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Building an understanding of EPFRs’ nature and role in human health for policy makers is also important, to create new standards for air pollution.

Outdoor air pollution is a major environmental hazard affecting human health across the globe. The link between inhalation of ambient particulate matter (PM) and various adverse health effects is documented extensively by epidemiological and toxicological studies. Large fractions of ambient air PM originate from combustion and thermal sources, where at low temperatures at the end of the process, interaction of products of incomplete combustion with transition metals form environmentally persistent free radicals (EPFRs). These are identified as crucial PM components triggering hydroxyl radical (•OH) generation via EPFR redox cycle ^{1, 2}. The oxidative potential of PM is therefore an important health-relevant metric.

These long-lived radicals are either carbon-centered polyaromatic soot radicals or oxygen-centered semiquinone or phenoxyl in type, the latter promoting the generation of reactive oxygen species (ROS). Their half-life varies from several days to several months and, on the internal surface of fine particles, can persist indefinitely. Semiquinones are also known to undergo redox cycling and produce biologically damaging superoxide and •OH radicals³.

Free Radical Detection with EPR
Identifying and monitoring generation of free radicals from ambient PM, and determining their oxidative potential, is of great importance due to adverse effects on human health. PM is one of the largest transporters of pollutants, such as transition metals and polycyclic aromatic hydrocarbons (PAHs), so understanding their behavior in the air can facilitate the design of more environmentally-friendly combustion systems, and understand the effects of pollutants on the body.

Electron paramagnetic resonance (EPR) spectroscopy is a technique that detects species with unpaired electrons, such as free radicals and many transition metal ions. As EPR data can be collected in seconds, and the analysis of the data delivers not only the identity, but also quantitative information about the species being measured, it provides a useful tool for environmental analysis.

EPR is the only method capable of the direct measurement of radicals. All other methods are indirect and based on the chemical reactivity of the radicals, and are therefore subject to kinetic limitations. Even more complex is the measurement of very short-lived radicals. For example, it is impossible to measure a hydroxyl radical directly – spin-traps are required which react with the radical and convert it to a different, more stable radical, which can then be measured.

In recent years, the quantitation of radicals in the analyzed samples has become more accurate and user-friendly. Where older instruments require standards for radical concentration of very stable radicals, newer EPR spectrometers, such as the Bruker EMX series, include built in reference standards. These instruments can quantify radicals automatically, without requiring a primary calibration curve using a known concentration of radicals.

Measuring Pollutants in the Lung
At the Louisiana State University (LSU) Department of Environmental Sciences, we are currently researching long-lived radicals present on ambient air PM. Particles formed in the combustion streams, if not properly captured by air pollution control devices, are emitted into the atmosphere. Due to the heterogenous reactions occurring on the PM surface at the cool zone of combustion systems, such particles have a characteristic radical signal associated with them. These radicals were discovered many years ago but it has only been in the past decade where researchers have found the chemical nature of this phenomena. These species are hybrid species of the organic molecules associated with the metal centers of the particles. The electronic interaction between the molecule and metal results in a stable radical. In fact, these conjugates cannot be considered as a separate organic or metal moieties, but as one unique entity – EPFR. These radicals, unlike others, can live for a very long time in the atmosphere as EPFRs.

The team at LSU has found that, upon inhalation, these EFPRs become active, as the redox cycle begins and generate •OH radicals in the lungs^4-7. Free radicals present on combustion-generated PM are also known to cause DNA damage⁸. Strong oxidating agents such as these can cause a lot of damage to tissues.

There is already a proven link between exposure to particulates containing radicals and asthma incidents³. This is aligned with the fact that high particulate pollution is causing higher hospital admission rates. One group at LSU has also associated EPFRs with cardiovascular problems, and found that animals which had an induced heart attached had significantly suppressed recovery if exposed to radicals on particulates. The mechanism of the redox cycle, which gives rise to •OH radicals, starts a cascade of different signaling pathways in the body, which can result in disease.

To better understand the observed PM effects on human health, more studies are necessary on EPFRs role. The key steps are the identification and characterization of the EPFRs present on the ambient particulates, and collaboration between chemists, epidemiologists and biomedical researchers to establish speciation-exposure-health implication relationships and likely outcomes of exposure to ambient particulates that contain radicals.

Influencing Policy with Research
LSU Superfund Research Center funded by the National Institute for Environmental Health Sciences Superfund Research Program is primarily focused on EPFRs. The role of the Center is not only to understand the chemistry and biology and health effects associated with EPFR formation, emission, and environmental presence, but also to find solutions for EPFR exposure prevention, how to warn communities of pollution issues, and how these communities can protect themselves from harmful EPFRs. This work on EPFRs has a global impact.

