Script for Tear Gas in Plaza de la Dignidad
Forensic Architecture

Since October 2019, the roundabout of Plaza de la Dignidad, in Santiago de Chile had been the focal point of nationwide protests.

Students, labor unions, queer and feminist advocacy groups and indigenous peoples gathered here to protest against neoliberal policies that had sustained decades of violent economic and social inequality.

The 20th of December 2019 turned out to be one of the most intense days of repression.

To suppress the protesters, the Chilean security forces used unprecedented quantities of tear gas within a relatively small area, against people engaging in peaceful and unarmed protest, affecting hundreds of local residents.

Forensic Architecture was asked by the Chilean organization No+Lacrimógenas to work together and build a timeline of the police repression of the protest on that day, investigate the scale of use of tear gas, and evaluate the resulting health risks to protesters, residents and the environment.

The entire battle for the roundabout was documented by a camera installed on a nearby building.

First, we reconstructed the camera’s cone of vision and confirmed its time code using shadows.

At 5:40 p.m., police deploy water cannons on protesters approaching the roundabout.

We noticed the first tear gas cloud at 5:50 p.m., on a traffic island where some protesters start to gather.

We marked the extent of the cloud with a frame, and the approximate location of the canister with a small cross, and added the event to our timeline.

Each canister takes about 25 seconds to discharge.

We developed an automatic method of video analysis to identify the location of discharged
canisters and offer a reliable count.

Over time, the scale of tear gas deployment escalates.

By 7:12 p.m., 186 canisters have already been deployed, as protesters continue their attempt to reach the symbolic centre of the roundabout.

Having removed the barricades, the protesters carry their flag to the center of the roundabout.

Within 15 minutes, an armored vehicle circles the roundabout, discharging a continuous spray of tear gas.

A second vehicle follows, but the protest persists.

At 7:37 p.m., a protester crossing the street was hit by the two armored vehicles moving fast
through the crowd.

The protest further escalates.

Twenty minutes later, protesters took over the entire space of the roundabout, bringing traffic to a standstill.

While the roundabout draws the protesters in, providing a space for a temporarily occupied
commons, the state agents’ cloud intoxicates the air to seek centrifugal dispersal.

From 8:30 p.m., police further escalated its use of tear gas. Our system identified at least 82 canisters deployed within a space of 10 minutes.

With the locations of 594 tear gas canisters accurately mapped within the model, we are able to measure levels of toxicity in air and on the ground.

While the discharge cloud is visible for only 20 seconds, toxic chemicals disperse invisibly through the air for much longer.

Collaborating with a fluid dynamics specialist, Dr. Salvador Navarro-Martínez, we simulated the airborne movement of toxins, taking into account temperature, wind and humidity.

Chilean Police’s Manual for Crowd Control states that exposure to tear gas, or CS, should be limited to 0.4 milligram per cubic meter, and concentrations above the 2 miligram threshold constitutes a serious danger.

This diagram shows that this threshold was surpassed at a sample point for a total duration of 185 seconds.

Between 8:30 and 8:40 p.m., the concentration of tear gas reached toxicity levels of 40 times the allowed limit, risking the lives of protesters.

The concentration of toxic CS particles here in this sample point exceeds the threshold of dangerous health consequences, and is close to the harmful threshold specified in the report.

The wind carries CS particles from southwest to northeast, across the roundabout, ultimately depositing them on the ground.

The surrounding buildings generate irregular turbulences that carry airborne toxin into the larger neighborhood, putting hundreds of residents at risk.

This combination of open source visual investigation and fluid dynamics simulation is the first to measure airborne and ground deposits of CS concentration, establishing a benchmark methodology for doing so.

This method shows that CS particles were carried beyond the perimeter of the roundabout and into the nearby Mapocho River, which feeds the farms on the southern outskirts of Santiago.

The coronavirus lockdowns temporarily suppressed protest activity in Santiago, but when civilians returned to the streets in October 2020, they faced the same tactics of police brutality.

This month, Chile’s Human Rights Commission filed a complaint against the country’s military police for the illegal use of chemical weapons against protesters in the plaza between October and December 2020, evidencing chemical infection and dermatitis.

The findings of our own investigation support this complaint, and urge for the outright ban of
tear gas as a chemical weapon.