Influence of environmental conditions on the dithiothreitol (DTT)-Based oxidative potential of size-resolved indoor particulate matter of ambient origin
Graphical abstract
Introduction
Epidemiological studies have consistently found associations between elevated concentrations of ambient particulate matter, including fine particulate matter with aerodynamic diameter less than 2.5 μm (PM2.5), and adverse health outcomes, including mortality (Bowe et al., 2019; Brook et al., 2010; Di et al., 2017; Pope et al., 2002; Pope and Dockery, 2006; Thurston et al., 2016). However, the heterogeneous and inconsistent nature of these epidemiological associations, including high spatiotemporal variability in the strengths of associations with adverse health outcomes, suggests substantial variability in human exposures and/or the intrinsic toxicities of ambient PM2.5 based on composition or other factors (Chen et al., 2012; Dai et al., 2014; Franklin et al, 2007, 2008; Zanobetti and Schwartz, 2009; Zhou et al., 2011). Moreover, much of human exposure to ambient PM2.5 likely occurs indoors (Azimi and Stephens, 2018) because people spend nearly 90% of their time inside buildings (Klepeis et al., 2001) and particles of ambient origin can infiltrate and persist in buildings with varying efficiencies (Allen et al., 2012; Chen and Zhao, 2011; Kearney et al., 2014; MacNeill et al, 2012, 2014).
Although the mechanisms of action for PM-related health effects are not completely understood, an increasing body of evidence has demonstrated that PM exposure can induce oxidative stress in the body and, therefore, the oxidative potential (OP) of PM, or the capability of particles to generate reactive oxygen species (ROS) in a biologically-relevant system, may be a useful indicator of the intrinsic toxicity of PM (Bates et al., 2019). Several recent epidemiological studies have shown stronger associations between respiratory and cardiovascular health endpoints and OP of ambient PM2.5 than with PM mass concentrations (Abrams et al., 2017; Bates et al., 2015; Delfino et al., 2013; Maikawa et al., 2016; Weichenthal et al., 2016; Yang et al., 2016).
To date, the OP of ambient PM has been characterized in numerous studies using a variety of approaches (Bates et al., 2019). The most commonly used chemical approach for characterizing the OP of PM is the dithiothreitol (DTT) assay. Under the presence of PM, DTT is oxidized by molecular oxygen, leading to the formation of DTT disulfide and superoxide radical (∙O2−). Thus, the DTT assay simulates a similar mechanism for the ROS generation as through the NADPH oxidation occurring in mitochondria (Kumagai et al., 1997). The consumption rate of DTT, also known as the DTT activity, has been found to be correlated with the production rate of H2O2 (Tong et al., 2018; Xiong et al., 2017). Both organic compounds [e.g. quinones, humic-like substances] as well as transition metals (e.g., Cu and Mn) have been shown to be active in the DTT assay (Charrier and Anastasio, 2012; Cho et al., 2005; Lin and Yu, 2011; Nicolas et al., 2015; Verma et al., 2012).
Fewer studies have investigated the OP of indoor PM of either indoor or outdoor origin, including in European office buildings (Mihucz et al., 2015; Szigeti et al, 2014, 2017) and both rural (Secrest et al., 2016) and urban (Zhan et al., 2018) homes in China. In the only study of which we are aware of the OP of PM in U.S. residences, only the particle-bound ROS (Khurshid et al, 2014, 2016) and the hydroxyl radical (•OH) generation rate in the presence of H2O2 (Khurshid et al., 2019) have been measured as indicators of OP. These metrics measure the ROS already present on the particle's surface or its ability to generate ROS under specific conditions. Conversely, OP is a generic property of the particle to consume cellular antioxidants directly or indirectly (i.e., by generating ROS in a biological or a surrogate-to-biological system).
Although many studies have investigated size-resolved OP of ambient aerosol (Cho et al., 2005; Godri et al., 2011; Lovett et al., 2018; Saffari et al., 2013), no studies to date have reported size-resolved OP of indoor PM of ambient origin or explored the impacts of varying environmental conditions that may influence the composition (Hodas and Turpin, 2013; Johnson et al., 2017; Avery et al., 2019) and/or size-resolution (Zhao and Stephens, 2017) of indoor PM (and therefore OP) of ambient origin. Although the U.S. Environmental Protection Agency's (EPA's) most recent Integrated Science Assessment on Particulate Matter (US EPA, 2019) acknowledges the importance of indoor exposures to PM of ambient origin, as well as the potential utility of OP as a health-relevant measure of PM, it does not make the explicit connection between ambient-to-indoor transformations of OP, largely because there is limited literature on the subject (Goldstein et al., 2020). Therefore, the goal of this work is to experimentally characterize the OP of size-resolved PM samples collected simultaneously inside and outside of an unoccupied indoor environment, free from indoor emission sources, and to explore the influence of environmental conditions on the OP of indoor PM of ambient origin.
Section snippets
Materials and methods
Sampling. Simultaneous indoor and outdoor size-resolved aerosol samples were collected using Sioutas Impactors placed inside an unoccupied apartment unit and on the roof of a mid-rise (9-story) dormitory building on the main campus of Illinois Institute of Technology, Chicago, IL USA (Carman Hall, constructed in 1953). The test apartment unit, which is described in detail in prior publications (Kunkel et al., 2017; Zhao and Stephens, 2016, 2017), is a corner unit on the 3rd floor with two
Results and discussion
Fig. 1 shows the resulting time-integrated indoor and outdoor size-resolved PM mass concentrations measured during each sampling campaign throughout the study duration. A full summary of indoor and outdoor sampling results is provided in the supplemental information (SI) (Table S1 to S4). The average indoor and outdoor total PM mass concentrations integrated across all sizes, including >2.5 μm, were 4.0 μg/m3 and 13.9 μg/m3, respectively, ranging 1.6–6.2 μg/m3 and 6.2–20.3 μg/m3, respectively.
Conclusion
Our study provides the very first comparison of indoor vs. outdoor levels of a biologically relevant property (i.e., OP) of ambient PM as it infiltrates indoors. However, there were several limitations in our work. First, we limited our sampling only to a single unoccupied environment with a relatively small sample size (N = 11). The OP of the particles in an occupied environment can be substantially altered by other factors, such as occupant movement and activities, including behaviors to
CRediT authorship contribution statement
Yicheng Zeng: Sampling, data collection, analysis, Writing – review & editing, Writing – original draft, Writing – review & editing. Haoran Yu: Sampling, data collection, analysis, Writing – review & editing. Haoran Zhao: Sampling, data collection, analysis, Writing – review & editing. Brent Stephens: Conceptualization of study design, Methodology, Writing – review & editing. Vishal Verma: Conceptualization of study design, Methodology, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported in part by an ASHRAE New Investigator Award to Brent Stephens and by the Armour College of Engineering at Illinois Tech. The OP analysis on the filters was supported from Vishal Verma's startup fund from UIUC.
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