Environmentally persistent free radicals decrease cardiac function before and after ischemia/reperfusion injury in vivo

First-principles study on the heterogeneous formation of environmentally persistent free radicals (EPFRs) over α-Fe2O3(0001) surface: Effect of oxygen vacancy

Metal oxides with oxygen vacancies have a significant impact on catalytic activity for the transformation of organic pollutants in waste-to-energy (WtE) incineration processes. This study aims to investigate the influence of hematite surface oxygen point defects on the formation of environmentally persistent free radicals (EPFRs) from phenolic compounds based on the first-principles calculations. Two oxygen-deficient conditions were considered: oxygen vacancies at the top surface and on the subsurface. Our simulations indicate that the adsorption strength of phenol on the α-Fe2O3(0001) surface is enhanced by the presence of oxygen vacancies. However, the presence of oxygen vacancies has a negative impact on the dissociation of the phenol molecule, particularly for the surface with a defective point at the top layer. Thermo-kinetic parameters were established over a temperature range of 300-1000 K, and lower reaction rate constants were observed for the scission of phenolic O-H bonds over the oxygen-deficient surfaces compared to the pristine surface. The negative effects caused by the oxygen-deficient conditions could be attributed to the local reduction of FeIII to FeII, which lower the oxidizing ability of surface reaction sites. The findings of this study provide us a promising approach to regulate the formation of EPFRs.

Inhalation of particulate matter containing environmentally persistent free radicals induces endothelial dysfunction mediated via AhR activation at the air-blood interface

Particulate matter (PM) containing environmentally persistent free radicals (EPFR) is formed by the incomplete combustion of organic wastes, resulting in the chemisorption of pollutants to the surface of PM containing redox-active transition metals. In prior studies in mice, EPFR inhalation impaired endothelium-dependent vasodilation. These findings were associated with aryl hydrocarbon receptor (AhR) activation in the alveolar type-II (AT-II) cells that form the air-blood interface in the lung. We thus hypothesized that AhR activation in AT-II cells promotes the systemic release of mediators that promote endothelium dysfunction peripheral to the lung. To test our hypothesis, we knocked down AhR in AT-II cells of male and female mice and exposed them to 280 µg/m3 EPFR lo (2.7e + 16 radicals/g) or EPFR (5.5e + 17 radicals/g) compared with filtered air for 4 h/day for 1 day or 5 days. AT-II-AhR activation-induced EPFR-mediated endothelial dysfunction, reducing endothelium-dependent vasorelaxation by 59%, and eNOS expression by 50%. It also increased endothelin-1 mRNA levels in the lungs and peptide levels in the plasma in a paracrine fashion, along with soluble vascular cell adhesion molecule-1 and iNOS mRNA expression, possibly via NF-kB activation. Finally, AhR-dependent increases in antioxidant response signaling, coupled to increased levels of 3-nitrotyrosine in the lungs of EPFR-exposed littermate control but not AT-II AhR KO mice suggested that ATII-specific AhR activation promotes oxidative and nitrative stress. Thus, AhR activation at the air-blood interface mediates endothelial dysfunction observed peripheral to the lung, potentially via release of systemic mediators.

Can the concentration of environmentally persistent free radicals describe its toxicity to Caenorhabditis elegans? Evidence provided by neurotoxicity and oxidative stress

Environmentally persistent free radicals (EPFRs) are emerging pollutants stabilized on or inside particles. Although the toxicity of EPFR-containing particles has been confirmed, the conclusions are always ambiguous because of the presence of various compositions. A clear dose-response relationship was always challenged by the fact that the concentrations of these coexisted components simultaneously changed with EPFR concentrations. Without these solid dose-response pieces of evidence, we could not confidently conclude the toxicity of EPFRs and the description of potential EPFR risks. In this study, we established a particle system with a fixed catechol concentration but different reaction times to obtain particles with different EPFR concentrations. Caenorhabditis elegans (C. elegans) in response to different EPFR concentrations was systematically investigated at multiple biological levels, including behavior observations and biochemical and transcriptome analyses. Our results showed that exposure to EPFRs disrupted the development and locomotion of C. elegans. EPFRs cause concentration-dependent neurotoxicity and oxidative damage to C. elegans, which could be attributed to reactive oxygen species (ROS) promoted by EPFRs. Furthermore, the expression of key genes related to neurons was downregulated, whereas antioxidative genes were upregulated. Overall, our results confirmed the toxicity from EPFRs and EPFR concentration as a rational parameter to describe the extent of toxicity.

Role of Electronegativity in Environmentally Persistent Free Radicals (EPFRs) Formation on ZnO

Environmentally persistent free radicals (EPFRs), a group of emerging pollutants, have significantly longer lifetimes than typical free radicals. EPFRs form by the adsorption of organic precursors on a transition metal oxide (TMO) surface involving electron charge transfer between the organic and TMO. In this paper, dihalogenated benzenes were incorporated to study the role of electronegativity in the electron transfer process to obtain a fundamental knowledge of EPFR formation mechanism on ZnO. Upon chemisorption on ZnO nanoparticles at 250 °C, electron paramagnetic resonance (EPR) confirms the formation of oxygen adjacent carbon-centered organic free radicals with concentrations between 1016 and 1017 spins/g. The radical concentrations show a trend of 1,2-dibromobenzene (DBB) > 1,2-dichlorobenzene (DCB) > 1,2-difluorobenzene (DFB) illustrating the role of electronegativity on the amount of radical formation. X-ray absorption spectroscopy (XAS) confirms the reduction of the Zn2+ metal center, contrasting previous experimental evidence of an oxidative mechanism for ZnO single crystal EPFR formation. The extent of Zn reduction for the different organics (DBB > DCB > DFB) also correlates to their polarity. DFT calculations provide theoretical evidence of ZnO surface reduction and exhibit a similar trend of degree of reduction for different organics, further building on the experimental findings. The lifetimes of the EPFRs formed confirm a noteworthy persistency.

Formation Mechanism of Environmentally Persistent Free Radicals on Alkaline Earth Oxide Surfaces

The formation of environmentally persistent free radicals (EPFRs) is usually related to transition-metal oxides in particulate matter (PM). However, recent studies suggest that alkaline-earth-metal oxides (AEMOs) in PM also influence EPFRs formation, but the exact mechanism remains unclear. Here, density functional theory calculations were performed to investigate the formation mechanism of EPFRs by C6H5OH on AEMO (MgO, CaO, and BaO) surfaces and compare it with that on transition-metal oxide (ZnO and CuO) surfaces. Results indicate that EPFRs can be rapidly formed on AEMOs by dissociative adsorption of C6H5OH, accompanied by electrons transfer. As the alkalinity of AEMOs increases, both adsorption energy and the number of electron transfers gradually increase. Also, the stability of the formed EPFRs is mainly attributed to the electrostatic and van der Waals interactions between the phenoxy radical and surfaces. Notably, the formation mechanism of EPFRs on AEMOs is similar to that on ZnO but differs from that on CuO, as suggested through geometric structure and charge distribution analyses. This study not only elucidates the formation mechanisms of EPFRs on AEMOs but also provides theoretical insights into addressing EPFRs pollution.

