1 Air Quality Processes Research Section, Environment and Climate Change Canada, Toronto, Ontario, Canada.
2 Department of Environmental Science and Engineering, University of Science and Technology of China, Hefei, China.
3 School of Public Health, University of Nevada Reno, Reno, NV, USA.
4 Laboratory Services Branch, Ontario Ministry of the Environment, Conservation and Parks, Toronto, Ontario, Canada.
5 Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada.
6 International Joint Research Center for Persistent Toxic Substances (IJRC-PTS), State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, China.
7 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), Villigen, Switzerland.
8 State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beiji

Commercial chemicals are used extensively across urban centres worldwide¹, posing a potential exposure risk to 4.2 billion people². 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 Convention³. 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 challenge⁴. 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 concern⁵, 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.