early experiments demonstrated that sectioning of the trigeminal nerve abrogated irritant 

effects on respiratory rate, heart rate and systolic arterial blood pressure (Alarie, 1973, 070967). 

These nasal reflexes were attributed to the ophthalmic branch of the trigeminal nerve since they were 

identical to reflex responses following diving or immersion of the face in water (Alarie, 1973, 

070967). Early experiments also demonstrated that non-nasal reflexes were mediated by cholinergic 

parasympathetic pathways involving the vagus nerve and inhibited by atropine (Grunstein et al., 

1977, 071445; Nadel et al., 1965, 014846). More recent experiments have shown that noncholinergic 

mechanisms may also be involved. For example, stimulation of C-fiber receptors can activate local 

axon reflexes. These local axon pathways are responsible for secretion of neuropeptides and the 

development of neurogenic inflammation (Widdicombe and Lee, 2001, 019049). It has been 

proposed that, in some cases, neurogenic pulmonary responses can switch from their normally 

protective function to one that perpetuates pulmonary inflammation (Wong et al., 2003, 097707). 

Differences in respiratory tract innervation between rodents and humans suggest that C-fiber 

mediated neurogenic inflammation may be more important in rodents than in humans (Groneberg et 

al., 2004, 138134; Widdicombe, 2003, 157145; Widdicombe and Lee, 2001, 019049) However, the 

role of neurogenic inflammation in mediating pulmonary responses in humans is an active area of 

investigation. 

VR1 receptors represent a subset of neuropeptide and acid-sensitive irritant receptors which 

belong to the transient receptor potential (TRP) family. They are located on the sensory C-fibers 

which lie underneath and between lung epithelial cells and on immune and non-immune airway 

cells. Some investigators have focused on the role played by these receptors in mediating 

inflammation following exposure to PM (Veronesi and Oortgiesen, 2001, 015977). Exposure of 

bronchial epithelial cells and neurons to PM in vitro has been shown to result in an immediate 

increase in intracellular calcium followed by the release of neuropeptides and inflammatory 

cytokines (Veronesi et al., 1999, 048764; Veronesi et al., 2000, 017062). In one study, this response 

was found to be due to an intrinsic property of the particle core and was not metal-dependent 

(Oortgiesen et al., 2000, 013998), while in another study electrostatic charge was found to activate 

VR1 receptors (Veronesi et al., 2003, 094384). PM-mediated activation of VR1 receptors which 

results in increases in intracellular calcium and apoptosis in epithelial cells has also been 

demonstrated (Agopyan et al., 2003, 155649). New studies, discussed in later chapters, provide 

evidence for the involvement of TRPV1 irritant receptor involvement in PM-dependent responses 

(Ghelfi et al., 2008, 156468; Rhoden et al., 2005, 087878). 

December 2009 5-16 
Recently it has been proposed that pulmonary reflex responses may be modulated by CNS 

plasticity (Bonham et al., 2006, 191140; Sekizawa et al., 2008, 191167). Plasticity is a property of 

neurons or synapses which allows change in response to previous events. In the case of the 

respiratory tract, visceral afferent inputs to the CNS are integrated primarily in the nucleus tractus 

soltarius (NTS) region of the brain (Bonham et al., 2006, 191140). Inputs from local networks, 

higher brain regions and circulating mediators contribute to the reflex output (Bonham et al., 2006, 

191140). This integration allows for plasticity in that repeated or prolonged exposure to a particular 

stimuli may lead to altered reflex responses to the same or subsequent stimuli. An exaggerated reflex 

response was recently observed in guinea pigs exposed to ETS for an extended period of time 

(Sekizawa et al., 2008, 191167). It is not known whether CNS plasticity influences responses 

following acute and chronic exposure to ambient PM, but it is a mechanism that may possibly 

explain hyperresponsiveness and/or adaptation of reflex-related responses. 

At this time, it is not clear how activation of the ANS by pulmonary reflexes contributes to the 

kinds of altered conduction and/or repolarization properties of the heart which may be linked to 

arrhythmias (Figure 5-5). Pulmonary reflexes, as they are currently understood, initially lead to 

increases in parasympathetic tone. However, decreased heart rate variability appears to be reflective 

of decreased parasympathetic tone and/or increased sympathetic tone. A PM-dependent sympathetic 

stress response mediated by cytokines has been postulated (Godleski et al., 2000, 000738), but there 

is little new information to support this mechanism. Thus, activation of the autonomic nervous 

system by mechanisms other than pulmonary reflexes seems likely in response to PM. Very little is 

known about putative alternative mechanisms leading to sympathoexcitatory responses although one 

study demonstrated a role for the olfactory bulb-NTS pathway in regulating cardiovascular functions 

following smoke exposure (Moffitt et al., 2002, 191160). In addition, sympathoexcitatory responses 

occur during myocardial ischemia, mediated by the release of adenosine or the production of ROS 

by the myocardium (Longhurst et al., 2001, 191158). Hence, PM-dependent effects leading to 

myocardial ischemia may stimulate sympathoexcitatory responses. Furthermore, the effects of 

pre-existing alterations in the ANS due to disease processes (e.g., increased sympathetic tone 

observed in cardiac diseases) on PM responses are not understood. Possibly, integration of neural 

signals resulting from pulmonary and cardiac reflexes at the level of the NTS may have an influence 

on ANS responses to PM. Further investigation will be required to clarify these mechanisms. 

5.4. Translocation of UFPs or Soluble PM Components 

UFPs can translocate across cell membranes by non-endocytotic mechanisms involving 

adhesive interactions and diffusion (Geiser et al., 2005, 087362), as described in Section 4.3.3.1. In 

this study, there was no measurable loss of PM from the lung over 24 h despite the rapid 

translocation of inhaled UFPs into alveolar epithelial cells and capillary endothelial cells. In another 

study, UFPs were localized in macrophage mitochondria as demonstrated by electron microscopy (Li 

et al., 2003, 042082). Other studies found extrapulmonary translocation of poorly soluble UFPs, but 

the process was slow and resulted in only a small amount leaving the lung. It is possible that in these 

studies PM gained access to the circulation after initial transport to the lymph nodes or the 

gastrointestinal system. Hence, there is limited evidence to date that UFPs or other PM size 

fractions access the circulation by traversing the epithelial barrier of the respiratory tract. 

However, soluble components from all size fractions of PM have the potential to translocate 

across the airway epithelium into the bronchial circulation or across the alveolar epithelium into the 

systemic circulation as depicted in Figure 5-5. Absorption across nasal epithelium may also occur 

(Illum, 2006, 191205). Factors affecting this process include the rate of dissolution of the solute 

from the particle and the molecular weight of the solute (Section 4.4). More rapid dissolution of 

soluble components may occur in the case of UFPs due to the higher surface/volume ratios compared 

with larger particles. 

Several interesting studies investigated the translocation of water-soluble metals from the lung. 

Gilmour et al. (2006, 156472) demonstrated the rapid appearance of Zn in the plasma of rats 

following IT instillation of zinc sulfate (ZnSO4). Similarly, Wallenborn et al. (2007, 156144) 

demonstrated the rapid appearance of water-soluble metals in the blood, heart and liver following IT 

instillation of oil combustion PM in rats. Using a more sensitive technique, these same investigators 

demonstrated the accumulation of 70Zn, a rare isotope of Zn, in blood, heart and liver following IT 

December 2009 5-17 
instillation of ZnSO4 (Wallenborn et al., 2009, 191172). In three other studies, soluble Zn and Cu 

were associated with cardiac effects following IT instillation of rats with different forms of Zn- and 

Cu-containing PM (Gilmour et al., 2006, 088489; Gottipolu et al., 2008, 191148; Kodavanti et al., 

2008, 155907). These results suggest the possibility that PM-derived soluble Zn and Cu translocated 

across the alveolar-capillary barrier into the circulation and exerted effects on the heart. However, in 

the two studies in which barrier function was measured it was found to be compromised (Gilmour et 

al., 2006, 088489; Gottipolu et al., 2008, 191148) suggesting an acute lung injury response to IT 

instillation of high concentrations of metal. Acute lung injury is not likely to occur in healthy 

individuals exposed to PM at concentrations relevant to ambient levels. Cardiac effects were also 

observed following subchronic inhalation exposure to low concentrations of aerosolized ZnSO4

(Wallenborn et al., 2008, 191171). Since it was not possible to measure extra-pulmonary Zn in this 

study, it remains unclear whether cardiac effects were a direct effect of translocated Zn or an indirect 

effect of exposure to Zn-containing PM. Nonetheless, translocation of soluble components derived 

from inhaled PM remains a viable hypothesis to explain some extra-pulmonary effects. 