A short review on environmental distribution and toxicity of the environmentally persistent free radicals

Environmentally Persistent Free Radicals (EPFRs) are usually generated by the electron transfer of a certain radical precursor on the surface of a carrier. They are characterized with high activity, wide migration range, and relatively long half-life period. In this review, we summarized the literature on EPFRs published since 2010, including their environmental occurrence and potential cytotoxicity and biotoxicity. The EPFRs in the atmosphere are the most abundant in the environment, mainly generated from the combustion of raw materials or biochar, and the C-center types (quinones, semiquinones radicals, etc.) may exist for a relatively long time. These EPFRs can transform into other substances (such as reactive oxygen species, ROS) under the influence of environmental factors, and partly enter soil and water by wet and dry deposition of particulate matter, which may promote the generation of EPFRs in those media. The wide distribution of EPFRs in the environment may lead to their exposure to biota including humans, resulting in cytotoxicity and biotoxicity. The EPFRs can influence the normal redox process of the biota, and generate a large number of free radicals like ROS. Exposure to EPFRs may change the expression of gene and activity of metabolic enzymes, and damage the cells, as well as some organs such as the lung, trachea, and heart. However, due to the difficulty in sample extraction, identification, and quantification of the specific EPFR individuals, the toxicity and exposure evaluation of biota are still limited which merits study in the future.

Comparative 6+studies of environmentally persistent free radicals on nano-sized coal dusts

Coal dust is the major hazardous pollutant in the coal mining environment. Recently environmentally persistent free radicals (EPFRs) were identified as one of the key characteristics which could impart toxicity to the particulates released into the environment. The present study used Electron Paramagnetic Resonance (EPR) spectroscopy to analyze the characteristics of EPFRs present in different types of nano-size coal dust. Further, it analyzed the stability of the free radicals in the respirable nano-size coal dust and compared their characteristics in terms of EPR parameters (spin counts and g-values). It was found that free radicals in coal are remarkably stable (can remain intact for several months). Also, Most of the EPFRs in the coal dust particles are either oxygenated carbon centered or a mixture of carbon and oxygen-centered free radicals. EPFRs concentration in the coal dust was found to be proportional to the carbon content of coal. The characteristic g-values were found to be inversely related to the carbon content of coal dust. The spin concentrations in the lignite coal dust were between 3.819 and 7.089 μmol/g, whereas the g-values ranged from 2.00352 to 2.00363. The spin concentrations in the bituminous coal dust were between 11.614 and 25.562 μmol/g, whereas the g-values ranged from 2.00295 to 2.00319. The characteristics of EPFRs present in coal dust identified by this study are similar to the EPFRs, which were found in other environmental pollutants such as combustion-generated particulates, PM_2.5, indoor dust, wildfires, biochar, haze etc., in some of the previous studies. Considering the toxicity analysis of environmental particulates containing EPFRs similar to those identified in the present study, it can be confidently hypothesized that the EPFRs in the coal dust might play a major role in modulating the coal dust toxicity. Hence, it is recommended that future studies should analyze the role of EPFR-loaded coal dust in mediating the inhalation toxicity of coal dust.

Scientific and regulatory challenges of environmentally persistent free radicals: From formation theory to risk prevention strategies

EPFRs (Environmentally Persistent Free Radicals) are a class of pollutants that have been identified as potential environmental contaminants due to their persistence and ability to generate reactive oxygen species (ROS) that can cause oxidative stress in living organisms. However, no study has comprehensively summarized the production conditions, influencing factors and toxic mechanisms of EPFRs, impeding exposure toxicity assessments and risk prevention strategies. To bridge the gap between theoretical research and practical application, a thorough literature review to summarize the formation, environmental effects, and biotoxicity of EPFRs are conducted. A total of 470 relevant papers were screened in Web of Science Core collection databases. The transfer of electrons between interfaces and the cleavage of covalent bonds of persistent organic pollutants is crucial to the generation of EPFRs, which is induced by external sources of energy, including thermal energy, light energy, transition metal ions, and others. In the thermal system, the stable covalent bond of organic matter can be destroyed by heat energy at low temperature to form EPFRs, while the formed EPFRs can be destroyed at high temperature. Light can also accelerate the production of free radicals and promote the degradation of organic matter. The persistence and stability of EPFRs are synergistically influenced by individual environmental factors such as environmental humidity, oxygen content, organic matter content, and environmental pH. Studying the formation mechanism of EPFRs and their biotoxicity is essential for fully understanding the hazards posed by these emerging environmental contaminants.

Direct toxicity of environmentally persistent free radicals to nematode Caenorhabditis elegans after excluding the concomitant chemicals

Environmentally persistent free radicals (EPFRs) have attracted extensive attention due to their potential toxicity. However, EPFRs-containing particles always coexist with their parent organic contaminants and intermediate degradation products (IM), which may have hindered the toxicity assessment of EPFRs. In this study, the toxicity of EFFRs was specifically verified after comparing the systems without EPFRs, such as the immediate mixture of catechol (CT) and particles, solutions of CT only, IM extracted from the particles, as well as particles after EPFRs quenching. Caenorhabditis elegans (C. elegans) were used as model organisms. Our results showed that EPFRs-containing particles (Si-Al-CT) exhibited significant toxicity to C. elegans, but not for the parent chemical CT and IM on the particles. Higher levels of reactive oxygen species (ROS) and malondialdehyde (MDA) in the Si-Al-CT system were attributed to the mediated generation of ·O₂^- and ·OH via EPFRs. EPFRs could increase gene expressions related not only to oxidative stress and biotransformation in C. elegans, but also to indications of disturbances in energy homeostasis, survival, proliferation, cell and embryonic development. Overall, these results confirmed the direct toxicity of EPFRs and highlighted the key role of EPFRs which may be neglected in assessing the environmental risks of organic contaminants.

A critical review of environmentally persistent free radical (EPFR) solvent extraction methodology and retrieval efficiency

Long-lived environmentally persistent free radical (EPFR) exposures have been shown in toxicology studies to lead to respiratory and cardiovascular effects, which were thought to be due to the persistence of EPFR and their ability to produce reactive oxygen species. To characterize EPFR exposure and resulting health impacts, it is necessary to identify and systematize analysis protocols. Both direct measurement and solvent extraction methods have been applied to analyze environmental samples containing EPFR. The use of different protocols and solvents in EPFR analyses makes it difficult to compare results among studies. In this work, we reviewed EPFR studies that involved solvent extraction and carefully reported the details of the extraction methodology and retrieval recovery. EPFR recovery depends on the structure of the radical species and the solvent. For the limited number of studies available for review, the polar solvents had superior recovery in more studies. Radicals appeared to be more oxygen-centered following extraction for fly ash and particulate matter (PM) samples. Different solvent extraction methods to retrieve EPFR may produce molecular products during the extraction, thus potentially changing the sample toxicity. The number of studies reporting detailed methodologies is limited, and data in these studies were not consistently reported. Thus, inference about the solvent and protocol that leads to the highest EPFR extraction efficiency for certain types of radicals is not currently possible. Based on our review, we proposed reporting criteria to be included for future EPFR studies.

Formation of Environmentally Persistent Free Radicals (EPFRs) on the Phenol-Dosed α-Fe2O3(0001) Surface

Environmentally persistent free radicals (EPFRs) are a class of toxic air pollutants that are found to form by the chemisorption of substituted aromatic molecules on the surface of metal oxides. In this study, we employ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to perform a temperature-dependent study of phenol adsorption on α-Fe₂O₃(0001) to probe the radical formation mechanism by monitoring changes in the electronic structure of both the adsorbed phenol and metal oxide substrate. Upon dosing at room temperature, new phenol-derived electronic states have been clearly observed in the UPS spectrum at saturation coverage. However, upon dosing at high temperature (>200 °C), both photoemission techniques have shown distinctive features that strongly suggest electron transfer from adsorbed phenol to Fe₂O₃ surface atoms and consequent formation of a surface radical. Consistent with the experiment, DFT calculations show that phenoxyl adsorption on the iron oxide surface at RT leads to a minor charge transfer to the adsorbed molecule. The experimental findings at high temperatures agree well with the EPFRs' proposed formation mechanism and can guide future experimental and computational studies.

