AS&T virtual collection: Toxicity of ambient particulate matter – impact of chemical composition, emission sources and atmospheric processes

An increasing number of studies in recent years have linked ambient particulate matter (PM) to diseases such as asthma (Fan et al. 2016), myocardial infarction (Madrigano et al. 2013), stroke (O’Donnell et al. 2011) and even Alzheimer’s (Shou et al. 2019). There have also been several epidemiological studies that predict a surge in global disease burden and mortality in the near future if the present trend in rising PM emissions continues (Burnett et al. 2018; Li et al. 2019; Apte et al. 2015). Most of these models use PM mass as a metric although a few complex models also include some of the key chemical species present in PM such as metals and organic compounds (Heo et al. 2014; Wang et al. 2022). However, these models still ignore the complexity of PM chemical composition, which is influenced not only by heterogeneous emission sources of PM but also by atmospheric processes (R€ onkk€ o et al. 2018). Moreover, these models ignore the interactions among various chemical components of PM which can alter the overall toxicity of PM as obtained by the simple summation of individual toxicities (Yu et al. 2018; Wang et al. 2020). Thus, the notion that higher PM mass implies higher mortality or prevalence of diseases is not completely true. It is clear that there is a necessity to search for alternative metric(s) than PM mass to represent PM toxicity in epidemiological models, which in turn requires clarity on the exact mechanisms underlying PM toxicity. Currently, some progress has been made in this regard in the past two decades and it has indicated that PM sources, atmospheric processes, chemical composition and pathology of diseases share a complex relationship with each other. Investigations for linking the PM toxicity with chemical composition have often shown varied and sometimes contradictory results and presently face several issues that need attention. For example, the traditional toxicity characterization methods are labor-intensive and time-consuming, and therefore, only a limited number of samples can be analyzed for the toxicity evaluation. Furthermore, there is also a range of toxicity endpoints that one could choose from, which has often led to debate on whether or not a single endpoint could adequately represent PM toxicity. Recently, oxidative potential (OP) has emerged as a proxy for PM toxicity, although currently there is a lack of consensus regarding the most appropriate method to measure PM OP (Yu, Puthussery, and Verma 2020). Moreover, there is a lack of standard protocols for the procedures of PM collection, extraction and exposure to lung cells, although these factors have been shown to heavily influence results (Daher et al. 2011), making comparison among different studies difficult. Overcoming the hurdles described in the previous paragraph would better equip us to answer some important questions that can be addressed by the aerosol community, and this is possible through focused research into certain areas. These include the development of methodologies that can better mimic physiological processes in the human body, leading to a more accurate assessment of the health impacts. Design of simple, inexpensive and high throughput methods to evaluate PM toxicity is needed to generate large datasets that can be incorporated into epidemiological models. Development of real-time instruments for field deployment could also greatly expand our capabilities to generate such large datasets. Investigations into spatiotemporal variations in PM toxicity and their correlations with chemical composition could refine our understanding of the role of emission sources and atmospheric processing in influencing PM toxicity. Such studies could also lead to development of machine learning based models to make predictions in data-sparse regions. Another research area that requires immediate attention is the interaction between various chemical components and their role in altering the overall toxicity of PM. Currently, little work has been undertaken in this area and, except for preliminary information regarding the interaction among a few prominent species, not much is known. We have collated a selection of articles published by AS&T to form a Virtual Collection as an attempt to highlight the efforts made by aerosol researchers so far to answer the questions posed here, addressing issues vital to further our understanding of PM toxicity. For example, Landreman et al. (2008) developed an assay to measure the PM OP in alveolar macrophages subjected to short-term exposure. This assay has been widely used in PM studies revealing important information regarding the spatiotemporal variations in OP. Yu, Puthussery, and Verma (2020) developed an instrument for measuring five commonly used OP endpoints to provide a rapid and comprehensive assessment of PM OP of a large number of PM samples. Such large-scale studies are necessary for understanding the relationship between