Tire-derived p-phenylenediamine antioxidants and their transformation products in particulate matter across Toronto: size distribution, inhalation bio-accessibility, and oxidative potential

Urban traffic is a major contributor to air pollution and associated health risks. Upon inhalation, pollutants can interact with the lung’s natural antioxidant defences, generating reactive oxygen species (ROS) that lead to oxidative stress, inflammation, and cellular damage. This capacity of pollutants to drive oxidative processes is defined as oxidative potential (OP), a metric increasingly used to assess particulate matter (PM) toxicity.
While exhaust emissions have been extensively studied, non-exhaust sources – specifically tire wear – remain underexplored despite their growing contribution to urban PM. Tire-derived emissions contain a complex mixture of trace metals and organic additives, including p-phenylenediamine (PPD) antioxidants. PPDs undergo oxidative transformation on tire surfaces or within the atmosphere to form PPD-quinones (PPD-Qs). Recent evidence identifies PPD-Qs, such as 6PPD-quinone, as potent redox-active agents with an OP comparable to established quinones. These compounds have been consistently detected in urban environments, ambient air, and road dust. Their demonstrated toxicity and recent detection in human biomonitoring studies underscore a critical need to evaluate their environmental occurrence and broader public health implications.
This study investigates the intrinsic OP of individual PPD-Qs and their binary mixtures with trace metals using chemical OP assays (ascorbate, hydroxyl radical, and dithiothreitol); characterizes the size-resolved distribution of PPDs and PPD-Qs in PM from three near-road sites in Toronto (with varying traffic volumes and distances from roadways); and estimates their inhalation bio-accessibility using chemical partitioning models. Our results demonstrate that PPDs and PPD-Qs are ubiquitous across urban sites, with the highest abundances found at traffic-dense locations and within the coarse PM fraction. These findings underscore how roadway proximity and particle size modulate exposure to non-exhaust emissions, providing evidence for policies aimed at mitigating tire-wear pollution and enhancing urban air quality and public health.