Epithelial permeability is a key determinant of translocation and is discussed in detail in 

Section 4.4.2. In brief, a number of studies have measured clearance of 99mTc-DTPA as an index of 

alveolar epithelial membrane integrity and permeability of alveolar-capillary barrier (Braude et al., 

1986, 155701). Endothelial integrity also contributes to the alveolar-capillary barrier and is measured 

by transvascular protein flux but is not discussed here (Braude et al., 1986, 155701). In laboratory 

animals, increased alveolar permeability was shown in terminally senescent mice (Tankersley et al., 

2003, 096363). In human volunteers, epithelial permeability was transiently increased following 3 h 

of moderate exercise but not following 24-h exposure to particle-rich urban air (Brauner et al., 2009, 

190244). A previous study found that the exercise-induced increase in epithelial permeability was 

transient and suggested that it was due to increased ventilation and elevated vascular pressure which 

altered the properties of tight junctions (Hanel et al., 2003, 155826). Smokers (Jones et al., 1983, 

155884) and individuals with acute respiratory distress syndrome (Braude et al., 1986, 155701) or 

interstitial lung disease (Rinderknecht et al., 1980, 191965) also exhibited increased alveolar 

epithelial permeability. The changes in smokers were reversible upon cessation of smoking. 

Increased airway epithelial permeability was found in asthmatics when 99mTc-DTPA clearance was 

used to measure the permeability of the bronchial mucosa (Ilowite et al., 1989, 156584). These 

studies demonstrate that epithelial permeability is increased following moderate exercise and in lung 

syndromes associated with inflammation and suggest that compromised epithelial barrier functions 

in the lung may contribute to PM-mediated effects. 

Interaction of circulating PM or soluble PM components with vascular endothelial cells, 

platelets, and other leukocytes is a potential mechanism underlying the cardiovascular and systemic 

effects of inhaled PM. A role for PM-derived ROS and/or cellular-derived ROS has been proposed. 

Furthermore, soluble metals that do not redox-cycle may activate cell signaling pathways without the 

generation of ROS. In this way, PM may promote adverse cardiovascular effects such as endothelial 

dysfunction, atherosclerosis and thrombosis. Circulating PM or soluble PM components also have 

the potential to impact other organ systems. However, convincing evidence that this occurs to an 

appreciable extent in healthy individuals following inhalation of PM at concentrations relevant to 

ambient exposures is lacking. 

5.5. Disease of the Cardiovascular and Other Organ 

Systems 

As discussed above, deposition of PM in the lung may lead not only to pulmonary disease but 

also to diseases of other systems (Figure 5-5). In the cardiovascular system, myocardial ischemia and 

MI may occur as a result of the above proposed effects of PM on atherosclerosis, plaque instability, 

thrombosis, plaque rupture and/or altered vasoreactivity of coronary vessels. Myocardial ischemia 

and MI may alter the conduction and depolarization properties of the heart and lead to arrhythmic 

events. In addition, thrombosis may lead to stroke and/or thromboembolic disease. Many of these 

processes may be interlinked and responses to ambient PM exposures may involve multiple 

mechanisms simultaneously with some variability depending on PM composition. Furthermore, it is 

not clear at this time whether PM initiates cardiovascular disease or whether it perturbs existing 

disease. 

December 2009 5-18 
In addition, recent studies which are discussed in later chapters have demonstrated PMdependent effects on the CNS (Campbell et al., 2005, 087217; Kleinman et al., 2008, 190074; 

Sirivelu et al., 2006, 111151; Veronesi et al., 2005, 087481; Win-Shwe et al., 2008, 190146). At this 

time, it is not known whether this is a direct or indirect consequence of PM exposure. Translocation 

of soluble and poorly soluble particles from the olfactory mucosa via the axons to the olfactory bulb 

of the brain has been proposed as a possible mechanism by which PM or its components may 

directly access the CNS. Evidence for this pathway is discussed in Section 4.3.3.2. Alternative 

mechanisms proposed for PM-mediated CNS effects involve systemic inflammation and autonomic 

responses. These are new and intriguing possibilities which warrant further investigation. 

PM-dependent effects on the reproductive system, reproductive outcomes and perinatal 

development have also been identified and are discussed in a later chapter. Mechanisms involved in 

these responses have not been determined. However, it seems possible that systemic inflammation 

and/or oxidative stress may play a role. Developmental windows of susceptibility may also be an 

important consideration. Furthermore, it has been hypothesized that oxygen gradients and redox 

status are key to cell differentiation and epigenetic processes occurring during development 

(Section 5.1.11) (Hitchler and Domann, 2007, 191151). 

5.6. Acute and Chronic Responses 

In general, repeated acute responses may lead to cumulative effects which manifest as chronic 

disease. Several examples relevant to the modes of action discussed in this chapter are that of 

allergic responses, atherosclerosis and lung development. Allergic responses require repeated 

exposures to antigen over time. Co-exposure to an adjuvant, possibly DEP or ultrafine concentrated 

ambient particles (CAPs), can enhance this response. Furthermore, the presence of oxidative stress, 

as may occur in response to PM, can contribute to allergic responses. Allergic sensitization often 

underlies allergic asthma, characterized by inflammation and AHR. In this way, repeated or chronic 

exposures involving multifactorial responses (immune system activation, oxidative stress, 

inflammation) can lead to irreversible outcomes. Similarly, the development of atherosclerosis 

involves inflammation and remodeling of the blood vessel wall. Factors contributing to this process 

include systemic inflammation, endothelial dysfunction, oxidative stress and high levels of 

circulating lipids. PM exposure is associated with three out of four of these processes. The role of 

PM in initiating, promoting or complicating this disease or its outcomes has yet to be determined. 

Critical windows of susceptibility during development also provide an opportunity for repeated 

exposures to injurious agents to lead to irreversible changes in organ structure and function. The 

extended period of postnatal lung development in humans and other species heightens this 

vulnerability. PM may serve as such an injurious agent as has been demonstrated previously for 

hyperoxia (Randell et al., 1990, 191956). 

Furthermore, adverse outcomes may be precipitated by acute events superimposed on chronic 

disease states. In the case of allergic asthma, acute PM exposure may provoke asthmatic responses 

through oxidative stress and inflammatory pathways. Additionally, PM can act as a carrier of 

aeroallergens and other biological materials which can potentially trigger asthma attacks. Similarly, 

PM exposure may provoke inflammatory or thrombotic responses leading to rupture of an 

atherosclerotic plaque which subsequently results in acute MI. In this way, the outcome of an acute 

exposure to PM may be drastically worsened by the underlying chronic disease. 

5.7. Results of New Inhalation Studies which Contribute 

to Modes of Action 

Prior to this review, much of the evidence for the proposed modes of action was obtained from 

animal studies involving IT instillation or inhalation of high concentrations of PM and from cell 

culture experiments. In many cases, the types of PM used were of questionable relevance to ambient 

exposures (i.e., high concentrations of ROFA, metals and ambient PM collected on filters). Since 

then, many inhalation studies have been conducted using CAPs, combustion-derived PM, urban air 

December 2009 5-19 
and carbon black, generally using concentrations of PM lower than 1 mg/m3

. Much of this research 

has been conducted in animal models of disease. These key new studies, described in detail in 

Chapters 6 and 7, add to the understanding of modes of action which are relevant to ambient PM 

exposure. A compilation of pertinent results is found below. 