Inhalation of particulate matter containing free radicals leads to decreased vascular responsiveness associated with an altered pulmonary function

Airborne particulate matter (PM) is associated with an increased risk for cardiovascular diseases. Although the goal of thermal remediation is to eliminate organic wastes through combustion, when incomplete combustion occurs, organics chemisorb to transition metals to generate PM-containing environmentally persistent free radicals (EPFRs). Similar EPFR species have been detected in PM found in diesel and gasoline exhaust, woodsmoke, and urban air. Prior in vivo studies demonstrated that EPFRs reduce cardiac function secondary to elevations in pulmonary arterial pressures. In vitro studies showed that EPFRs increase ROS and cytokines in pulmonary epithelial cells. We thus hypothesized that EPFR inhalation would promote lung inflammation and oxidative stress, leading to systemic inflammation, vascular endothelial injury, and a decline in vascular function. Mice were exposed to EPFRs for either 4 h or for 4 h/day for 10 days and lung and vascular function were assessed. After a 4-h exposure, plasma nitric oxide (NO) was reduced while endothelin-1 (ET-1) was increased, however lung function was not altered. After 10 day, plasma NO and ET-1 levels were again altered and lung tidal volume was reduced. These time course studies suggested the vasculature may be an early target of injury. To test this hypothesis, an intermediate time point of 3 days was selected. Though the mice exhibited no marked inflammation in either the lung or the blood, we did note significantly reduced endothelial function concurrent with a reduction in lung tidal volume and an elevation in annexin V protein levels in the lung. Although vascular dysfunction was not dependent upon inflammation, it may be associated with an injury at the air-blood interface. Gene expression analysis suggested roles for oxidative stress and aryl hydrocarbon receptor (Ahr) signaling. Studies probing the relationship between pulmonary oxidative stress and AhR signaling at the air-blood interface with vascular dysfunction seem warranted.NEW & NOTEWORTHY Particulate matter (PM) resulting from the combustion of organic matter is known to contribute to cardiopulmonary disease. Despite hypotheses that cardiovascular dysfunction occurring after PM exposures is secondary to lung or systemic inflammation, these studies investigating exposures to PM-containing environmentally persistent free radicals (EPFRs) demonstrate that cardiovascular dysfunction precedes pulmonary inflammation. The cardiopulmonary health consequences of EPFRs have yet to be thoroughly evaluated, especially in healthy, adult mice. Our data suggest the vasculature as a direct target of PM exposure, and our studies aimed to elucidate the mechanisms contributing to EPFR-induced vascular dysfunction.

Phytosampling-a supplementary tool for particulate matter (PM) speciation characterization

Ambient air particulate matter (PM) and PM-associated environmentally persistent free radicals (EPFRs) have been documented to contribute to pollution-related health effects. Studies of ambient air PM potentially bear artifacts stemming from the collection methods. We have investigated the applicability of PM phytosampling (PHS) as a supplementary tool to a classic PM sampler in respect of achieving better PM chemical composition assessment (primarily organic fraction). Phytosampling is a static PM collection method relying on the particle entrapment by the plant's leaf through electrostatic forces and surface trichomes. We have investigated the differences in the EPFR and polycyclic aromatic hydrocarbon (PAH) speciation and concentration on ambient air PM for PHS and high-volume PM sampler (HVS). The advantages of PHS are easy particle recovery from the matrix, collection under natural environmental conditions, and the ability to apply a dense collection network to accurately represent spatial pollutant distribution. The experimental results show that the PHS can provide valuable speciation information, sometimes different from that observed for HVS. For PM collected by PHS, we detected the larger contribution of oxygen-centered EPFRs, different decay behavior, and more consistent PAH distribution between different PM sizes compared to the PM from HVS. These results indicate that the isolation of samples from the ambient during HVS sampling and exposure to high-volume airflow may alter the chemical composition of the samples, while the PHS method could provide details on the original speciation and concentration and be more representative of the PM surface. However, PHS cannot evaluate an absolute air concentration of PM, so it serves as an excellent supplementary tool to work in conjunction with the standard PM collection method.

Formation of Environmentally Persistent Free Radicals during Thermochemical Processes and their Correlations with Unintentional Persistent Organic Pollutants

Attention is increasingly being paid to environmentally persistent free radicals (EPFRs), which are organic pollutants with the activities of free radicals and stabilities of organic pollutants. EPFRs readily form during thermal processes through the decomposition of organic precursors such as phenols, halogenated phenols, and quinone-type molecules, which are also important precursors of toxic unintentionally produced persistent organic pollutants (UPOPs). We have found that EPFRs are important intermediates for UPOP formation during thermal-related processes. However, interest in EPFRs is currently mostly focused on the toxicities and formation mechanisms of EPFRs themselves. Little information is available on the important roles EPFRs play in toxic UPOP formation during thermal processes. Here, we review the mechanisms involved in EPFR formation and transformation into UPOPs during thermal processes. The review is focused on typical EPFRs, including cyclopentadiene, phenoxy, and semiquinone radicals. The reaction temperature, metal species present, and oxygen concentration strongly affect EPFR and UPOP formation during thermal-related processes. Gaps in current knowledge and future directions for research into EPFR and UPOP formation, transformation, and control are presented. Understanding the relationships between EPFRs and UPOPs will allow synergistic control strategies to be developed for thermal-related industrial sources of EPFRs and UPOPs.

Lomnicki S, L. Tanguay R, A. Cormier S. Developmental Hazard of Environmentally Persistent Free Radicals and Protective Effect of TEMPOL in Zebrafish Model

Environmentally persistent free radicals (EPFRs) can be detected in ambient PM_2.5, cigarette smoke, and soils and are formed through combustion and thermal processing of organic materials. The hazards of EPFRs are largely unknown. In this study, we assess the developmental toxicity of EPFRs and the ability of TEMPOL (4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl) to protect against such hazards using zebrafish embryos. Particles containing EPFRs were acquired by dosing dichlorobenzene (DCB) vapor on the Cab-o-sil/5% CuO particles at 230 °C in vacuo (referred to as DCB-230). The particles were suspended in ultrapure water to make 1 mg/mL of stock solution from which series dilution was undertaken to obtain 10, 20, 30, 40, 50, 60, 80, and 100 µg/mL final test solutions, which were then placed in individual wells with a 4 h postfertilization (hpf) zebrafish embryo. Plates were run in duplicate to obtain a sample size of 24 animals per concentration; 12 embryos were exposed per concentration per plate. Statistical analysis of the morphology endpoints was performed. We investigated overt toxicity responses to DCB-230 in a 22-endpoint battery that included developing zebrafish from 24–120 hpf. Exposure to concentrations greater than 60 µg/mL of DCB-230 induced high mortality in the developmental zebrafish model. Exposure to EPFRs induced developmental hazards that were closely related to the concentrations of free radicals and EPFRs. The potential protective effects of TEMPOL against EPFRs’ toxicity in zebrafish were investigated. Exposure to EPFRs plus TEMPOL shifted the concentration to an induced 50% adverse effect (EC50), from 23.6 to 30.8 µg/mL, which verifies TEMPOL’s protective effect against EPFRs in the early phase of zebrafish development.

