Prevention has to start early, before the behavior does.

Choking means something is blocking the airway from the inside, like food or a small object. What people call choking during sex is pressure on the outside of the neck, and doctors, forensic nurses, prosecutors and state criminal laws call that strangulation.

Pressure on the neck closes the blood vessels before it closes the airway. A person whose carotid arteries are closed can lose consciousness within seconds while they are still breathing, so they may not cough, gasp or look like they are struggling.

Doing it right would mean knowing how much pressure a particular person’s neck can take before it is injured. Nobody knows that number. It differs from person to person, it changes with position, and there is no way to measure it while a hand is on someone’s neck. The Institute for Addressing Strangulation, the UK organization that trains doctors and police on strangulation, titled its position statement “There Is No Safe Way to Strangle.”

People who teach kink say the same thing. Scarleteen, a sex-education site written for young people, states that there is no 100 percent safe way to engage in breath play.

In a review of 300 strangulation cases sent to the San Diego City Attorney’s office, half of the people strangled had no visible neck injury, and another 35 percent had marks too small to photograph. The injuries that matter most happen inside the neck: swelling, bruised blood vessels, and small tears in the wall of an artery that can lead to a stroke.

The law reflects this. Federal law and several states do not require a visible injury to prove the crime of strangulation.

Strangulation is a crime in every US state. All 50 states, Washington, D.C., and three US territories have added strangulation or suffocation to their criminal codes, most of them under aggravated assault. If a partner is accidentally hurt or killed, the person who choked them may be held legally and criminally responsible, even when both agreed to it. Laws differ from state to state.

A yes to sex is not a yes to choking. A yes given at the start also has to be one the person can take back, and taking it back requires being able to decide and to communicate. Unconsciousness can follow within five to ten seconds of the carotid arteries closing, before a safe word or a hand signal can be used. Almost all of the physical risk is to the person being strangled.

The two people involved describe the consent differently. In a survey of 4,702 Australians aged 18 to 35, 79.1 percent of people who had strangled a partner said the partner actively agreed to it; 56.6 percent of people who had been strangled said the same. The Training Institute on Strangulation Prevention’s guidance for young people states: “Because choking is so dangerous, neither teenagers nor adults can legally consent to being choked/strangled.”

Swelling inside the neck can develop several hours after strangulation and can close the airway. Pressure can also tear the inner wall of a carotid artery, and a clot that forms at the tear can travel to the brain and cause a stroke days or weeks later. The Training Institute on Strangulation Prevention’s medical guidance lists the symptoms that call for a CT scan of the blood vessels in the neck, and tells people to come back if symptoms start or get worse after they go home.

Researchers are also studying what repeated choking does to the brain. In one study at Indiana University, 40 young women took memory tests. The women who had been choked frequently scored the same as the women who had not, but brain scans showed their brains working differently to produce those scores. The study measured each woman once, so it cannot show that choking caused the difference.