 Altered lung function including changes in respiratory frequency and AHR following 

short-term exposures to CAPs and combustion-derived PM (Section 6.3.2.3) 

 Mild pulmonary inflammation in response to short-term exposures to CAPs, urban air, 

combustion-derived PM and carbon black (Section 6.3.3.3) 

 Mild pulmonary injury in response to short-term exposure to CAPs and combustionderived PM (Section 6.3.5.3) 

 Inhibition of cell proliferation in the proximal alveolar region of neonatal animals 

following short-term exposure to iron-soot (Section 6.3.5.3) 

 Pulmonary oxidative stress in response to short-term exposure to CAPs, urban air, 

combustion-derived PM, carbon black and iron-soot; pulmonary nitrosative stress in 

response to titanium dioxide (TiO2) (Section 6.3.4.2) 

 Antioxidant intervention which ameliorates PM effects on oxidative stress, allergic 

responses, and AHR (Sections 6.3.4.2 ) 

 Allergic sensitization and exacerbation of allergic responses in response to CAPs and 

combustion-derived PM (Section 6.3.6.3) 

 Altered methylation of promoter regions of IFN-γ and IL-4 genes suggestive of proallergic Th2 gene activation following short-term exposure to combustion-derived PM in 

an allergy model (Section 6.3.6.3) 

 Increased susceptibility to respiratory infection following exposure to combustionderived PM (Section 6.3.7.2) 

 Effects on nasal epithelial mucosubstances, airway morphology and airway 

mucosubstances following chronic exposure to urban air-derived PM and woodsmoke 

(Section 7.3.5.1) 

 Worsening of papain-induced emphysema following chronic exposure to urban airderived PM (Section 7.3.5.1) 

 Effects on lung development following chronic exposure to urban air-derived PM 

(Sections 7.3.2.2 and 7.3.5.1) 

 Prolonged exposure to CAPs and combustion-derived PM leading sometimes to mild 

pulmonary inflammation, oxidative stress and injury and sometimes to loss of 

inflammatory, oxidative stress and AHR responses which were observed after short-term 

exposures (Sections 7.3.2.2, 7.3.3.2, 7.3.4.1, 7.3.5.1 and 7.3.6.2) 

 Hypermethylation of lung DNA following chronic exposure to combustion-derived PM 

(Section 7.3.5.1) 

 A role for TRPV1 irritant receptors in activating local axon and CNS reflexes following 

short-term exposure to CAPs and combustion-derived PM (Section 6.2.9.3) 

December 2009 5-20 
 A role for TRPV1 irritant receptors in mediating lung and heart oxidative stress through 

increased parasympathetic and sympathetic activity in response to CAPs 

(Sections 6.2.9.3 and 6.3.4.2) 

 Altered heart rate variability in response to CAPs, combustion-derived PM and carbon 

black (Section 6.2.1.3) 

 Arrhymthmic events in response to CAPs and combustion-derived PM (Section 6.2.2.2) 

 Altered cardiac contractility following short-term exposure to CAPs and carbon black 

(Section 6.2.6.1) 

 Enhanced myocardial ischemia following short-term exposure to CAPs (Section 6.2.3.3) 

 Endothelial dysfunction and altered vascular reactivity following short-term exposure to 

CAPs, combustion-derived PM and TiO2 (Section 6.2.4.3) 

 Increases in blood pressure following short-term exposure to CAPs and carbon black 

(Section 6.2.5.3) 

 Changes in blood leukocyte counts following short-term exposure to CAPs and carbon 

black (Section 6.2.7.3) 

 Increased levels of blood coagulation factors following short-term exposure to CAPs and 

on-road highway aerosols (Section 6.2.8.3) 

 Systemic and cardiovascular oxidative stress in response to short-term exposure to CAPs, 

road dust and combustion-derived PM (Section 6.2.9.3) 

 Progression of atherosclerosis, induction of TF in aortic plaques, vascular oxidative 

stress and altered vasomotor function following long-term exposure to CAPs in a 

susceptible animal model (Section 7.2.1.2) 

 Vascular remodeling following chronic exposure to urban air-derived PM 

(Section 7.2.1.2). 

 Enhanced angiotensin II-induced hypertension accompanied by vascular oxidative stress 

and altered vasoreactivity in response to chronic exposure to CAPs (Section 7.2.5.2) 

 Exaggerated insulin resistance, visceral adiposity and systemic inflammation in response 

to chronic exposure to CAPs and a high-fat diet (Section 7.2.3.1) 

 CNS responses following short- and long-term exposures to CAPs and combustionderived PM (Section 6.4.3) 

 Effects on the reproductive system, reproductive outcomes and developmental outcomes 

following chronic exposure to urban-air derived PM (Section 7.4.2) 

 DNA adducts in nose, lung and liver following chronic exposure to urban air 

(Section 7.5.2.1) 

 Germ line mutations, DNA strand breaks and global hypermethylation in sperm 

following chronic exposure to urban air-derived PM (Section 7.5.3) 

December 2009 5-21 
5.8. Gaps in Knowledge 

The new studies highlighted in Section 5.7 confirm and extend findings from older studies. 

However, this increasing body of evidence does not provide a complete picture of the biological 

pathways involved in mediating PM effects. For example, a lack of information regarding the timedependence of many responses makes it difficult to understand the underlying biological 

mechanisms. Existing gaps in knowledge include: 

 The spatial distribution of retained particles in the lung and its impact 

 The deposition, uptake and clearance of UFPs in the lung 

 Effects of ambient PM exposures on epithelial barrier function in the lung 

 Time dependence of responses 

 The putative modulation of neural reflexes by pre-existing disease or other factors 

 The putative role of neural reflexes besides those involving pulmonary irritant receptors 

 The putative role of ET in altering vasomotor tone following PM exposure 

 The putative translocation of PM or soluble components across the epithelial barrier of 

the lung into the circulation 

 The putative translocation of PM from olfactory epithelium to the olfactory bulb and 

other brain regions 

Additional studies will be required to clarify the biological mechanisms underlying the health 

effects of PM. 

December 2009 5-22 
Chapter 5 References 

Adamson I; Prieditis H (1995). Response of mouse lung to carbon deposition during injury and repair. Environ Health 

Perspect, 103: 72-76. 189982

Adcock IM; Tsaprouni L; Bhavsar P; Ito K (2007). Epigenetic regulation of airway inflammation. Curr Opin Immunol, 19: 

694-700. 191178

Agopyan N; Bhatti T; Yu S; Simon SA (2003). Vanilloid receptor activation by 2-and 10-μm particles induces responses 

leading to apoptosis in human airway epithelial cells. Toxicol Appl Pharmacol, 192: 21-35. 155649

Ahsan MK; Nakamura H; Tanito M; Yamada K; Utsumi H; Yodoi J (2005). Thioredoxin-1 suppresses lung injury and 

apoptosis induced by diesel exhaust particles (DEP) by scavenging reactive oxygen species and by inhibiting DEPinduced downregulation of Akt. Free Radic Biol Med, 39: 1549-1559. 156200

Alarie Y (1973). Sensory irritation by airborne chemicals. Crit Rev Toxicol, 2: 299-363. 070967

Amara N; Bachoual R; Desmard M; Golda S; Guichard C; Lanone S; Aubier M; Ogier-Denis E; Boczkowski J (2007). 

Diesel exhaust particles induce matrix metalloprotease-1 in human lung epithelial cells via a NADP(H) 

oxidase/NOX4 redox-dependent mechanism. Am J Physiol Lung Cell Mol Physiol, 293: L170-181. 156212

Araujo JA; Barajas B; Kleinman M; Wang X; Bennett BJ; Gong KW; Navab M; Harkema J; Sioutas C; Lusis AJ; Nel AE 

(2008). Ambient particulate pollutants in the ultrafine range promote early atherosclerosis and systemic oxidative 

stress. Circ Res, 102: 589-596. 156222

Ayres JG; Borm P; Cassee FR; Castranova V; Donaldson K; Ghio A; Harrison RM; Hider R; Kelly F; Kooter IM; Marano 

F; Maynard RL; Mudway I; Nel A; Sioutas C; Smith S; Baeza-Squiban A; Cho A; Duggan S; Froines J (2008). 

Evaluating the Toxicity of Airborne Particulate Matter and Nanoparticles by Measuring Oxidative Stress Potential - 

A Workshop Report and Consensus Statement. Inhal Toxicol, 20: 75 - 99. 155666

Baccarelli A; Wright RO; Bollati V; Tarantini L; Litonjua AA; Suh HH; Zanobetti A; Sparrow D; Vokonas PS; Schwartz J 

(2009). Rapid DNA Methylation Changes after Exposure to Traffic Particles . Am J Respir Crit Care Med, 179: 

572-578. 192155

Bachoual R; Boczkowski J; Goven D; Amara N; Tabet L; On D; Lecon-Malas V; Aubier M; Lanone S (2007). Biological 

effects of particles from the Paris subway system. Chem Res Toxicol, 20: 1426-1433. 155667

Barnes PJ (2007). New molecular targets for the treatment of neutrophilic diseases. J Clin Immunol, 119: 1055-62. 191139

Bayram H; Ito K; Issa R; Ito M; Sukkar M; Chung KF (2006). Regulation of human lung epithelial cell numbers by diesel 

exhaust particles. Eur Respir J, 27: 705-713. 088439

Becher R; Bucht A; Ovrevik J; Hongslo JK; Dahlman HJ; Samuelsen JT; Schwarze PE (2007). Involvement of NADPH 

oxidase and iNOS in rodent pulmonary cytokine responses to urban air and mineral particles. Inhal Toxicol, 19: 

645-655. 097125

Beck-Speier I; Dayal N; Karg E; Maier KL; Schumann G; Schulz H; Semmler M; Takenaka S; Stettmaier K; Bors W; Ghio 

A; Samet JM; Heyder J (2005). Oxidative stress and lipid mediators induced in alveolar macrophages by ultrafine 

particles. Free Radic Biol Med, 38: 1080-1092. 156262

Becker S; Fenton MJ; Soukup JM (2002). Involvement of microbial components and toll-like receptors 2 and 4 in cytokine 

responses to air pollution particles. Am J Respir Cell Mol Biol, 27: 611-618. 052419

Becker S; Mundandhara S; Devlin RB; Madden M (2005). Regulation of cytokine production in human alveolar 

macrophages and airway epithelial cells in response to ambient air pollution particles: further mechanistic studies. 

