Evaluation of PM2.5 Oxidative Potential (OP) as a Proxy for Aerosol Toxicity
Advisor: Professor Vishal Verma
Abstract
Fine ambient particulate matter (PM2.5) is one of the leading causes of mortality worldwide. Although mass is currently used to assess the health risks of PM2.5, its effectiveness in capturing PM2.5 toxicity is debatable, raising the need for alternate health metrics. One such metric is oxidative potential (OP), which is the ability of PM2.5 to cause an imbalance between reactive oxygen species (ROS) and the antioxidant systems in our body. Studies in the past two decades have provided crucial insights into various aspects of OP although there are still many unanswered questions. There is also a need to demonstrate the advantages of OP over PM2.5 mass as a health metric and provide a systematic evaluation of the biological relevance of OP. This dissertation addresses some of these issues.
First, to provide a better comparison between cellular and acellular OP, we developed a first-of-its-kind semi-automated instrument for cellular oxidative potential evaluation (SCOPE) of water-soluble extracts of ambient PM using rat alveolar macrophage cells. The instrument is equipped to operate continuously for 24 h with minimal manual intervention and has a high analytical precision. The results obtained from the instrument were in good agreement with manual measurements. We further demonstrated the ability of this instrument to analyze a large number of both ambient and laboratory samples and developed a dataset on the intrinsic cellular OP of compounds commonly known to be present in ambient PM2.5 such as metals, quinones, PAHs, and inorganic salts.
Second, we evaluated the relationship between PM2.5 mass and OP on a large spatial extent using 385 PM2.5 samples collected from 14 different sites across 4 different continents and using 5 different endpoints (3 acellular and 2 cellular) to assess the toxicity of water-soluble PM2.5 extracts. Our results show that the relationships between cytotoxicity, OP, and PM2.5 mass were non-proportional with substantial heterogeneity in the slope of OP (or cytotoxicity) vs. PM2.5 mass, attributable to the differences in intrinsic toxicity resulting from the heterogeneous chemical composition of the PM2.5. These results emphasize the need to develop localized CR functions incorporating other measures of PM2.5 properties (e.g., intrinsic OP) to better predict the PM2.5 attributed health burdens. We also demonstrated how integrating OP measurements into global PM monitoring networks such as SPARTAN, could provide better insights into PM toxicity. We measured two acellular OP endpoints using 95 PM2.5 samples collected as a part of the SPARTAN campaign from 9 different sites on 3 different continents. Analysis of the spatiotemporal variations in OP and the non-linear relationship between PM2.5 mass and OP further demonstrate the importance of intrinsic OP in determining the relationship between PM2.5 mass and health effects.
Finally, we explored the biological relevance of current acellular OP assays. We first assessed the statistical correlation between different OP endpoints (OPDTT, OPGSH, OPOH, OPAA, and cellular ROS in A549 cells) and cytotoxicity in A549 cells for a large number of samples collected from different regions of the world. We next attempted to explain the observed statistical relationships between OP and cytotoxicity using a mechanistic investigation of the role of antioxidants in cytotoxicity and ROS consumption. Overall, our results validate the relevance of PM2.5-induced oxidative stress in cytotoxicity and indicate that our current approach of using single antioxidant depletion measurements and total ROS measurements as a proxy for aerosol toxicity may need a rethinking.