Cardioprotective effects of garcinol following myocardial infarction in rats with isoproterenol-induced heart failure

Myocardial infarction is a clinical form of necrosis in the myocardium caused by an imbalance between the coronary blood supply and myocardial demand. Garcinol is a polyisoprenylated benzophenone found in the fruit of Garcinia indica, which is abundant in tropical regions. This fruit contains high levels of garcinol, isoxanthochymol, isogarcinol, hydroxycitric acid and xanthochymol. Garcinol and hydroxycitric acid have been shown to have antioxidant effects. In this study, rats were assigned to sham, control, low-dose, high-dose and positive control groups. Hemodynamic and apoptotic markers were evaluated, and histopathological analysis was conducted. The mRNA and protein levels of caspase-3, Bax, Bcl-2 and cleaved caspase-3 were quantified. Garcinol treatment increased the heart rate and improved the maximum rate of increase in left-ventricle (LV) pressure (+dp/dt_max), maximum rate of decrease in LV pressure (–dp/dt_max), LV ejection fraction and LV systolic pressure in rats with induced heart failure. Garcinol treatment reversed body, liver and heart weight changes, resulting in returns to near-normal levels. In the garcinol treatment group, the number of broken fibers, extent of inflammatory cell infiltration and rate of apoptosis remained within normal ranges. Garcinol reduced the cross-sectional areas of cardiomyocytes, and reduced interstitial fibrosis to a normal level. The mRNA and protein levels of cleaved caspase-3, caspase-3 and Bax were reduced, whereas those of Bcl-2 were increased, following high-dose (100 mg/kg) garcinol treatment. These findings suggest that garcinol effectively prevents apoptosis in rats with isoproterenol-induced heart failure and in cardiac H9C2 cells.

Risk evaluation of environmentally persistent free radicals in airborne particulate matter and influence of atmospheric factors

Environmentally persistent free radicals (EPFRs) was considered unrecognized composition of air pollutants and might help explain the long-standing medical mystery of why non-smokers develop tobacco-related diseases like lung cancer. EPFRs in airborne fine particulate matter (PM_2.5) can induce oxidative and DNA damage when inhaled. We assessed the inhalation risk of EPFRs in PM_2.5 and factors influencing this risk in Beijing as a large city with frequent haze events. The average concentration of EPFRs in PM_2.5 was 6.00 × 10¹⁷ spins/m³ in spring, autumn, and winter; lower concentrations were recorded in the summer. To estimate the daily inhalation risk of EPFRs in PM_2.5, we used the equivalent EPFRs in cigarette tar. The average daily inhalation exposure of EPFRs in PM_2.5 was estimated to be the equivalent of 33.1 cigarette tar EPFRs per day (range: 0.53-226.9) during both haze and non-haze days. The major factors influencing EPFR concentrations in the atmosphere were precipitation and humidity, which reduced airborne concentrations. Levels of PM_2.5 and carbon monoxide were positively correlated with EPFR concentrations. The health risks of inhaling airborne EPFRs could be significant and should be recognized and quantified.

Comparative Studies of Environmentally Persistent Free Radicals on Total Particulate Matter Collected from Electronic and Tobacco Cigarettes

In the current study, electron paramagnetic resonance (EPR) spectroscopy was employed to measure environmentally persistent free radicals (EPFRs) in the total particulate matter (TPM) of mainstream and sidestream TPM of conventional cigarettes and the TPM of e-cigarettes. Comparable concentrations of EPFRs were detected in both sidestream (8.05 ± 1.32) × 104 pmol/g and mainstream TPM (7.41 ± 0.85) × 104 pmol/g of conventional cigarettes. TPM exposure to air resulted in long-lived oxygen centered, secondary radicals with EPR g values of 2.0041 for mainstream and 2.0044 for sidestream. Surprisingly, despite no combustion process, the TPM from e-cigarettes (menthol flavor of NJOY and V2 brands) also contain EPFRs with g values of 2.0031-2.0033, characteristic of carbon centered radicals, while the radical signal in the vanilla flavor of V2 brand was remarkably similar to semiquinones in cigarette smoke with a higher g value (2.0063). The radical concentration in e-cigarettes was much lower as compared to tobacco TPM. Although the production of ROS generated by e-cigarettes is comparatively lower than ROS generated by conventional cigarettes, EPFRs in e-cigarettes appear to be more potent than those in tobacco TPM with respect to hydroxyl radical generation yield per unit EPFR. EPFRs in e-cigarette TPM may be a potential source of health impacts.

Levels, spatial distribution, and source identification of airborne environmentally persistent free radicals from tree leaves

Environmentally persistent free radicals (EPFRs) are receiving increasing concern due to their toxicity and ubiquity in the environment. To avoid restrictions imposed when using a high-volume active sampler, this study uses tree leaves to act as passive samplers to investigate the spatial distribution characteristics and sources of airborne EPFRs. Tree leaf samples were collected from 120 sites in five areas around China (each approximately 4 km × 4 km). EPFR concentrations in particles (<2 μm) on the surface of 110 leaf samples were detected, ranging from 7.5 × 10¹⁶ to 4.5 × 10¹⁹ spins/g. For the 10 N.D. samples, they were all collected from areas inaccessible by vehicles. The g-values of EPFRs on 68% leaf samples were larger than 2.004, suggesting the electron localized on the oxygen atom, and they were consistent with the road dust sample (g-value: 2.0042). Significant positive correlation was found between concentrations of elemental carbon (tracer of vehicle emissions) and EPFRs. Spatial distribution mapping showed that EPFR levels in various land uses differed noticeably. Although previous work has linked atmospheric EPFRs to waste incineration, the evidence in this study suggests that vehicle emissions, especially from heavy-duty vehicles, are the main sources. While waste incinerators with low emissions or effective dust-control devices might not be an important EPFR contributor. According to our estimation, over 90% of the EPFRs deposited on tree leaves might be attributed to automotive exhaust emissions, as a synergistic effect of primary exhausts and degradation of aromatic compounds in road dust. With adding the trapping agent into the particle samples (<2 μm), signals of hydroxyl radicals were observed. This indicates that EPFRs collected from this phytosampling method can lead to the release of reactive oxygen species (ROS) once they are inhaled by human beings. Thus, this study helps highlight EPFR "hotspots" for potential health risk identification.

Occurrence, formation, environmental fate and risks of environmentally persistent free radicals in biochars

Biochars are used globally in agricultural crop production and environmental remediation. However, environmentally persistent free radicals (EPFRs), which are stable emerging pollutants, are generated as a characteristic feature during biomass pyrolysis. EPFRs can induce the formation of reactive oxygen species, which poses huge agro-environmental and human health risks. Their half-lives and persistence in both biochar residues and in the atmosphere may lead to potentially adverse risks in the environment. This review highlights the comprehensive research into these bioreactive radicals, as well as the bottlenecks of biochar production leading up to the formation and persistence of EPFRs. Additionally, a way forward has been proposed, based on two main recommendations. A global joint initiative to create an all-encompassing regulations policy document that will improve both the technological and the quality control aspects of biochars to reduce EPFR generation at the production level. Furthermore, environmental impact and risk assessment studies should be conducted in the extensive applications of biochars in order to protect the environmental and human health. The highlighted key research directions proposed herein will shape the production, research, and adoption aspects of biochars, which will mitigate the considerable concerns raised on EPFRs.

Ecological risk analysis of the solid residues collected from the thermal disposal process of hyperaccumulator Pteris vittata including heavy metals and environmentally persistent free radicals

To conduct a comprehensive ecological analysis on the solid residues derived from the thermal disposal of hyperaccumulator Pteris vittata, this study focused on the behaviors of As and Pb and the characteristics of environmentally persistent free radicals (EPFRs) in the solid residues under different thermal treatment conditions. The analysis results revealed that the concentrations of As in the biochars and bio-slag were approximately 350 and 1100 mg/kg, respectively. Moreover, the concentrations of Pb in the solid residues varied from 34 to 1050 mg/kg. According to the results of the modified BCR sequential extractions, As is more stable in the biochar while Pb is more stable in the combustion slags. In addition, As showed a higher volatilization temperature compared with Pb. The ecological risk assessment indicated that the correlation index between the contamination factor (C_f) of As and the risk index (R² = 0.995) is considerably larger than the correlation index between the contamination factor of Pb and the risk index (R² = 0.117), which implies that the pyrolysis method should be selected at priority. Moreover, the EPFR concentrations of the biochar declined by approximately 75 times when the pyrolysis temperature increased from 500 to 600 °C. This behavior indicated that high-temperature pyrolysis (> 600 °C) could simultaneously control both the heavy metal behavior and EPFR concentrations.