Toxicol Appl Pharmacol, 207: 269-275. 088590

Blanchet S; Ramgolam K; Baulig A; Marano F; Baeza-Squiban A (2004). Fine particulate matter induces amphiregulin 

secretion by bronchial epithelial cells. Am J Respir Cell Mol Biol, 30: 421-427. 087982

 

Note: Hyperlinks to the reference citations throughout this document will take you to the NCEA HERO database (Health and 

Environmental Research Online) at http://epa.gov/hero. HERO is a database of scientific literature used by U.S. EPA in the process of 

developing science assessments such as the Integrated Science Assessments (ISA) and the Integrated Risk Information System (IRIS). 

December 2009 5-23 
Blank F; Rothen-Rutishauser B; Gehr P (2007). Dendritic cells and macrophages form a transepithelial network against 

foreign particulate antigens. Am J Respir Cell Mol Biol, 36: 669-77. 096521

Bleck B; Tse Doris B; Jaspers I; de Lafaille Maria AC; Reibman J (2006). Diesel exhaust particle-exposed human bronchial 

epithelial cells induce dendritic cell maturation. J Immunol, 176: 7431-7437. 096560

Bonham AC; Chen CY; Sekizawa S; Joad JP (2006). Plasticity in the nucleus tractus solitarius and its influence on lung and 

airway reflexes. J Appl Physiol, 101: 322-7. 191140

Braude S; Nolop KB; Hughes JMB; Barnes PJ; Royston D (1986). Comparison of lung vascular and epithelial permeability 

indices in the adult respiratory distress syndrome. Am Rev Respir Dis, 133: 1002-1005. 155701

Brauner EV; Mortensen J; Moller P; Bernard A; Vinzents P; Wahlin P; Glasius M; Loft S (2009). Effects of ambient air 

particulate exposure on blood-gas barrier permeability and lung function. Inhal Toxicol, 21: 38-47. 190244

Brown DM; Donaldson K; Borm PJ; Schins RP; Dehnhardt M; Gilmour P; Jimenez LA; Stone V (2004). Calcium and 

ROS-mediated activation of transcription factors and TNF-alpha cytokine gene expression in macrophages exposed 

to ultrafine particles. Am J Physiol, 286: L344-L353. 155705

Brown DM; Donaldson K; Stone V (2004). Effects of PM10 in human peripheral blood monocytes and J774 macrophages. 

Respir Res, 5: 29. 088663

Caligiuri G;Levy B;Pernow J;Thoren P;Hansson GK (1999). Myocardial infarction mediated by endothelin receptor 

signaling in hypercholesterolemic mice. PNAS, 96: 6920-4. 157365

Campbell A; Oldham M; Becaria A; Bondy SC; Meacher D; Sioutas C; Misra C; Mendez LB; Kleinman M (2005). 

Particulate matter in polluted air may increase biomarkers of inflammation in mouse brain. Neurotoxicology, 26: 

133-140. 087217

Cao D; Tal TL; Graves LM; Gilmour I; Linak W; Reed W; Bromberg PA; Samet JM (2007). Diesel exhaust particulateinduced activation of Stat3 requires activities of EGFR and Src in airway epithelial cells. Am J Physiol Lung Cell 

Mol Physiol, 292: L422-L429. 156322

Chan RC-F; Wang M; Li N; Yanagawa Y; Onoe K; Lee JJ; Nel AE (2006). Pro-oxidative diesel exhaust particle chemicals 

inhibit LPS-induced dendritic cell responses involved in T-helper differentiation. J Allergy Clin Immunol, 118: 455-

465. 097468

Chang C-C; Chiu H-F; Wu Y-S; Li Y-C; Tsai M-L; Shen C-K; Yang C-Y (2005). The induction of vascular endothelial 

growth factor by ultrafine carbon black contributes to the increase of alveolar-capillary permeability. Environ 

Health Perspect, 113: 454-460. 097776

Chen J; Tan M; Nemmar A; Song W; Dong M; Zhang G; Li Y (2006). Quantification of extrapulmonary translocation of 

intratracheal-instilled particles in vivo in rats: effect of lipopolysaccharide. Toxicology, 222: 195-201. 147267

Chen LC; Nadziejko C (2005). Effects of subchronic exposures to concentrated ambient particles (CAPs) in mice V CAPs 

exacerbate aortic plaque development in hyperlipidemic mice. Inhal Toxicol, 17: 217-224. 087219

Cho AK; Sioutas C; Miguel AH; Kumagai Y; Schmitz DA; Singh M; Eiguren-Fernandez A; Froines JR (2005). Redox 

activity of airborne particulate matter at different sites in the Los Angeles Basin. Environ Res, 99: 40-47. 087937

Cho H-Y; Reddy SP; Kleeberger SR (2006). Nrf2 defends the lung from oxidative stress. Antioxid Redox Signal, 8: 76-87. 

156345

Chu AJ (2005). Tissue factor mediates inflammation. Arch Biochem Biophys, 440: 123-132. 155730

Coleridge HM; Coleridge JCG (1994). Pulmonary Reflexes: Neural Mechanisms of Pulmonary Defense. Annu Rev 

Physiol, 56: 69-91. 156362

Cowburn AS; Condliffe AM; Farahi N; Summers C; Chilvers ER (2008). Advances in neutrophil biology: clinical 

implications. Chest, 134: 606-12. 191142

de Haar C; Hassing I; Bol M; Bleumink R; Pieters R (2006). Ultrafine but not fine particulate matter causes airway 

inflammation and allergic airway sensitization to co-administered antigen in mice. Clin Exp Allergy, 36: 1469-

1479. 144746

De Kok TM; Hogervorst JG; Briede JJ; Van Herwijnen MH; Maas LM; Moonen EJ; Driece HA; Kleinjans JC (2005). 

Genotoxicity and physicochemical characteristics of traffic-related ambient particulate matter. Environ Mol 

Mutagen, 46: 71-80. 088656

December 2009 5-24 
Dick CAJ; Brown DM; Donaldson K; Stone V (2003). The role of free radicals in the toxic and inflammatory effects of 

four different ultrafine particle types. Inhal Toxicol, 15: 39-52. 036605

Donaldson K; Borm PJA; Oberdorster G; Pinkerton KE; Stone V; Tran CL (2008). Concordance between in vitro and in 

vivo dosimetry in the proinflammatory effects of low-toxicity, low-solubility particles: the key role of the proximal 

alveolar region. Inhal Toxicol, 20: 53-62. 190217

Donaldson K; Stone V; Borm PJA; Jimenez LA; Gilmour PS; Schins RPF; Knaapen AM; Rahman I; Faux SP; Brown DM; 

MacNee W (2003). Oxidative stress and calcium signaling in the adverse effects of environmental particles (PM10). 

Free Radic Biol Med, 34: 1369-1382. 156408

Dostert C; Petrilli V; Van Bruggen R; Steele C; Mossman BT; Tschopp J (2008). Innate Immune Activation Through Nalp3 

Inflammasome Sensing of Asbestos and Silica. Science, 320: 674-677. 155753

Foley DL; Craig JM; Morley R; Olsson CA; Dwyer T; Smith K; Saffery R (2009). Prospects for epigenetic epidemiology. 

