Despite mounting criticism about the use of tear gas and other crowd-control agents, law enforcement agencies have resisted disclosing exactly what they're spraying on protesters.

During recent protests in Brooklyn Center over then-officer Kimberly Potter's killing of Daunte Wright, some demonstrators and police exchanged bricks and fireworks for rubber bullets and tear gas. Residents of nearby apartment complexes were trapped in the crossfire.

In response, the Brooklyn Center City Council quickly banned chemical weapons and Mayor Mike Elliott publicly criticized their use. However, in a letter to the Hennepin County sheriff, who assumed the role of incident commander after Brooklyn Center Police Chief Tim Gannon resigned under pressure, Elliott requested the continued support of law enforcement agencies that do use tear gas.

Beyond the immediate, incapacitating effects of tear gas, medical researchers suspect the chemicals may cause long-term health damage, though research is sparse.

Law enforcement agencies say tear gas and other less-lethal techniques are a substitute for a more violent response in a chaotic situation.

"The alternative to chemical munitions would be physical force," including baton strikes, kicks and punches, said Andy Skoogman, spokesman for Hennepin County Sheriff David Hutchinson.

The Hennepin County Sheriff's Office was one of the only agencies that offered details on its use of tear gas.

The Sheriff's Office said its deputies used two hand-thrown CS gas canisters and two 37-mm launched rounds on April 11, the day of Wright's death, before the Brooklyn Center Police Department was fenced.

In response to a Star Tribune data request for the make and model of chemical weapons in its arsenal, the Sheriff's Office provided a purchasing catalog from Safariland, which along with Nonlethal Technologies and Combined Systems, is a major American manufacturer of less-lethal weapons.

Other agencies involved in Operation Safety Net, the law enforcement coalition created to provide security around the Derek Chauvin trial, include the Minnesota National Guard, which doesn't carry less-than-lethal munitions. The University of Minnesota Police, which assisted Operation Safety Net in Brooklyn Center, chose not to use them.

The Minneapolis and St. Paul police departments, Department of Natural Resources and Ramsey County Sheriff's Office did not respond to Star Tribune requests for information about their riot-control agents.

Metro Transit cited a state law that restricts the release of security information. "We're looking to find out if we revealed some of these specifics if that would inform a crowd that is knowledgeable about, you know, how far back they might need to stand based on what we say we're using," spokeswoman Laura Baenen said.

The Department of Public Safety also declined to provide a list of specific chemical agents, distraction devices and impact munitions that certain State Patrol troopers carried. The department said that 75 to 175 troopers assisted Brooklyn Center, depending on the day.

"The State Patrol had more than 100 documented cases of troopers being struck by objects thrown by protesters, exposure to bear spray used by protesters or being spat on," said spokesman Bruce Gordon.

In the chaos of police crackdowns in Brooklyn Center, residents also have been gassed, while clearly identified journalists have been arrested, suffered broken bones and been sprayed in the face with chemical irritants. Finding out who's responsible has been difficult. Despite the unified command structure of Operation Safety Net, each agency is responsible for investigating use-of-force complaints against its own officers.

Twin Cities emergency room doctors, who have treated many protesters over the past year, criticized police for violating United Nations guidance and their own policy manuals by aiming projectiles, including tear gas canisters, at the head and neck.

Tear gas was created during World War I for trench warfare and was banned by the Geneva Protocol of 1925, said Timothy Monko, a neuroscientist doctoral student at the University of Minnesota Medical School who studied the history of tear gas in American policing.

Military surpluses of tear gas then were sold to law enforcement agencies throughout the country. Even after the United States ratified the Geneva Protocol in 1975 following the Vietnam War, local police continued to use it for riot control.

The FDA, CDC and ATF lack regulatory authority over the production of chemical irritants, according to the agencies. However, the EPA sets guidance levels for airborne chemicals, which states that for all concentrations of tear gas, the "general population, including susceptible individuals, could experience irreversible or other serious long lasting adverse health effects or an impaired ability to escape."

Monko's research, using discarded devices picked up off the street and photos of police utility belts, has found that pepper spray as well as the chemicals OC (found in Mace) and CS (found in tear gas canisters) have all been used in the Twin Cities over the past year.

A more precise understanding of what local police are using is complicated by agencies' refusal to provide information, Monko said.

Doctors in Bahrain have anecdotally observed that miscarriages increase following tear gas attacks. A Chilean study also found a probable link. The CDC recognizes that immediate exposure to high doses of tear gas can result in blindness, coma, respiratory failure and death.

However, quantitative research on the long-term health effects of chemical weapons used on protesters is lacking because there's no ethical way to experimentally gas people, said Asha Hassan, a research manager at Planned Parenthood studying the effects of chemical weapons on reproductive health.

"The way it is being deployed makes it virtually impossible for us to know how much tear gas somebody has consumed," she said.

After the use of riot control weapons on protesters following the murder of George Floyd, Hassan started hearing from people who normally wouldn't menstruate — such as trans men on testosterone and people with certain types of IUDs — who were experiencing spontaneous uterine bleeding.

Planned Parenthood is conducting a nationwide survey that already has collected several thousand respondents' stories of menstrual problems to determine whether tear gas is playing a role.

Last week, the journal BMC Public Health published a peer-reviewed study based on a self-reported survey of 2,257 adults exposed to tear gas in Portland, Ore., last summer. Of the 1,650 respondents who potentially menstruate, more than half reported some disruption, including increased cramping, unusual spotting and increased bleeding.

Susan Du • 612-673-4028