'Tear Gas Tuesday' in Downtown Portland

Despite being broadly banned in warfare under the terms of the 1925 Geneva Protocol, tear gas as an agent for so-called ‘riot control’ has become the preferred means for police, in the US and around the world, to clear dissenting voices from public spaces. But the toxic chemicals contained in tear gas and other widely-used chemical munitions can cause serious short- and long-term side effects, from asthma and chemical burns to lung injury and neurodegeneration.

In 2020, as protests swept across the US in response to the police killing of George Floyd and Breonna Taylor—just two in a long trajectory of other Black Americans killed with impunity by American law enforcement—police forces in cities from Philadelphia to Minneapolis responded with excessive force to suppress these demonstrations.

In Portland, Oregon, the demonstrations went on for months, and were consistently met with repeated and excessive use of tear gas by the city’s police force, the Portland Police Bureau, or PPB.

On 2 June 2020, the PPB deployed large amounts of tear gas in an attempt to force protestors to withdraw from the streets and squares of downtown Portland, unleashing a highly toxic cloud of airborne chemicals onto the same citizens it was tasked with protecting. That day would become known locally as ‘Tear Gas Tuesday’.

Using computer fluid dynamics, FA and Imperial College London simulated the movement of CS particles across downtown Portland as a result of the actions of Portland’s police, the PPB. (Forensic Architecture, 2023)

Forensic Architecture worked with the Department of Mechanical Engineering at Imperial College London to analyse the PPB’s use of chemical munitions that day using computer fluid dynamics (CFD) simulation.

Drawing on publicly available information, we counted, located and identified the different munitions used that night by the PPB, and created a simulation of the toxic tear gas cloud in order to measure the exposure, spread, and concentration of tear gas throughout the city.

Our analysis shows how airborne chemical concentrations were in serious excess of recognised safe thresholds, raising important questions about the proportional nature of the PPB’s response, their understanding of the potential harms of the chemical munitions that they use, and the risks inherent in how they use them. The findings of this investigation reinforce concerns raised and legal action taken by local activists and civil rights groups to restrict and ban the use of tear gas.

Our investigation shows how the use of tear gas by the PPB on 2 June exposed civilians exercising their democratic right to protest and assembly to dangerous and toxic levels of CS—or o-chlorobenzylidene malononitrile—one of the most commonly used chemicals found in tear gas.

Within a timeframe of only three hours, between 21:00 and 0:00 that night, the PPB used at least 148 munitions (of which 138 were likely chemical munitions). With our model, we are able to show that these munitions caused airborne chemical concentration levels to exceed the 2mg/m³ threshold beyond which exposure is ‘immediately dangerous to life and health’ (IDLH), as defined by federal US agencies (specifically the US Department of Labor’s Operational Safety and Health Administration). According to their own internal documents, the PPB is aware of these established IDLH thresholds and the ‘severe risks’ caused by exposing civilians to ‘high concentrations’ of such chemical agents—yet these regulations appear to have been actively ignored by the PPB officers on duty.

The results of the simulation, and the ‘volume measurements’, show that in all sample locations, federally-recognised safe concentrations thresholds for CS exposure were breached significantly. (Forensic Architecture, 2023)

We evaluated the airborne concentrations of CS in fifteen different locations across downtown Portland, some in immediate proximity to protest sites and others in locations further away. At all fifteen measurement points the IDLH threshold of 2mg/m³ was exceeded for at least thirty seconds.

In nineteen separate instances across the sampling locations, the 2mg/m³ threshold was exceeded continuously for more than a minute.

Throughout the night, across all locations, concentrations of 2mg/m³ were exceeded for a total of more than 73 minutes.

At sampling location A (highlighted in image above), which simulates conditions at the intersection of SW 5th Avenue and SW Yamhill Street, a key location in the 2 June protests, the IDLH value was continuously exceeded for a period of six minutes and twenty seconds. The maximum airborne concentration recorded at sampling location A was 443.9 mg/m³.

At sampling location H (highlighted in image above), just one block north of sampling location A, at the intersection of SW 5th Avenue and SW Morrison Street, our simulation recorded the highest airborne concentration value across the sampled locations: 4,568 mg/m³, more than 2,000 times the IDLH threshold.

CS particles deposit in varying concentrations on the ground after their discharge and dispersion through the air. Our model shows that CS particles spread widely across downtown Portland, reaching and polluting the nearby Willamette River. (Forensic Architecture, 2023)

In addition to airborne chemical concentrations, our analysis also reveals the quantities of CS particles deposited on the ground after their discharge and dispersion in the air. Our simulation shows that teargas particles travelled up to 809 metres, accumulating in areas close to heavily frequented commercial areas and offices, as well as in the nearby Willamette River—home to a variety of aquatic species for whom CS is highly toxic—where up to 2kg of CS particles were likely deposited.

Our simulation focused on the dispersion of CS, as its health impacts have been studied in more detail than other similarly weaponised chemical compounds. It is worth noting, however, that the impact of exposure to combinations of high concentrations of CS and OC (oleoresin capsicum, the active ingredient in pepper spray)—to which Portland residents were likely subjected on 2 June and regularly throughout the following months—have not been extensively studied.