Am J Epidemiol, 169: 389-400. 191144

Gabelová A; Valovicova Z; Labaj J; Bacova G; Binkova B; Farmer Peter B (2007). Assessment of oxidative DNA damage 

formation by organic complex mixtures from airborne particles PM(10). Mutat Res, 620: 135-144. 156457

Gallagher J; Sams R; Inmon J; Gelein R; Elder A; Oberdorster G; Prahalad AK (2003). Formation of 8-oxo-7,8-dihydro-2'-

deoxyguanosine in rat lung DNA following subchronic inhalation of carbon black. Toxicol Appl Pharmacol, 190: 

224-231. 140171

Geiser M; Rothen-Rutishauser B; Kapp N; Schurch S; Kreyling W; Schulz H; Semmler M; Im Hof V; Heyder J; Gehr P 

(2005). Ultrafine particles cross cellular membranes by nonphagocytic mechanisms in lungs and in cultured cells. 

Environ Health Perspect, 113: 1555-1560. 087362

Geng H; Meng Z; Zhang Q (2005). Effects of blowing sand fine particles on plasma membrane permeability and fluidity, 

and intracellular calcium levels of rat alveolar macrophages. Toxicol Lett, 157: 129-137. 096689

Geng H; Meng Z; Zhang Q (2006). In vitro responses of rat alveolar macrophages to particle suspensions and water-soluble 

components of dust storm PM(2.5). Toxicol In Vitro, 20: 575-584. 097026

Ghelfi E; Rhoden CR; Wellenius GA; Lawrence J; Gonzalez-Flecha B (2008). Cardiac oxidative stress and 

electrophysiological changes in rats exposed to concentrated air particles are mediated by TRP-dependent 

pulmonary reflexes. Toxicol Sci, 102: 328-336. 156468

Ghio AJ; Churg A; Roggli VL (2004). Ferruginous bodies: Implications in the mechanism of fiber and particle toxicity. 

Toxicol Pathol, 32: 643-649. 155790

Ghio AJ; Cohen MD (2005). Disruption of iron homeostasis as a mechanism of biologic effect by ambient air pollution 

particles. Inhal Toxicol, 17: 709-716. 088272

Gilmour PS; Morrison ER; Vickers MA; Ford I; Ludlam CA; Greaves M; Donaldson K; MacNee W (2005). The 

procoagulant potential of environmental particles (PM10). Occup Environ Med, 62: 164-171. 087410

Gilmour PS; Nyska A; Schladweiler MC; McGee JK; Wallenborn JG; Richards JH; Kodavanti UP (2006). Cardiovascular 

and blood coagulative effects of pulmonary zinc exposure. Toxicol Appl Pharmacol, 211: 41-52. 088489

Gilmour PS; Rahman I; Donaldson K; MacNee W (2003). Histone acetylation regulates epithelial IL-8 release mediated by 

oxidative stress from environmental particles. Am J Physiol Lung Cell Mol Physiol, 284: L533-L540. 096959

Gilmour PS; Schladweiler MC; Nyska A; McGee JK; Thomas R; Jaskot RH; Schmid J; Kodavanti UP (2006). Systemic 

imbalance of essential metals and cardiac gene expression in rats following acute pulmonary zinc exposure. J 

Toxicol Environ Health A Curr Iss, 69: 2011-2032. 156472

Godleski JJ; Verrier RL; Koutrakis P; Catalano P; Coull B; Reinisch U; Lovett EG; Lawrence J; Murthy GG; Wolfson JM; 

Clarke RW; Nearing BD; Killingsworth C (2000). Mechanisms of morbidity and mortality from exposure to 

ambient air particles. Res Rep Health Eff Inst, 91: 5-88; discussion 89-103. 000738

Gopalakrishnan S; Van Emburgh BO; Robertson KD (2008). DNA methylation in development and human disease. Mutat 

Res, 647: 30-38. 191147

Gottipolu RR; Landa ER; Schladweiler MC; McGee JK; Ledbetter AD; Richards JH; Wallenborn GJ; Kodavanti UP 

(2008). Cardiopulmonary responses of intratracheally instilled tire particles and constituent metal components. 

Inhal Toxicol, 20: 473-84. 191148

December 2009 5-25 
Groneberg DA; Quarcoo D; Frossard N; Fischer A (2004). Neurogenic mechanisms in bronchial inflammatory diseases. 

Allergy, 59: 1139-52. 138134

Grunstein MM; Hazucha M; Sorli J; Milic-Emili J (1977). Effect of SO2 on control of breathing in anesthetized cats. J 

Appl Physiol, 43: 844-851. 071445

Halvorsen B; Otterdal K; Dahl TB; Skjelland M; Gullestad L; Oie E; Aukrust P (2008). Atherosclerotic plaque stability--

what determines the fate of a plaque? Prog Cardiovasc Dis, 51: 183-94. 191149

Hanel B; Law I; Mortensen J (2003). Maximal rowing has an acute effect on the blood-gas barrier in elite athletes. J Appl 

Physiol, 95: 1076-1082. 155826

Hitchler MJ; Domann FE (2007). An epigenetic perspective on the free radical theory of development. Free Radic Biol 

Med, 43: 1023-1036. 191151

Huang Y-CT; Soukup J; Harder S; Becker S (2003). Mitochondrial oxidant production by a pollutant dust and NOmediated apoptosis in human alveolar macrophage. Am J Physiol Lung Cell Mol Physiol, 284: C24-C32. 156573

Illum L (2006). Nasal clearance in health and disease. J Aerosol Med, 19: 92-99. 191205

Ilowite JS; Bennett WD; Sheetz MS; Groth ML; Nierman DM (1989). Permeability of the bronchial mucosa to 99mTcDTPA in asthma. Am Rev Respir Dis, 139: 1139-1143. 156584

Imrich A; Ning Y; Lawrence J; Coull B; Gitin E; Knutson M; Kobzik L (2007). Alveolar macrophage cytokine response to 

air pollution particles: oxidant mechanisms. Toxicol Appl Pharmacol, 218: 256-264. 155859

Jaspers I; Ciencewicki JM; Zhang W; Brighton LE; Carson JL; Beck MA; Madden MC (2005). Diesel exhaust enhances 

influenza virus infections in respiratory epithelial cells. Toxicol Sci, 85: 990-1002. 088115

Jiang J; Oberdrster G; Elder A; Gelein R; Mercer P; Biswas P (2008). Does nanoparticle activity depend upon size and 

crystal phase? Nanotoxicology, 2: 33-42. 156609

Jones JG; Minty BD; Royston D; Royston JP (1983). Carboxyhaemoglobin and pulmonary epithelial permeability in man. 

Thorax, 38: 129-133. 155884

Jones PA; Baylin SB (2007). The epigenomics of cancer. Cell, 128: 683-92. 191153

Kaan PM; Hegele RG (2003). Interaction between respiratory syncytial virus and particulate matter in guinea pig alveolar 

macrophages. Am J Respir Cell Mol Biol, 28: 697-704. 095753

Karoly ED; Li Z; Dailey LA; Hyseni X; Huang YCT (2007). Up-regulation of Tissue Factor in Human Pulmonary Artery 

Endothelial Cells after Ultrafine Particle Exposure. Environ Health Perspect, 115: 535-540. 155890

Kaul N; Forman HJ (1996). Activation of NF?B by the respiratory burst of macrophages. Free Radic Biol Med, 21: 401-

405. 155892

Keverne EB; Curley JP (2008). Epigenetics, brain evolution and behaviour. Front Neuroendocrinol, 29: 398-412. 191154

Kleinman MT; Araujo JA; Nel A; Sioutas C; Campbell A; Cong PQ; Li H; Bondy SC (2008). Inhaled ultrafine particulate 

matter affects CNS inflammatory processes and may act via MAP kinase signaling pathways. Toxicol Lett, 178: 

127-130. 190074

Kodavanti UP; Schladweiler MC; Gilmour PS; Wallenborn JG; Mandavilli BS; Ledbetter AD; Christiani DC; Runge MS; 

Karoly ED; Costa DL; Peddada S; Jaskot R; Richards JH; Thomas R; Madamanchi NR; Nyska A (2008). The role 

of particulate matter-associated zinc in cardiac injury in rats. Environ Health Perspect, 116: 13-20. 155907

Koike E; Hirano S; Furuyama A; Kobayashi T (2004). cDNA microarray analysis of rat alveolar epithelial cells following 

exposure to organic extract of diesel exhaust particles. Toxicol Appl Pharmacol, 201: 178-185. 058555

Koike E; Kobayashi T (2005). Organic extract of diesel exhaust particles stimulates expression of Ia and costimulatory 

molecules associated with antigen presentation in rat peripheral blood monocytes but not in alveolar macrophages. 