Environmentally persistent free radicals: Occurrence, formation mechanisms and implications

Environmentally persistent free radicals (EPFRs) are defined as organic free radicals stabilized on or inside particles. They are persistent because of the protection by the particles and show significant toxicity to organisms. Increasing research interests have been attracted to study the potential environmental implications of EPFRs. Because of their different physical forms from conventional contaminants, it is not applicable to use the commonly used technique and strategy to predict and assess the behavior and risks of EPFRs. Current studies on EPFRs are scattered and not systematic enough to draw clear conclusions. Therefore, this review is organized to critically discuss the current research progress on EPFRs, highlighting their occurrence and transport, generation mechanisms, as well as their environmental implications (including both toxicity and reactivity). EPFR formation and stabilization as affected by the precursors and environmental factors are useful breakthrough to understand their formation mechanisms. To better understand the major differences between EPFRs and common contaminants, we identified the unique processes and/or mechanisms related to EPFRs. The knowledge gaps will be also addressed to highlight the future research while summarizing the research progress. Quantitative analysis of the interactions between organic contaminants and EPFRs will greatly improve the predictive accuracy of the multimedia environmental fate models. In addition, the health risks will be better evaluated when considering the toxicity contributed by EFPRs.

Dominant Fraction of EPFRs from Nonsolvent-Extractable Organic Matter in Fine Particulates over Xi'an, China

To understand the nature and possible sources of environmentally persistent free radicals (EPFRs) in atmospheric aerosols, the present study used a solvent extraction method to fractionate aerosol components with different polarities and solvent resistance in fine particulate matter (PM_2.5) from Xi'an, China. The characteristics of EPFRs, that is., their concentration, type and lifetime, were obtained based on their electron paramagnetic resonance spectra. The results showed that the EPFRs in the PM_2.5 samples were carbon-centered with a nearby heteroatom ( g = 2.0031) and had a long half-life of more than 3 years. Nearly all of the extractable EPFRs were detected in the water-insoluble organic fraction and showed characteristics indicating that may contain oxygen-centered radical ( g = 2.0038). Most of the total EPFRs in the PM_2.5 were derived from solvent-resistant organic matter (88%), which likely consisted of graphene oxide analogues. The results suggest that previous studies may have missed the major proportion of EPFRs in atmospheric particulates if they only focused on solvent-extractable or metallic oxide-formed EPFRs. Our results showed that the EPFR concentration was significantly and positively correlated with the elemental carbon and NO₂ concentrations, suggesting that traffic emissions may be an important source of EPFRs in PM_2.5 over Xi'an.

Environmentally Persistent Free Radicals: Insights on a New Class of Pollutants

Environmentally persistent free radicals, EPFRs, exist in significant concentration in atmospheric particulate matter (PM). EPFRs are primarily emitted from combustion and thermal processing of organic materials, in which the organic combustion byproducts interact with transition metal-containing particles to form a free radical-particle pollutant. While the existence of persistent free radicals in combustion has been known for over half-a-century, only recently that their presence in environmental matrices and health effects have started significant research, but still in its infancy. Most of the experimental studies conducted to understand the origin and nature of EPFRs have focused primarily on nanoparticles that are supported on a larger micrometer-sized particle that mimics incidental nanoparticles formed during combustion. Less is known on the extent by which EPFRs may form on engineered nanomaterials (ENMs) during combustion or thermal treatment. In this critical and timely review, we summarize important findings on EPFRs and discuss their potential to form on pristine ENMs as a new research direction. ENMs may form EPFRs that may differ in type and concentration compared to nanoparticles that are supported on larger particles. The lack of basic data and fundamental knowledge about the interaction of combustion byproducts with ENMs under high-temperature and oxidative conditions present an unknown environmental and health burden. Studying the extent of ENMs on catalyzing EPFRs is important to address the hazards of atmospheric PM fully from these emerging environmental contaminants.

Environmentally Persistent Free Radicals Cause Apoptosis in HL-1 Cardiomyocytes

Samples of environmental particulate matter contain environmentally persistent free radicals (EPFRs) capable of sustained generation of oxygen radicals. While exposure to EPFRs produces cardiac toxicity and oxidative stress in experimental animals, the underlying mechanisms are largely unknown. To determine whether EPFRs could directly damage cardiomyocytes, cultured mouse cardiomyocytes (HL-1) and primary rat adult left ventricular myocytes (ALVM) were incubated with an EPFR consisting of 1,2-dichlorobenzene chemisorbed to CuO-coated silica beads (DCB230). Treatment with DCB230 killed both HL-1 and ALVM in a dose- and time-dependent manner. The cytotoxic effects of DCB230 were significantly attenuated by treatment with α-tocopherol. One to 2 h after exposure to DCB230, there were significant reductions in mitochondrial membrane potential and significant increases in cleaved caspase-9, but no significant increases in DNA damage or cell death. After 8 h of treatment, there were significant increases in caspase-3, caspase-9, DNA damage and PARP cleavage associated with significant cell death. Together, these data indicate that DCB230 kills HL-1 myocytes by inducing oxidative stress that initiates apoptosis, with the intrinsic or mitochondrial pathway acting early in the apoptotic signaling process.

Unraveling mechanisms of toxicant-induced oxidative stress in cardiovascular disease

To date, numerous clinical studies examining correlations between oxidative stress biomarkers and cardiovascular diseases (CVD) have repeatedly suggested a role for oxidant injury in the pathogenesis of diseases such as atherosclerosis. Despite this, antioxidant supplementation trials have not demonstrated a reduction in disease progression. Nevertheless, small animal and epidemiological studies have linked exposures to certain toxicants with increased CVD risk involving putative oxidative stress mechanisms. A few prototypical vascular toxicants will be discussed as examples of toxicants that likely act via oxidative stress mechanisms. For discussion, we will classify these toxicants as those that induce direct (e.g., arsenic, nucleoside reverse transcriptase inhibitors) versus indirect (particulate matter, ozone) oxidative stress mechanisms, and those that likely induce CVD through both direct and indirect mechanisms (cigarette smoke). Finally, new findings in oxidative stress research, including the emerging importance of reactive sulfur species, hydrogen peroxide as a presumed endothelium-derived hyperpolarizing factors, etc., will be discussed, as well as the need to determine the role of toxicants in modulating these newly identified pathways. Moreover, given the lack of success in conclusively demonstrating the roles of oxidative stress in CVD risk stratification, research probing the roles of toxicant exposures in propagating CVD pathogenesis may be a novel approach for more conclusively delineating the causal role of oxidative stress in CVD initiation and progression.

A Scalable Field Study Protocol and Rationale for Passive Ambient Air Sampling: A Spatial Phytosampling for Leaf Data Collection

Stable, bioreactive, radicals known as environmentally persistent free radicals (EPFRs) have been found to exist on the surface of airborne PM2.5. These EPFRs have been found to form during many combustion processes, are present in vehicular exhaust, and persist in the environment for weeks and biological systems for up to 12 h. To measure EPFRs in PM samples, high volume samplers are required and measurements are less representative of community exposure; therefore, we developed a novel spatial phytosampling methodology to study the spatial patterns of EPFR concentrations using plants. Leaf samples for laboratory PM analysis were collected from 188 randomly drawn sampling sites within a 500-m buffer zone of pollution sources across a sampling grid measuring 32.9 × 28.4 km in Memphis, Tennessee. PM was isolated from the intact leaves and size fractionated, and EPFRs on PM quantified by electron paramagnetic resonance spectroscopy. The radical concentration was found to positively correlate with the EPFR g-value, thus indicating cumulative content of oxygen centered radicals in PM with higher EPFR load. Our spatial phytosampling approach reveals spatial variations and potential "hotspots" risk due to EPFR exposure across Memphis and provides valuable insights for identifying exposure and demographic differences for health studies.

