An excessive amount of teargas was used to control crowds in 2019-2020 protests in Santiago, Chile. With Forensic Architecture we chose the 20th of December 2019 as a day representative of these protests. Within this investigation I did the research, spatial research, 3D modelling, photo-matching and conceptualisation. I interviewed witnesses and most notably - created a workflow for tracking and 3D reconstructing all 594 instances of teargas over a 3 hour span. The reconstruction allowed for the first ever accurate measurement of CS concentration in the air during a protest.