Toxicol Appl Pharmacol, 209: 277-285. 088303

Lee CC; Cheng YW; Kang JJ (2005). Motorcycle exhaust particles induce IL-8 production through NF-kappaB activation 

in human airway epithelial cells. J Toxicol Environ Health A Curr Iss, 68: 1537-1555. 156682

Lee SS; Woo CH; Chang JD; Kim JH (2003). Roles of Rac and cytosolic phospholipase A2 in the intracellular signalling in 

response to titanium particles. Cell Signal, 15: 339-345. 156678

December 2009 5-26 
Li J; Ghio AJ; Cho SH; Brinckerhoff CE; Simon SA; Liedtke W (2009). Diesel exhaust particles activate the matrixmetalloproteinase-1 gene in human bronchial epithelia in a beta-arrestin-dependent manner via activation of RAS. 

Environ Health Perspect, 117: 400-409. 190424

Li N; Nel AE (2006). Role of the Nrf2-mediated signaling pathway as a negative regulator of inflammation: Implications 

for the impact of particulate pollutants on asthma. Antioxid Redox Signal, 8: 88-98. 156694

Li N; Sioutas C; Cho A; Schmitz D; Misra C; Sempf J; Wang M; Oberley T; Froines J; Nel A (2003). Ultrafine particulate 

pollutants induce oxidative stress and mitochondrial damage. Environ Health Perspect, 111: 455-460. 042082

Li N; Wang M; Bramble LA; Schmitz DA; Schauer JJ; Sioutas C; Harkema JR; Nel AE (2009). The adjuvant effect of 

ambient particulate matter is closely reflected by the particulate oxidant potential . Environ Health Perspect, 117: 

1116-1123. 190457

Li Y-J; Kawada T; Matsumoto A; Azuma A; Kudoh S; Takizawa H; Sugawara I (2007). Airway inflammatory responses to 

oxidative stress induced by low-dose diesel exhaust particle exposure differ between mouse strains. Exp Lung Res, 

33: 227-244. 155929

Libby P (2002). Inflammation in atherosclerosis. Nature, 420: 868-874 . 192009

Lindbom J; Gustafsson M; Blomqvist G; Dahl A; Gudmundsson A; Swietlicki E; Ljungman AG (2007). Wear particles 

generated from studded tires and pavement induces inflammatory reactions in mouse macrophage cells. Chem Res 

Toxicol, 20: 937-946. 155934

Liu J; Ballaney M; Al-Alem U; Quan C; Jin X; Perera F; Chen LC; Miller RL (2008). Combined Inhaled Diesel Exhaust 

Particles and Allergen Exposure Alter Methylation of T Helper Genes and IgE Production In Vivo. Toxicol Sci, 102: 

76-81. 156709

Long JF; Waldman WJ; Kristovich R; Williams M; Knight D; Dutta PK (2005). Comparison of ultrastructural cytotoxic 

effects of carbon and carbon/iron particulates on human monocyte-derived macrophages. Environ Health Perspect, 

113: 170-174. 087454

Longhurst JC; Tjen-a-looi SC; Fu LW (2001). Cardiac sympathetic afferent activation provoked by myocardial ischemia 

and reperfusion. Mechanisms and reflexes. Ann N Y Acad Sci, 940: 74-95. 191158

Lund AK; Lucero J; Lucas S; Madden MC; McDonald JD; Seagrave JC; Knuckles TL; Campen MJ (2009). Vehicular 

emissions induce vascular MMP-9 expression and activity associated with endothelin-1 mediated pathways. 

Arterioscler Thromb Vasc Biol, 29: 511-517. 180257

Mackman N (2005). Tissue-Specific Hemostasis in Mice. Arterioscler Thromb Vasc Biol, 25: 2273-2281. 156722

Mercer BA; Kolesnikova N; Sonett J; D'Armiento J (2004). Extracellular regulated kinase/mitogen activated protein kinase 

is up-regulated in pulmonary emphysema and mediates matrix metalloproteinase-1 induction by cigarette smoke. J 

Biol Chem, 279: 17690-17696. 191180

Moffitt JA; Grippo AJ; Holmes PV; Johnson AK (2002). Olfactory bulbectomy attenuates cardiovascular 

sympathoexcitatory reflexes in rats. Am J Physiol Heart Circ Physiol, 283: H2575-H2583. 191160

Molinelli AR; Madden MC; McGee JK; Stonehuerner JG; Ghio AJ (2002). Effect of metal removal on the toxicity of 

airborne particulate matter from the Utah Valley. Inhal Toxicol, 14: 1069-1086. 035347

Moller W; Brown DM; Kreyling WG; Stone V (2005). Ultrafine particles cause cytoskeletal dysfunctions in macrophages: 

role of intracellular calcium. Part Fibre Toxicol, 2: 7. 156770

Monn C; Naef R; Koller T (2003). Reactions of macrophages exposed to particles <10 "micron"m. Environ Res, 91: 35-44. 

052418

Mutlu GM; Green D; Bellmeyer A; Baker CM; Burgess Z; Rajamannan N; Christman JW; Foiles N; Kamp DW; Ghio AJ; 

Chandel NS; Dean DA; Sznajder JI; Budinger GR (2007). Ambient particulate matter accelerates coagulation via an 

IL-6-dependent pathway. J Clin Invest, 117: 2952-2961. 121441

Nadel JA; Salem H; Tamplin B; Tokiwa Y (1965). Mechanism of bronchoconstriction during inhalation of sulfur dioxide. J 

Appl Physiol, 20: 164-167. 014846

Newby AC (2008). Metalloproteinase expression in monocytes and macrophages and its relationship to atherosclerotic 

plaque instability. Arterioscler Thromb Vasc Biol, 28: 2108-2114. 191161

December 2009 5-27 
Nurkiewicz TR; Porter DW; Barger M; Millecchia L; Rao KMK; Marvar PJ; Hubbs AF; Castranova V; Boegehold MA 

(2006). Systemic microvascular dysfunction and inflammation after pulmonary particulate matter exposure. 

Environ Health Perspect, 114: 412-419. 088611

Nurkiewicz TR; Porter DW; Hubbs AF; Stone S; Chen BT; Frazer DG; Boegehold MA; Castranova V (2009). Pulmonary 

nanoparticle exposure disrupts systemic microvascular nitric oxide signaling. Toxicol Sci, 110: 191-203. 191961

Oberdörster G (1988). Lung clearance of inhaled insoluble and soluble particles. J Aerosol Med, 1: 289-330. 006857

Oortgiesen M; Veronesi B; Eichenbaum G; Kiser PF; Simon SA (2000). Residual oil fly ash and charged polymers activate 

epithelial cells and nociceptive sensory neurons. Am J Physiol, 278: L683-L695. 013998

Owen CA (2008). Roles for proteinases in the pathogenesis of chronic obstructive pulmonary disease. Int J of COPD, 3: 

253-268. 191162

Pardo A; Selman M (2005). MMP-1: the elder of the family. Int J Biochem Cell Biol, 37: 283-8. 191163

Penn A; Murphy G; Barker S; Henk W; Penn L (2005). Combustion-derived ultrafine particles transport organic toxicants 

to target respiratory cells. Environ Health Perspect, 113: 956-963. 088257

Pinkerton KE; Joad JP (2006). Influence of air pollution on respiratory health during perinatal development. Clin Exp 

Pharmacol Physiol, 33: 269-272. 091237

Rahman I; Biswas SK; Kode A (2006). Oxidant and antioxidant balance in the airways and airway diseases. Eur J 

Pharmacol, 533: 222-39. 191165

Ramage L; Guy K (2004). Expression of C-reactive protein and heat-shock protein-70 in the lung epithelial cell line A549, 

in response to PM10 exposure. Inhal Toxicol, 16: 447-452. 055640

Randell SH; Mercer RR; Young SL (1990). Neonatal hyperoxia alters the pulmonary alveolar and capillary structure of 40-

day-old rats. Am J Pathol, 136: 1259-1266. 191956

Reed MD; Barrett EG; Campen MJ; Divine KK; Gigliotti AP; McDonald JD; Seagrave JC; Mauderly JL; Seilkop SK; 

Swenberg JA (2008). Health effects of subchronic inhalation exposure to gasoline engine exhaust. Inhal Toxicol, 

20: 1125-1143. 156903

Rengasamy A; Barger MW; Kane E; Ma JKH; Castranova V; Ma JYC (2003). Diesel exhaust particle-induced alterations 

of pulmonary phase I and phase II enzymes of rats. J Toxicol Environ Health A Curr Iss, 66: 153-167. 156907

