Elsevier

Free Radical Biology and Medicine

Volume 31, Issue 9, 1 November 2001, Pages 1132-1138
Free Radical Biology and Medicine

Hypothesis paper
Quinoid redox cycling as a mechanism for sustained free radical generation by inhaled airborne particulate matter

https://doi.org/10.1016/S0891-5849(01)00703-1Get rights and content

Abstract

The health effects of airborne fine particles are the subject of government regulation and scientific debate. The aerodynamics of airborne particulate matter, the deposition patterns in the human lung, and the available experimental and epidemiological data on health effects lead us to focus on airborne particulate matter with an aerodynamic mean diameter less than 2.5 μm (PM2.5) as the fraction of the particles with the largest impact in health. In this article we present a novel hypothesis to explain the continuous production of reactive oxygen species produced by PM2.5 when it is deposited in the lung. We find PM2.5 contains abundant persistent free radicals, typically 1016 to 1017 unpaired spins/gram, and that these radicals are stable for several months. These radicals are consistent with the stability and electron paramagnetic resonance spectral characteristics of semiquinone radicals. Catalytic redox cycling by semiquinone radicals is well documented in the literature and we had studied in detail its role on the health effects of cigarette smoke particulate matter. We believe that we have for the first time shown that the same, or similar radicals, are not confined to cigarette smoke particulate matter but are also present in PM2.5. We hypothesize that these semiquinone radicals undergo redox cycling, thereby reducing oxygen and generating reactive oxygen species while consuming tissue-reducing equivalents, such as NAD(P)H and ascorbate. These reactive oxygen species generated by particles cause oxidative stress at sites of deposition and produce deleterious effects observed in the lung.

Introduction

In this article we present a novel hypothesis to explain the continuous production of reactive oxygen species produced by fine airborne particulate matter when it is deposited in the lung. We find airborne particulate matter with an aerodynamic mean diameter less than 2.5 μm (PM2.5) contains abundant persistent free radicals, typically about 1016 to 1017 unpaired spins/gram. The stability and electron paramagnetic resonance spectral characteristics of these radicals are consistent with those of semiquinone radicals. We hypothesize that these semiquinone radicals undergo redox cycling, thereby reducing oxygen and generating reactive oxygen species while consuming reducing equivalents (e.g., NAD(P)H or ascorbate). These reactive oxygen species generated by particles cause oxidative stress in the adjoining tissues and are conducive to the deleterious effects observed in the lung [1], [2], [3].

Section snippets

Particle size and related descriptors of airborne particulate matter

Airborne particulate matter (PM) is an important pollutant in urban atmospheres that has been linked to various adverse health effects. PM is usually divided into three size classes (ultrafine, fine, and coarse), according to its aerodynamic diameter (Table 1). Ultrafine and fine particles originate primarily from combustion sources (e.g., motor vehicles and power plants), while coarse particles are generated by mechanical processes.

Airborne particulate matter-health: cause-effect relationships

The winter fog episodes in the Meuse Valley in Belgium

PM2.5 contains large amounts of semiquinone radicals

Because the principal source of airborne fine particles is combustion [15], [22] (Table 1) and combustion of organic materials, such as diesel fuel and tobacco, generate particles that contain persistent free radicals [23], [24], [25], we suspected that PM2.5 may contain radicals [26] and examined samples of PM2.5 from five sites distributed about the U.S. using electron paramagnetic resonance (EPR).

The EPR spectra of these samples are shown in Fig. 1 and the principal EPR parameters are given

Summary and perspectives

We have presented evidence that PM2.5 contains quinoid substances and that it produces reactive oxygen species (ROS). We associate production of ROS by PM2.5 with the well-known redox cycling capability of quinoid substances. These quinoid substances present in PM2.5 may be in part chemically bound to the particle matrices and leach out with extreme difficulty, contributing to their stability and striking persistence. During redox cycling, quinoid substances in their reduced states can reduce

Acknowledgements

We thank the Louisiana Department of Environmental Quality for furnishing samples of PM2.5. We acknowledge the cooperation of Dr. Paul Solomon of the U.S. Environmental Protection Agency for providing samples of PM2.5 that were obtained as part of the EPA Chemical Speciation Sampler Evaluation Study. This work was supported by grants from NSF and the Patrick F. Taylor Chair to B.D. and NIH to W.A.P.

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