One of the more gripping images to emerge from the ongoing protests in Brazil is of a young woman, her face clenched shut as a police officer in tactical armor blasts her with chemical spray from a couple of feet away. (A zoomed-out iteration is starker still: the corner she stands on is deserted, and there is not one officer but three.) Her face is slick with the substance, and the top of her dress seems saturated. The photographer told New York magazine that she began “screaming and cursing” as she moved back.

Modern aerosol pepper sprays typically consist of a propellant, a carrier or dispersant, and oleoresin capsicum, a reddish-brown, oily material that contains many related chemicals and is extracted from chili peppers in the family Capsicum. The primary irritant is the chemical capsaicin. Technically known as 8-methyl-N-vanillyl-trans-6-nonenamide, it works by triggering nociceptors—the body’s pain sensors—creating a sensation of intense heat. The Scoville Heat Unit, a measure of a given pepper’s spiciness, is based on capsaicin content. Tabasco peppers rate in the tens of thousands of S.H.U.s; habaneros are in the low hundreds of thousands. A typical pepper spray used by police might be rated in the low millions.

Patents for implementations of pepper spray often refer to it as a lachrymator, meaning it can cause teary eyes and a runny nose. Capsaicin compounds induce the release of anti-inflammatory substances, like histamine, and can cause facial and lung-tissue swelling. In certain cases, they lead to bronchial spasms and constriction, in which the body limits breathing to keep them out. Elevated heart rate, blurred vision, and minor cornea damage are typical. While a single dose is not likely to damage eyes permanently, repeated exposure can alter corneal sensitivity. There is debate over whether pepper spray can cause death. A report by the National Institute of Justice cites it as a contributing factor in the deaths of two people in policy custody, both of whom had asthma.

Little can be done to alleviate the pain of pepper spray. A 2008 study at Fort Sam Houston exposed forty-nine volunteer-law-enforcement trainees—their faces, specifically—to pepper spray as part of their normal training. Researchers then separated them into five groups, each of which attempted to alleviate the effects with a different substance: water, milk, baby shampoo, lidocaine cream, or Maalox. The only effective treatment was time.

Pepper spray wasn’t always so common. Oleoresin capsicum was made available as an aerosol in 1973, and it saw widespread use in the United States shortly thereafter as an alternative to synthetic tear gases: forest workers and hunters outfitted themselves with pepper sprays to fend off bears, and Cincinnati Magazine, in 1980, explained that mail carriers were armed with what it called “mustard and pepper spray” to deter vicious dogs. The F.B.I. worked to develop it into “weapons-grade material” in the eighties, and issued a study approving it as a means to control unruly people. It was later revealed that the agent who led the study had accepted fifty-seven thousand dollars from a pepper-spray manufacturer, and he was sentenced to two months in jail. Nonetheless, the spray’s use grew among police departments, and the N.Y.P.D. decided to adopt pepper spray as an “anti-personnel” weapon in 1994. By 1999, the Department of Justice noted, the police were reaching for their pepper spray six times more often than their guns.

That same year, the American Civil Liberties Union asked a federal appeals court to declare the use of pepper spray “dangerous and cruel.” In North Carolina, the Department of Labor and the Department of Health and Human Services declared that exposure constituted “an unacceptable health risk.” In many countries, pepper spray is highly regulated; the United Kingdom, for instance, classifies it as a prohibited weapon, along with explosive bullets and rocket launchers.

Pepper spray was once thought to be a non-lethal alternative to guns. But other, less harmful incapacitating agents, using light and sound, may soon be thought of as alternatives to pepper spray.

Photograph by Victor R. Caivano/AP.