Rhoden CR; Lawrence J; Godleski JJ; Gonzalez-Flecha B (2004). N-acetylcysteine prevents lung inflammation after shortterm inhalation exposure to concentrated ambient particles. Toxicol Sci, 79: 296-303. 087969

Rhoden CR; Wellenius GA; Ghelfi E; Lawrence J; Gonzalez-Flecha B (2005). PM-induced cardiac oxidative stress and 

dysfunction are mediated by autonomic stimulation. Biochim Biophys Acta, 1725: 305-313. 087878

Rinderknecht J; Shapiro L; Krauthammer M; Taplin G; Wasserman K; Uszler JM; Effros RM (1980). Accelerated clearance 

of small solutes from the lungs in interstitial lung disease. Am Rev Respir Dis, 121: 105-117. 191965

Risom L Moller P; Loft S (2005). Oxidative stress-induced DNA damage by particulate air pollution. Mutat Res, 592: 119-

137. 089070

Roberts ES; Richards JH; Jaskot R; Dreher KL (2003). Oxidative stress mediates air pollution particle-induced acute lung 

injury and molecular pathology. Inhal Toxicol, 15: 1327-1346. 156051

Roberts JR; Young S-H; Castranova V; Antonini JM (2007). Soluble metals in residual oil fly ash alter innate and adaptive 

pulmonary immune responses to bacterial infection in rats. Toxicol Appl Pharmacol, 221: 306-319. 097623

Rouse RL; Murphy G; Boudreaux MJ; Paulsen DB; Penn AL (2008). Soot nanoparticles promote biotransformation, 

oxidative stress, and inflammation in murine lungs. Am J Respir Cell Mol Biol, 39: 198-207. 156930

Ruckerl R; Greven S; Ljungman P; Aalto P; Antoniades C; Bellander T; Berglind N; Chrysohoou C; Forastiere F; 

Jacquemin B; von Klot S; Koenig W; Kuchenhoff H; Lanki T; Pekkanen J; Perucci CA; Schneider A; Sunyer J; 

Peters A (2007). Air pollution and inflammation (interleukin-6, C-reactive protein, fibrinogen) in myocardial 

infarction survivors. Environ Health Perspect, 115: 1072-1080. 156931

Sakamoto N; Hayashi S; Gosselink J; Ishii H; Ishimatsu Y; Mukae H; Hogg JC; van Eeden SF (2007). Calcium dependent 

and independent cytokine synthesis by air pollution particle-exposed human bronchial epithelial cells. Toxicol Appl 

Pharmacol, 225: 134-141. 096282

December 2009 5-28 
Sarin S; Undem B; Sanico A; Togias A (2006). The role of the nervous system in rhinitis. J Allergy Clin Immunol, 118: 

999-1016. 191166

Schins RPF; Knaapen AM (2007). Genotoxicity of poorly soluble particles. Inhal Toxicol, 19 Suppl 1: 189-198. 156074

Seaton A; MacNee W; Donaldson K; Godden D (1995). Particulate air pollution and acute health effects. Lancet, 345: 176-

178. 045721

Sekizawa S; Chen CY; Bechtold AG; Tabor JM; Bric JM; Pinkerton KE; Joad JP; Bonham AC (2008). Extended 

secondhand tobacco smoke exposure induces plasticity in nucleus tractus solitarius second-order lung afferent 

neurons in young guinea pigs. Eur J Neurosci, 28: 771-81. 191167

Semmler-Behnke M; Takenaka S; Fertsch S; Wenk A; Seitz J; Mayer P; Oberdörster G; Kreyling WG (2007). Efficient 

elimination of inhaled nanoparticles from the alveolar region: evidence for interstitial uptake and subsequent 

reentrainment onto airways epithelium. Environ Health Perspect, 115: 728-733. 156080

Shi T; Knaapen AM; Begerow J; Birmili W; Borm PJA; Schins RPF (2003). Temporal variation of hydroxyl radical 

generation and 8-hydroxy-2'-deoxyguanosine formation by coarse and fine particulate matter. Occup Environ Med, 

60: 315-321. 088248

Sirivelu MP; MohanKumar SMJ; Wagner JG; Harkema JR; MohanKumar PS (2006). Activation of the stress axis and 

neurochemical alterations in specific brain areas by concentrated ambient particle exposure with concomitant 

allergic airway disease. Environ Health Perspect, 114: 870-874. 111151

Snipes MB (1996). Current information on lung overload in nonrodent mammals: contrast with rats. Inhal Toxicol, 8: 91-

109. 076041

Snipes MB; Harkema JR; Hotchkiss JA; Bice DE (1997). Neutrophil Involvement in the Retention and Clearance of Dust 

Intratracheally Instilled into the LUNGS of F344/N Rats. Exp Lung Res, 23: 65-84. 156092

Soberanes S; Panduri V; Mutlu GM; Ghio A; Bundinger GRS; Kamp DW (2006). p53 mediates particulate matter-induced 

alveolar epithelial cell mitochondria-regulated apoptosis. Am J Respir Crit Care Med, 174: 1229-1238. 156991

Soberanes S; Urich D; Baker CM; Burgess Z; Chiarella SE; Bell EL; Ghio AJ; De Vizcaya-Ruiz A; Liu J; Ridge KM; 

Kamp DW; Chandel NS; Schumacker PT; Mutlu GM; Budinger GRS (2009). Mitochondrial complex III-generated 

oxidants activate ASK1 and JNK to induce alveolar epithelial cell death following exposure to particulate matter air 

pollution. J Biol Chem, 284: 2176-2186. 190483

Steerenberg PA; Withagen CET; van Dalen WJ; Dormans JAMA; van Loveren H (2004). Adjuvant Activity of Ambient 

Particulate Matter in Macrophage Activity-Suppressed, N-Acetylcysteine-Treated, iNOS- and IL-4-Deficient Mice. 

Inhal Toxicol, 16: 835-843. 087981

Sun Q; Wang A; Jin X; Natanzon A; Duquaine D; Brook RD; Aguinaldo JG; Fayad ZA; Fuster V; Lippmann M; Chen LC; 

Rajagopalan S (2005). Long-term air pollution exposure and acceleration of atherosclerosis and vascular 

inflammation in an animal model. JAMA, 294: 3003-3010. 087952

Takizawa H; Abe S; Okazaki H; Kohyama T; Sugawara I; Saito Y; Ohtoshi T; Kawasaki S; Desaki M; Nakahara K; 

Yamamoto K; Matsushima K; Tanaka M; Sagai M; Kudoh S (2003). Diesel exhaust particles upregulate eotaxin 

gene expression in human bronchial epithelial cells via nuclear factor-kappa B-dependent pathway. Am J Physiol 

Lung Cell Mol Physiol, 284: L1055-L1062. 157039

Tal TL; Graves LM; Silbajoris R; Bromberg PA; Wu W; Samet JM (2006). Inhibition of protein tyrosine phosphatase 

activity mediates epidermal growth factor receptor signaling in human airway epithelial cells exposed to Zn2+. 

Toxicol Appl Pharmacol, 214: 16-23. 108588

Tamaoki J; Isono K; Takeyama K; Tagaya E; Nakata J; Nagai A (2004). Ultrafine carbon black particles stimulate 

proliferation of human airway epithelium via EGF receptor-mediated signaling pathway. Am J Physiol Lung Cell 

Mol Physiol, 287: L1127-L1133. 157040

Tankersley CG; Shank JA; Flanders SE; Soutiere SE; Rabold R; Mitzner W; Wagner EM (2003). Changes in lung 

permeability and lung mechanics accompany homeostatic instability in senescent mice. J Appl Physiol, 95: 1681-

1687. 096363

Tao F; Gonzalez-Flecha B; Kobzik L (2003). Reactive oxygen species in pulmonary inflammation by ambient particulates. 