Formation of environmentally persistent free radicals (EPFRs) on ZnO at room temperature: Implications for the fundamental model of EPFR generation

Environmentally persistent free radicals (EPFRs) have significant environmental and public health impacts. In this study, we demonstrate that EPFRs formed on ZnO nanoparticles provide two significant surprises. First, EPR spectroscopy shows that phenoxy radicals form readily on ZnO nanoparticles at room temperature, yielding EPR signals similar to those previously measured after 250°C exposures. Vibrational spectroscopy supports the conclusion that phenoxy-derived species chemisorb to ZnO nanoparticles under both exposure temperatures. Second, DFT calculations indicate that electrons are transferred from ZnO to the adsorbed organic (oxidizing the Zn), the opposite direction proposed by previous descriptions of EPFR formation on metal oxides.

Addressing Emerging Risks: Scientific and Regulatory Challenges Associated with Environmentally Persistent Free Radicals

Airborne fine and ultrafine particulate matter (PM) are often generated through widely-used thermal processes such as the combustion of fuels or the thermal decomposition of waste. Residents near Superfund sites are exposed to PM through the inhalation of windblown dust, ingestion of soil and sediments, and inhalation of emissions from the on-site thermal treatment of contaminated soils. Epidemiological evidence supports a link between exposure to airborne PM and an increased risk of cardiovascular and pulmonary diseases. It is well-known that during combustion processes, incomplete combustion can lead to the production of organic pollutants that can adsorb to the surface of PM. Recent studies have demonstrated that their interaction with metal centers can lead to the generation of a surface stabilized metal-radical complex capable of redox cycling to produce ROS. Moreover, these free radicals can persist in the environment, hence their designation as Environmentally Persistent Free Radicals (EPFR). EPFR has been demonstrated in both ambient air PM_2.5 (diameter < 2.5 µm) and in PM from a variety of combustion sources. Thus, low-temperature, thermal treatment of soils can potentially increase the concentration of EPFR in areas in and around Superfund sites. In this review, we will outline the evidence to date supporting EPFR formation and its environmental significance. Furthermore, we will address the lack of methodologies for specifically addressing its risk assessment and challenges associated with regulating this new, emerging contaminant.

Catechol degradation on hematite/silica-gas interface as affected by gas composition and the formation of environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) formed on a solid particle surface have received increasing attention because of their toxic effects. However, organic chemical fate regulated by EPFRs has rarely been investigated, and this information may provide the missing link in understanding their environmental behavior. Previous studies have suggested that the reduction of transition metals is involved in EPFRs formation. We thus hypothesize that an oxidative environment may inhibit EPFRs formation in particle-gas interface, which will consequently release free radicals and accelerate organic chemical degradation. Our result indicates that a 1% hematite coating on a silica surface inhibited catechol degradation in N₂, especially at low catechol loadings on solid particles (S_CT). However, under an O₂ environment, catechol degradation decreased when S_CT was <1 μg/mg but increased when S_CT was >1 μg/mg. Stable organic free radicals were observed in the N₂ system with g factors in the 2.0035–2.0050 range, suggesting the dominance of oxygen-centered free radicals. The introduction of O₂ into the catechol degradation system substantially decreased the free radical signals and decreased the Fe(II) content. These results were observed in both dark and light irradiation systems, indicating the ubiquitous presence of EPFRs in regulating the fate of organic chemicals.

Sarcoendoplasmic reticulum Ca(2+) ATPase. A critical target in chlorine inhalation-induced cardiotoxicity

Autopsy specimens from human victims or experimental animals that die due to acute chlorine gas exposure present features of cardiovascular pathology. We demonstrate acute chlorine inhalation-induced reduction in heart rate and oxygen saturation in rats. Chlorine inhalation elevated chlorine reactants, such as chlorotyrosine and chloramine, in blood plasma. Using heart tissue and primary cardiomyocytes, we demonstrated that acute high-concentration chlorine exposure in vivo (500 ppm for 30 min) caused decreased total ATP content and loss of sarcoendoplasmic reticulum calcium ATPase (SERCA) activity. Loss of SERCA activity was attributed to chlorination of tyrosine residues and oxidation of an important cysteine residue, cysteine-674, in SERCA, as demonstrated by immunoblots and mass spectrometry. Using cardiomyocytes, we found that chlorine-induced cell death and damage to SERCA could be decreased by thiocyanate, an important biological antioxidant, and by genetic SERCA2 overexpression. We also investigated a U.S. Food and Drug Administration-approved drug, ranolazine, used in treatment of cardiac diseases, and previously shown to stabilize SERCA in animal models of ischemia-reperfusion. Pretreatment with ranolazine or istaroxime, another SERCA activator, prevented chlorine-induced cardiomyocyte death. Further investigation of responsible mechanisms showed that ranolazine- and istaroxime-treated cells preserved mitochondrial membrane potential and ATP after chlorine exposure. Thus, these studies demonstrate a novel critical target for chlorine in the heart and identify potentially useful therapies to mitigate toxicity of acute chlorine exposure.

Probing environmentally significant surface radicals: Crystallographic and temperature dependent adsorption of phenol on ZnO

Environmentally persistent free radicals (EPFRs) are toxic organic/metal oxide composite particles that have been discovered to form from substituted benzenes chemisorbed to metal oxides. Here, we perform photoelectron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction of phenol chemisorbed to ZnO(1 0 1̱ 0) and (0 0 0 1̱)-Zn to observe electronic structure changes and charge transfer as a function adsorption temperature. We show direct evidence of charge transfer from the ZnO surfaces to the phenol. This evidence can help gain a better understanding of EPFRs and be used to develop possible future remediation strategies.

Environmentally persistent free radical-containing particulate matter competitively inhibits metabolism by cytochrome P450 1A2

Combustion processes generate different types of particulate matter (PM) that can have deleterious effects on the pulmonary and cardiovascular systems. Environmentally persistent free radicals (EPFRs) represent a type of particulate matter that is generated after combustion of environmental wastes in the presence of redox-active metals and aromatic hydrocarbons. Cytochromes P450 (P450/CYP) are membrane-bound enzymes that are essential for the phase I metabolism of most lipophilic xenobiotics. The EPFR formed by chemisorption of 2-monochlorophenol to silica containing 5% copper oxide (MCP230) has been shown to generally inhibit the activities of different forms of P450s without affecting those of cytochrome P450 reductase and heme oxygenase-1. The mechanism of inhibition of rat liver microsomal CYP2D2 and purified rabbit CYP2B4 by MCP230 has been shown previously to be noncompetitive with respect to substrate. In this study, MCP230 was shown to competitively inhibit metabolism of 7-benzyl-4-trifluoromethylcoumarin and 7-ethoxyresorufin by the purified, reconstituted rabbit CYP1A2. MCP230 is at least 5- and 50-fold more potent as an inhibitor of CYP1A2 than silica containing 5% copper oxide and silica, respectively. Thus, even though PM generally inhibit multiple forms of P450, PM interacts differently with the forms of P450 resulting in different mechanisms of inhibition. P450s function as oligomeric complexes within the membrane. We also determined the mechanism by which PM inhibited metabolism by the mixed CYP1A2-CYP2B4 complex and found that the mechanism was purely competitive suggesting that the CYP2B4 is dramatically inhibited when bound to CYP1A2.