Building an understanding of EPFRs' nature and role in human health for policy makers is also important, to create new standards for air pollution. Currently, particulate pollution measurement is based on mass concentration in µg/m³, as regulated by the Environmental Protection Agency. The group at LSU has found that, if radicals are one of the main reasons for the toxicity of ambient particles for humans, then radical concentration on a particle should be measured and regulated, rather than pure mass. It was found that special distribution of EPFRs does not correlate with the overall PM concentration in ambient air². This implies, that one particle can be more toxic than another, thus PM mass reference alone does not provide sufficient information and protection to population.

The Future of Air Pollution Research
With the recognition of EPFRs as the important health factors in PM, EPR undoubtedly has an important new role in air pollution studies and the presence of radicals in the air. By correlating this work with that of biomedical scientists, it is possible to understand the way these chemical species interact with human tissue and potentially cause disease. This, in turn, may influence environmental policy and how to curb air pollution, particularly in urban areas.

References

1. Vejerano, E. P.; Rao, G.; Khachatryan, L.; Cormier, S. A.; Lomnicki, S. (2018) Environmentally Persistent Free Radicals: Insights on a New Class of Pollutants. Environ Sci Technol, 52(5), 2468-2481.
2. Oyana, T. J.; Lomnicki, S. M.; Guo, C. Q.; Cormier, S. A. (2017) A Scalable Field Study Protocol and Rationale for Passive Ambient Air Sampling: A Spatial Phytosampling for Leaf Data Collection. Environ Sci Technol, 51(18), 10663-10673.
3. Cormier SA., Lomnicki S, Backes W and Dellinger B (2006) Origin and Health Impacts of Emissions of Toxic By-Products and Fine Particles from Combustion and Thermal Treatment of Hazardous Wastes and Materials, Environmental Health Perspectives, 114(6), 810-817, doi:10.1289/ehp.8629.
4. Kelley, M. A.; Hebert, V. Y.; Thibeaux, T. M.; Orchard, M. A.; Hasan, F.; Cormier, S. A.; Thevenot, P. T.; Lomnicki, S. M.; Varner, K. J.; Dellinger, B.; Latimer, B. M.; Dugas, T. R. (2013) Model Combustion-Generated Particulate Matter Containing Persistent Free Radicals Redox Cycle to Produce Reactive Oxygen Species. Chem Res Toxicol, 26(12), 1862-1871.
5. Lee, G. I.; Saravia, J.; You, D. H.; Shrestha, B.; Jaligama, S.; Hebert, V. Y.; Dugas, T. R.; Cormier, S. A. (2014) Exposure to combustion generated environmentally persistent free radicals enhances severity of influenza virus infection. Part Fibre Toxicol, 11.
6. Balakrishna, S.; Saravia, J.; Thevenot, P.; Ahlert, T.; Lomnicki, S.; Dellinger, B.; Cormier, S. A. (2011) Environmentally persistent free radicals induce airway hyperresponsiveness in neonatal rat lungs. Part Fibre Toxicol, 8.
7. Balakrishna, S.; Lomnicki, S.; McAvey, K. M.; Cole, R. B.; Dellinger, B.; Cormier, S. A. (2009) Environmentally persistent free radicals amplify ultrafine particle mediated cellular oxidative stress and cytotoxicity. Part Fibre Toxicol, 6.
8. Dellinger B, Pryor WA, Cueto R, Squadrito GL0, Hegde V and Deutsch WA (2001) Role of Free Radicals in the Toxicity of Airborne Fine Particulate Matter, Chemical Research in Toxicology, 14(10), 1371-1377, doi: 10.1021/tx010050x.

Professor Slawo Lomnicki, Ph.D., is an Assistant Professor at the Department of Environmental Sciences, Louisiana State University (LSU) and his research is centered around air pollution and in particular, emissions from combustion sources. He also investigates the potential impact of these emissions on human health, and researches general environmental pollution in soil and groundwater, and how to remediate this. Prof. Lomnicki joined LSU as a post doctorate and research associate in chemistry in 2001, where he worked with the famous combustion chemist Dr. Barry Dellinger, before joining the Department of Environmental Sciences in 2014.

For more information, please visit Department of Environmental Sciences at Louisiana State University and LSU Superfund Research Center. For more information on Bruker's EMXnano EPR spectrometer, please visit https://www.bruker.com/products/mr/epr/emxnano/overview.html.

Address: Department of Environmental Sciences, Louisiana State University, 1251 Energy Coast & Environment Bldg., Baton Rouge, LA 70803

Phone: 225-578-8147 

Email: slomni1@lsu.edu