Free Radic Biol Med, 35: 327-340. 156111

December 2009 5-29 
Tao F; Kobzik L (2002). Lung macrophage-epithelial cell interactions amplify particle-mediated cytokine release. Am J 

Respir Cell Mol Biol, 26: 499-505. 157044

Tarantini L; Bonzini M; Apostoli P; Pegoraro V; Bollati V; Marinelli B; Cantone L; Rizzo G; Hou L; Schwartz J; Bertazzi 

PA; Baccarelli A (2009). Effects of particulate matter on genomic DNA methylation content and iNOS promoter 

methylation. Environ Health Perspect, 117: 217-222. 192010

Tarantini L; Bonzini M; Apostoli P; Pegoraro V; Bollati V; Marinelli B; Cantone L; Rizzo G; Hou L; Schwartz J; Bertazzi 

PA; Baccarelli A (2009). Errata to Effects of particulate matter on genomic DNA methylation content and iNOS 

promoter methylation . Environ Health Perspect, 117: A143. 192153

Thomson E; Kumarathasan P; Goegan P; Aubin RA; Vincent R (2005). Differential regulation of the lung endothelin 

system by urban particulate matter and ozone. Toxicol Sci, 88: 103-113. 087554

Upadhyay D; Panduri V; Ghio A; Kamp DW (2003). Particulate matter induces alveolar epithelial cell DNA damage and 

apoptosis: role of free radicals and the mitochondria. Am J Respir Cell Mol Biol, 29: 180-187. 097370

Valinluck V; Tsai HH; Rogstad DK; Burdzy A; Bird A; Sowers LC (2004). Oxidative damage to methyl-CpG sequences 

inhibits the binding of the methyl-CpG binding domain (MBD) of methyl-CpG binding protein 2 (MeCP2). Nucleic 

Acids Res, 32: 4100-8. 191170

Van Eeden SF; Hogg JC (2002). Systemic inflammatory response induced by particulate matter air pollution: the 

importance of bone-marrow stimulation. J Toxicol Environ Health A Curr Iss, 65: 65:1597-1613. 088111

Van Eeden SF; Tan WC; Suwa T; Mukae H; Terashima T; Fujii T; Qui D; Vincent R; Hogg JC (2001). Cytokines involved 

in the systemic inflammatory response induced by exposure to particulate matter air pollutants (PM10). Am J 

Respir Crit Care Med, 164: 826-830. 019018

van Eeden SF; Yeung A; Quinlam K; Hogg JC (2005). Systemic response to ambient particulate matter: relevance to 

chronic obstructive pulmonary disease. Proc Am Thorac Soc, 2: 61-67. 157086

Veronesi B; Makwana O; Pooler M; Chen LC (2005). Effects of subchronic exposures to concentrated ambient particles: 

VII. Degeneration of dopaminergic neurons in Apo E-/- mice. Inhal Toxicol, 17: 235-241. 087481

Veronesi B; Oortgiesen M (2001). Neurogenic inflammation and particulate matter (PM) air pollutants. Neurotoxicology, 

22: 795-810. 015977

Veronesi B; Oortgiesen M; Carter JD; Devlin RB (1999). Particulate matter initiates inflammatory cytokine release by 

activation of capsaicin and acid receptors in a human bronchial epithelial cell line. Toxicol Appl Pharmacol, 154: 

106-115. 048764

Veronesi B; Oortgiesen M; Roy J; Carter JD; Simon SA; Gavett SH (2000). Vanilloid (capsaicin) receptors influence 

inflammatory sensitivity in response to particulate matter. Toxicol Appl Pharmacol, 169: 66-76. 017062

Veronesi B; Wei G; Zeng JQ; Oortgiesen M (2003). Electrostatic charge activates inflammatory vanilloid (VR1) receptors. 

Neurotoxicology, 24: 463-473. 094384

Wallenborn JG; Evansky P; Shannahan JH; Vallanat B; Ledbetter AD; Schladweiler MC; Richards JH; Gottipolu RR; 

Nyska A; Kodavanti UP (2008). Subchronic inhalation of zinc sulfate induces cardiac changes in healthy rats. 

Toxicol Appl Pharmacol, 232: 69-77. 191171

Wallenborn JG; Kovalcik KD; McGee JK; Landis MS; Kodavanti UP (2009). Systemic translocation of (70)zinc: kinetics 

following intratracheal instillation in rats. Toxicol Appl Pharmacol, 234: 25-32. 191172

Wallenborn JG; McKee JK; Schladweiler MC; Ledbetter AD; Kodavanti UP (2007). Systemic translocation of particulate 

matter-associated metals following a single intratracheal instillation in rats. J Toxicol Sci, 98: 231-239. 156144

Wan H; Winton HL; Soeller C; Tovey ER; Gruenert DC; Thompson PJ; Stewart GA; Taylor GW; Garrod DR; Cannell MB; 

Robinson C (1999). Der p 1 facilitates transepithelial allergen delivery by disruption of tight junctions. J Clin 

Invest, 104: 123-133. 191903

Wan J; Diaz-Sanchez D (2006). Phase II enzymes induction blocks the enhanced IgE production in B cells by diesel 

exhaust particles. J Immunol, 177: 3477-3483. 097399

Wan J; Diaz-Sanchez D (2007). Antioxidant enzyme induction: a new protective approach against the adverse effects of 

diesel exhaust particles. Inhal Toxicol, 19 Suppl 1: 177-182. 156145

December 2009 5-30 
December 2009 5-31 

Wang L; Pinkerton KE (2007). Air pollutant effects on fetal and early postnatal development. Birth Defects Res C Embryo 

Today, 81: 144-154. 179975

Warheit DB; Webb TR; Colvin VL; Reed KL; Sayes CM (2007). Pulmonary bioassay studies with nanoscale and finequartz particles in rats: toxicity is not dependent upon particle size but on surface characteristics. Toxicol Sci, 95: 

270-280. 090482

Widdicombe J (2006). Reflexes from the lungs and airways: historical perspective. J Appl Physiol, 101: 628-634. 155519

Widdicombe J; Lee L-Y (2001). Airway reflexes, autonomic function, and cardiovascular responses. Environ Health 

Perspect, 4: 579-584. 019049

Widdicombe JG (2003). Overview of neural pathways in allergy and asthma. Pulm Pharmacol Ther, 16: 23-30. 157145

Win-Shwe TT; Yamamoto S; Fujitani Y; Hirano S; Fujimaki H (2008). Spatial learning and memory function-related gene 

expression in the hippocampus of mouse exposed to nanoparticle-rich diesel exhaust. Neurotoxicology, 29: 940-

947. 190146

Wong SS; Sun NN; Keith I; Kweon C-B; Foster DE; Schauer James J; Witten ML (2003). Tachykinin substance P signaling 

involved in diesel exhaust-induced bronchopulmonary neurogenic inflammation in rats. Arch Toxicol, 77: 638-650. 

097707

Wright RO; Baccarelli A (2007). Metals and neurotoxicology. J Nutr, 137: 2809-2813. 191173

Xia T; Korge P; Weiss JN; Li N; Venkatesen MI; Sioutas C; Nel A (2004). Quinones and aromatic chemical compounds in 

particulate matter induce mitochondrial dysfunction: implications for ultrafine particle toxicity. Environ Health 

Perspect, 112: 1347-1358. 087486

Xiao GG; Nel AE; Loo JA (2005). Nitrotyrosine-modified proteins and oxidative stress induced by diesel exhaust particles. 

Electrophoresis, 26: 280-292. 156164

Yauk C; Polyzos A; Rowan-Carroll A; Somers CM; Godschalk RW; Van Schooten FJ; Berndt ML; Pogribny IP; Koturbash 

I; Williams A; Douglas GR; Kovalchuk O (2008). Germ-line mutations, DNA damage, and global hypermethylation 

in mice exposed to particulate air pollution in an urban/industrial location. PNAS, 105: 605-610. 157164

Yin XJ; Ma JYC; Antonini JM; Castranova V; Ma JKH (2004). Roles of reactive oxygen species and heme oxygenase-1 in 

modulation of alveolar macrophage-mediated pulmonary immune responses to listeria monocytogenes by diesel 

exhaust particles. Toxicol Sci, 82: 143-153. 087983

Ying Z; Kampfrath T; Thurston G; Farrar B; Lippmann M; Wang A; Sun Q; Chen LC; Rajagopalan S (2009). Ambient 

particulates alter vascular function through induction of reactive oxygen and nitrogen species. Toxicol Sci, 111: 80-

88. 190111

Zhang J; Ghio AJ; Chang W; Kamdar O; Rosen GD; Upadhyay D (2007). Bim mediates mitochondria-regulated particulate 

matter-induced apoptosis in alveolar epithelial cells. FEBS Lett, 581: 4148-4152. 156179

Zhao H; Barger MW; Ma JKH; Castranova V; Ma JYC (2006). Cooperation of the inducible nitric oxide synthase and 

cytochrome P450 1A1 in mediating lung inflammation and mutagenicity induced by diesel exhaust particles. 

Environ Health Perspect, 114: 1253-1258. 100996