Exposure to Deepwater Horizon Crude Oil Burnoff Particulate Matter Induces Pulmonary Inflammation and Alters Adaptive Immune Response

The ″in situ burning" of trapped crude oil on the surface of Gulf waters during the 2010 Deepwater Horizon (DWH) oil spill released numerous pollutants, including combustion-generated particulate matter (PM). Limited information is available on the respiratory impact of inhaled in situ burned oil sail particulate matter (OSPM). Here we utilized PM collected from in situ burn plumes of the DWH oil spill to study the acute effects of exposure to OSPM on pulmonary health. OSPM caused dose-and time-dependent cytotoxicity and generated reactive oxygen species and superoxide radicals in vitro. Additionally, mice exposed to OSPM exhibited significant decreases in body weight gain, systemic oxidative stress in the form of increased serum 8-isoprostane (8-IP) levels, and airway inflammation in the form of increased macrophages and eosinophils in bronchoalveolar lavage fluid. Further, in a mouse model of allergic asthma, OSPM caused increased T helper 2 cells (Th2), peribronchiolar inflammation, and increased airway mucus production. These findings demonstrate that acute exposure to OSPM results in pulmonary inflammation and alteration of innate/adaptive immune responses in mice and highlight potential respiratory effects associated with cleaning up an oil spill.

Chlorine inhalation-induced myocardial depression and failure

Victims of chlorine (Cl2) inhalation that die demonstrate significant cardiac pathology. However, a gap exists in the understanding of Cl2-induced cardiac dysfunction. This study was performed to characterize cardiac dysfunction occurring after Cl2 exposure in rats at concentrations mimicking accidental human exposures (in the range of 500 or 600 ppm for 30 min). Inhalation of 500 ppm Cl2 for 30 min resulted in increased lactate in the coronary sinus of the rats suggesting an increase in anaerobic metabolism by the heart. There was also an attenuation of myocardial contractile force in an ex vivo (Langendorff technique) retrograde perfused heart preparation. After 20 h of return to room air, Cl2 exposure at 500 ppm was associated with a reduction in systolic and diastolic blood pressure as well echocardiographic/Doppler evidence of significant left ventricular systolic and diastolic dysfunction. Cl2 exposure at 600 ppm (30 min) was associated with biventricular failure (observed at 2 h after exposure) and death. Cardiac mechanical dysfunction persisted despite increasing the inspired oxygen fraction concentration in Cl2-exposed rats (500 ppm) to ameliorate hypoxia that occurs after Cl2 inhalation. Similarly ex vivo cardiac mechanical dysfunction was reproduced by sole exposure to chloramine (a potential circulating Cl2 reactant product). These results suggest an independent and distinctive role of Cl2 (and its reactants) in inducing cardiac toxicity and potentially contributing to mortality.

Inhibition of cytochrome P450 2B4 by environmentally persistent free radical-containing particulate matter

Combustion processes generate particulate matter (PM) that can affect human health. The presence of redox-active metals and aromatic hydrocarbons in the post-combustion regions results in the formation of air-stable, environmentally persistent free radicals (EPFRs) on entrained particles. Exposure to EPFRs has been shown to negatively influence pulmonary and cardiovascular functions. Cytochromes P450 (P450/CYP) are endoplasmic reticulum resident proteins that are responsible for the metabolism of foreign compounds. Previously, it was shown that model EPFRs, generated by exposure of silica containing 5% copper oxide (CuO-Si) to either dicholorobenzene (DCB230) or 2-monochlorophenol (MCP230) at ≥ 230 °C, inhibited six forms of P450 in rat liver microsomes (Toxicol. Appl. Pharmacol. (2014) 277:200-209). In this study, the inhibition of P450 by MCP230 was examined in more detail by measuring its effect on the rate of metabolism of 7-ethoxy-4-trifluoromethylcoumarin (7EFC) and 7-benzyloxyresorufin (7BRF) by the purified, reconstituted CYP2B4 system. MCP230 inhibited the CYP2B4-mediated metabolism of 7EFC at least 10-fold more potently than non-EPFR controls (CuO-Si, silica, and silica generated from heating silica and MCP at 50 °C, so that EPFRs were not formed (MCP50)). The inhibition by EPFRs was specific for the P450 and did not affect the ability of the redox partner, P450 reductase (CPR) from reducing cytochrome c. All of the PM inhibited CYP2B4-mediated metabolism noncompetitively with respect to substrate. When CYP2B4-mediated metabolism of 7EFC was measured as a function of the CPR concentration, the mechanism of inhibition was competitive. EPFRs likely inhibit CYP2B4-mediated substrate metabolism by physically disrupting the CPR·P450 complex.

Environmentally persistent free radicals compromise left ventricular function during ischemia/reperfusion injury

Increases in airborne particulate matter (PM) are linked to increased mortality from myocardial ischemia. PM contains environmentally persistent free radicals (EPFRs) that form as halogenated hydrocarbons chemisorb to transition metal oxide-coated particles, and are capable of sustained redox cycling. We hypothesized that exposure to the EPFR DCB230 would increase cardiac vulnerability to subsequent myocardial ischemia-reperfusion (MI/R) injury. Rats were exposed to DCB230 or vehicle via nose-only inhalation (230 μg max/day) over 30 min/day for 7 days. MI/R or sham MI/R (sham) was initiated 24 h after the final exposure. Following 1 or 7 days of reperfusion, left ventricular (LV) function was assessed and infarct size measured. In vehicle-exposed rats, MI/R injury did not significantly reduce cardiac output (CO), stroke volume (SV), stroke work (SW), end-diastolic volume (EDV), or end-systolic volume (ESV) after 1 day of reperfusion, despite significant reductions in end-systolic pressure (ESP). Preload-recruitable SW (PRSW; contractility) was elevated, presumably to maintain LV function. MI/R 1-day rats exposed to DCB230 also had similarly reduced ESP. Compared with vehicle controls, CO, SV, and SW were significantly reduced in DCB230-exposed MI/R 1-day rats; moreover, PRSW did not increase. DCB230's effects on LV function dissipated within 8 days of exposure. These data show that inhalation of EPFRs can exacerbate the deficits in LV function produced by subsequent MI/R injury. Infarct size was not different between the MI/R groups. We conclude that inhalation of EPFRs can compromise cardiac function during MI/R injury and may help to explain the link between PM and MI/R-related mortality.

Environmentally persistent free radicals inhibit cytochrome P450 activity in rat liver microsomes

Combustion processes generate particulate matter that affects human health. When incineration fuels include components that are highly enriched in aromatic hydrocarbons (especially halogenated varieties) and redox-active metals, ultrafine particulate matter containing air-stable, environmentally persistent free radicals (EPFRs) is generated. The exposure to fine EPFRs (less than 2.5 μm in diameter) has been shown to negatively influence pulmonary and cardiovascular functions in living organisms. The goal of this study was to determine if these EPFRs have a direct effect on cytochrome P450 function. This was accomplished by direct addition of the EPFRs to rat liver microsomal preparations and measurement of several P450 activities using form-selective substrates. The EPFRs used in this study were formed by heating vapors from an organic compound (either monochlorophenol (MCP230) or 1,2-dichlorobenzene (DCB230)) and 5% copper oxide supported on silica (approximately 0.2 μm in diameter) to 230°C under vacuum. Both types of EPFRs (but not silica, physisorbed silica, or silica impregnated with copper oxide) dramatically inhibited the activities of CYP1A, CYP2B, CYP2E1, CYP2D2 and CYP3A when incubated at concentrations less than 0.1 mg/ml with microsomes and NADPH. Interestingly, at the same concentrations, the EPFRs did not inhibit HO-1 activity or the reduction of cytochrome c by NADPH-cytochrome P450 reductase. CYP2D2-selective metabolism by rat liver microsomes was examined in more detail. The inhibition of CYP2D2-selective metabolism by both DCB230- and MCP230-EPFRs appeared to be largely noncompetitive and was attenuated in the presence of catalase suggesting that reactive oxygen species may be involved in the mechanism of inhibition.

