Oxidative potential of atmospheric aerosols

Atmospheric particulate matter (PM) is one of the leading health risks worldwide [1,2]. Several epidemiological studies have provided evidence of the association between exposure to PM and the onset of cardiovascular and respiratory diseases [3], as well as cardiopulmonary diseases and other adverse health effects [4]. The exact mechanisms leading to PM toxicity are not fully known, however, several studies suggest that the generation of reactive oxygen species (ROS) could be a major mechanism by which PM leads to both chronic and acute adverse health effects [5,6]. For this reason, in recent years, the oxidative potential (OP) of PM, defined as its ability to generate oxidative stress in biological systems, has been proposed as a relevant metric for addressing PM exposure [7,8]. However, the link between OP and adverse health effects is still uncertain [9–11], and contrasting results have been obtained when PM oxidative potential has been compared with the results of in-vivo and in-vitro toxicological tests or the outcomes of epidemiological studies [12]. The OP can be evaluated through several in vitro assays, but protocols employing chemical (acellular) assays have become common as well. Acellular assays can be useful for investigating the PM properties which are responsible for oxidative stress: ROS compounds can either be carried by components of the aerosol itself (particle-bound ROS) or induced by the catalytic activity exerted by aerosol constituents (PM-induced ROS). The diverse OP assays developed so far have certainly improved our knowledge of the mechanisms underlying PM oxidative stress. At the same time, they pose the issue of comparability between the different assays and protocols, as well as problems surrounding the actual correlation between acellular OP and in vitro (or in vivo) toxicity. Measurements of PM oxidative potential are influenced by the chemical composition of the aerosol, by its size distribution, and by the weight of different natural and anthropogenic sources of PM leading to temporal and spatial variabilities that need investigation in current research. Moreover, recent studies show that photochemical aging increases the oxidative potential of atmospheric aerosols. However, several aspects regarding the specific chemical species, aerosol sources, and atmospheric processes that affect OP are not well established, and further research is needed [13–15]. Another topic that needs extensive research is the characterization of the OP of indoor aerosols. This special issue includes five research papers and two review papers discussing recent advances in the studies of the oxidative potential of atmospheric particulate matter.