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Commercial chemicals are used extensively across urban centres worldwide.sup.1, posing a potential exposure risk to 4.2 billion people.sup.2. Harmful chemicals are often assessed on the basis of their environmental persistence, accumulation in biological organisms and toxic properties, under international and national initiatives such as the Stockholm Convention.sup.3. However, existing regulatory frameworks rely largely upon knowledge of the properties of the parent chemicals, with minimal consideration given to the products of their transformation in the atmosphere. This is mainly due to a dearth of experimental data, as identifying transformation products in complex mixtures of airborne chemicals is an immense analytical challenge.sup.4. Here we develop a new framework--combining laboratory and field experiments, advanced techniques for screening suspect chemicals, and in silico modelling--to assess the risks of airborne chemicals, while accounting for atmospheric chemical reactions. By applying this framework to organophosphate flame retardants, as representative chemicals of emerging concern.sup.5, we find that their transformation products are globally distributed across 18 megacities, representing a previously unrecognized exposure risk for the world's urban populations. More importantly, individual transformation products can be more toxic and up to an order-of-magnitude more persistent than the parent chemicals, such that the overall risks associated with the mixture of transformation products are also higher than those of the parent flame retardants. Together our results highlight the need to consider atmospheric transformations when assessing the risks of commercial chemicals. A new framework is proposed for assessing the risks of the atmospheric transformation products of commercial chemicals, combining laboratory and field experiments, advanced techniques for screening suspect chemicals, and in silico modelling., Author(s): Qifan Liu [sup.1] [sup.2] , Li Li [sup.3] , Xianming Zhang [sup.1] [sup.4] [sup.5] , Amandeep Saini [sup.1] , Wenlong Li [sup.1] [sup.6] , Hayley Hung [sup.1] , Chunyan [...]

The inhalation of fine particulate matter (PM 2.5 ) is a major contributor to adverse health effects from air pollution worldwide. An important toxicity pathway is thought to follow oxidative stress from the formation of exogenous reactive oxygen species (ROS) in the body, a proxy of which is oxidative potential (OP). As redox-active transition metals and organic species are important drivers of OP in urban environments, we investigate how seasonal changes in emission sources, aerosol chemical composition, acidity, and metal dissolution influence OP dynamics. Using a kinetic model of the lung redox chemistry, we predicted ROS (O 2 •- , H 2 O 2 , • OH) formation with input parameters comprising the ambient concentrations of PM 2.5 , water-soluble Fe and Cu, secondary organic matter, nitrogen dioxide, and ozone across two years and two urban sites in Canada. Particulate species were the largest contributors to ROS production. Soluble Fe and Cu had their highest and lowest values in summer and winter, and changes in Fe solubility were closely linked to seasonal variations in chemical aging, the acidity of aerosol, and organic ligand levels. The results indicate three conditions that influence OP across various seasons: (a) low aerosol pH and high organic ligand levels leading to the highest OP in summer, (b) opposite trends leading to the lowest OP in winter, and (c) intermediate conditions corresponding to moderate OP in spring and fall. This study highlights how atmospheric chemical aging modifies the oxidative burden of urban air pollutants, resulting in a seasonal cycle with a potential effect on population health., Competing Interests: The authors declare no competing financial interest., (Crown © 2024. Published by American Chemical Society.)