Combustion By-Products and their Health Effects--combustion engineering and global health in the 21st century: issues and challenges

The 13th International Congress on Combustion By-Products and their Health Effects was held in New Orleans, Louisiana from May 15 to 18, 2013. The congress, sponsored by the Superfund Research Program, National Institute of Environmental Health Sciences, and National Science Foundation, brought together international academic and government researchers, engineers, scientists, and policymakers. With industrial growth, increased power needs and generation and coal consumption and their concomitant emissions, pernicious health effects associated with exposures to these emissions are on the rise. This congress provides a unique platform for interdisciplinary exchange and discussion of these topics. The formation, conversion, control, and health effects of combustion by-products, including particulate matter and associated heavy metals, persistent organic pollutants, and environmentally persistent free radicals, were discussed during the congress. This review will summarize and discuss the implications of the data presented.

Model combustion-generated particulate matter containing persistent free radicals redox cycle to produce reactive oxygen species

Particulate matter (PM) is emitted during thermal decomposition of waste. During this process, aromatic compounds chemisorb to the surface of metal-oxide-containing PM, forming a surface-stabilized environmentally persistent free radical (EPFR). We hypothesized that EPFR-containing PM redox cycle to produce ROS and that this redox cycle is maintained in biological environments. To test our hypothesis, we incubated model EPFRs with the fluorescent probe dihydrorhodamine (DHR). Marked increases in DHR fluorescence were observed. Using a more specific assay, hydroxyl radicals ((•)OH) were also detected, and their level was further increased by cotreatment with thiols or ascorbic acid (AA), known components of epithelial lining fluid. Next, we incubated our model EPFR in bronchoalveolar lavage fluid (BALF) or serum. Detection of EPFRs and (•)OH verified that PM generate ROS in biological fluids. Moreover, incubation of pulmonary epithelial cells with EPFR-containing PM increased (•)OH levels compared to those in PM lacking EPFRs. Finally, measurements of oxidant injury in neonatal rats exposed to EPFRs by inhalation suggested that EPFRs induce an oxidant injury within the lung lining fluid and that the lung responds by increasing antioxidant levels. In summary, our EPFR-containing PM redox cycle to produce ROS, and these ROS are maintained in biological fluids and environments. Moreover, these ROS may modulate toxic responses of PM in biological tissues such as the lung.

Environmentally persistent free radicals and their lifetimes in PM2.5

For the first time, an expansive study into the concentration and extended decay behavior of environmentally persistent free radicals in PM2.5 was performed. Results from this study revealed three types of radical decay-a fast decay, slow decay, and no decay-following one of four decay patterns: a relatively fast decay exhibiting a 1/e lifetime of 1-21 days accompanied by a slow decay with a 1/e lifetime of 21-5028 days (47% of samples); a single slow decay including a 1/e lifetime of 4-2083 days (24% of samples); no decay (18% of samples); and a relatively fast decay displaying an average 1/e lifetime of 0.25-21 days followed by no decay (11% of samples). Phenol correlated well with the initial radical concentration and fast decay rate. Other correlations for common atmospheric pollutants (ozone, NOx, SO2, etc.) as well as meteorological conditions suggested photochemical processes impact the initial radical concentration and fast decay rate. The radical signal in PM2.5 was remarkably similar to semiquinones in cigarette smoke. Accordingly, radicals inhaled from PM2.5 were related to the radicals inhaled from smoking cigarettes, expressed as the number of equivalent cigarettes smoked. This calculated to 0.4-0.9 cigarettes per day for nonextreme air quality in the United States.

Lifetime of combustion-generated environmentally persistent free radicals on Zn(II)O and other transition metal oxides

Previous studies indicated that Environmentally Persistent Free Radicals (EPFRs) are formed in the post-flame, cool zone of combustion. They result from the chemisorption of gas-phase products of incomplete combustion (particularly hydroxyl- and chlorine-substituted aromatics) on Cu(II)O, Fe(III)(2)O(3), and Ni(II)O domains of particulate matter (fly ash or soot particles). This study reports our detailed laboratory investigation on the lifetime of EPFRs on Zn(II)O/silica surface. Similarly, as in the case of other transition metals, chemisorption of the adsorbate on the Zn(II)O surface and subsequent transfer of electron from the adsorbate to the metal forms a surface-bound EPFR and a reduced metal ion center. The EPFRs are stabilized by their interaction with the metal oxide domain surface. The half-lives of EPFRs formed on Zn(II)O domains were the longest observed among the transition metal oxides studied and ranged from 3 to 73 days. These half-lives were an order of magnitude longer than those formed on nickel and iron oxides, and were 2 orders of magnitude longer compared to the EPFRs on copper oxide which have half-lives only on the order of hours. The longest-lived radicals on Zn(II)O correspond to the persistency in ambient air particles of almost a year. The half-life of EPFRs was found to correlate with the standard reduction potential of the associated metal.

Formation and stabilization of combustion-generated, environmentally persistent radicals on Ni(II)O supported on a silica surface

Previous studies have indicated environmentally persistent free radicals (EPFRs) are formed when hydroxyl- and chlorine-substituted aromatics chemisorbed on Cu(II)O and Fe(III)(2)O(3) surfaces and were stabilized through their interactions with the surface metal cation. The current study reports our laboratory investigation on the formation and stabilization of EPFRs on a Ni(II)O surface. The EPFRs were produced by the chemisorption of adsorbates on the supported metal oxide surface and transfer of an electron from the adsorbate to the metal center, resulting in reduction of the metal cation. Depending on the temperature and the nature of the adsorbate, more than one type of organic radical was formed. A phenoxyl-type radical, with g-value between 2.0029 and 2.0044, and a semiquinone-type radical, with g-value from 2.0050 to as high as 2.0081, were observed. The half-lives on Ni(II)O were long and ranged from 1.5 to 5.2 days, which were similar to what were observed on Fe(III)(2)O(3). The yields of the EPFRs formed on Ni(II)O were ~8× higher than on Cu(II)O and ~50× higher than on Fe(III)(2)O(3).

Environmentally persistent free radicals decrease cardiac function and increase pulmonary artery pressure

Epidemiological studies have consistently linked inhalation of particulate matter (PM) to increased cardiac morbidity and mortality, especially in at risk populations. However, few studies have examined the effect of PM on baseline cardiac function in otherwise healthy individuals. In addition, airborne PM contain environmentally persistent free radicals (EPFR) capable of redox cycling in biological systems. The purpose of this study was to determine whether nose-only inhalation of EPFRs (20 min/day for 7 days) could decrease baseline left ventricular function in healthy male Sprague-Dawley rats. The model EPFR tested was 1,2-dichlorobenzene chemisorbed to 0.2-μm-diameter silica/CuO particles at 230°C (DCB230). Inhalation of vehicle or silica particles served as controls. Twenty-four hours after the last exposure, rats were anesthetized (isoflurane) and ventilated (3 l/min), and left ventricular function was assessed using pressure-volume catheters. Compared with controls, inhalation of DCB230 significantly decreased baseline stroke volume, cardiac output, and stroke work. End-diastolic volume and end-diastolic pressure were also significantly reduced; however, ventricular contractility and relaxation were not changed. DCB230 also significantly increased pulmonary arterial pressure and produced hyperplasia in small pulmonary arteries. Plasma levels of C-reactive protein were significantly increased by exposure to DCB230, as were levels of heme oxygenase-1 and SOD2 in the left ventricle. Together, these data show that inhalation of EPFRs, but not silica particles, decreases baseline cardiac function in healthy rats by decreasing cardiac filling, secondary to increased pulmonary resistance. These EPFRs also produced systemic inflammation and increased oxidative stress markers in the left ventricle.