Dippin’ Dots deliveryman left dry ice in the back seat of his car. It may have asphyxiated his mother.

Around 4 a.m. on Friday morning, police in Tacoma, Washington, got a frantic call: A 51-year-old man had just found his wife and his mother unconscious inside his car.

The man, whose name has not yet been released, runs a business delivering Dippin’ Dots ice cream and had left four coolers full of dry ice on the back seat of the car, local authorities told the News Tribune. He smashed the car’s window with a rock and dragged out his wife, who was taken to the hospital in critical condition. But he was too late to save his mother.

Jeff Polanco, an investigator at the Pierce County Medical Examiner’s Office, told the Washington Post that the 77-year-old woman has been identified as Hildegard Whiting. She likely died of asphyxiation caused by the dry ice, which is simply carbon dioxide in its solid form. Since dry ice turns into a gas rather than melting into a liquid when it heats up, it’s often used as a less-messy alternative for transporting ice cream and keeping frozen food cold. But in a small enclosed space, too much carbon dioxide can be lethal. The invisible, odorless gas takes the place of oxygen, making it impossible to breathe.

“At this point we’re just looking at this as horrific accident,” Ed Troyer, a spokesman for the Pierce County Sheriff’s Office, told the News Tribune.

Troyer said that the man’s wife, who is also 51 years old and has not yet been named, had offered to drive his mother home at around 11 p.m. the previous evening. When he woke up early the next morning to go to work, he realized that she hadn’t come back. Then he spotted the car parked a few blocks away from his house in University Place, a suburb of Tacoma.

Death by dry ice is fairly unusual, but a few other examples have been recorded in medical journals. In one case that took place in Thailand, an otherwise healthy 20-year-old man who had been running away from a fight hid in a small, unmarked plastic container used to hold dry ice. When a security guard found him roughly five minutes later, he was having convulsions, according to a 2009 article in the Journal of Forensic Sciences. By the time EMTs arrived, he was dead.

Another case cited by the Journal of Emergency Medicine involved a walk-in freezer that contained dry ice. After a 59-year-old man stepped into the freezer, which had recently been repaired, he went into cardiac arrest. The article’s authors concluded that his death “illustrates the lethal consequences of improper storage of dry ice and the need to consider toxic environmental exposure as a cause of sudden cardiac arrest.”

For people in the ice cream business, dealing with dry ice can be an occupational hazard. Justin Woolverton, the founder of Halo Top Creamery, told Inc. magazine that he nearly died of asphyxiation a few months after starting the company. Driving through Los Angeles with 40 pints of ice cream on the back seat, he started having trouble breathing, then began to hyperventilate. When he got to a stoplight, he spotted an ambulance in the next lane, and frantically gestured to the driver. The paramedics pulled over to help him, but he had already started to recover once he got out of the car. As it turned out, he only needed fresh air: The dry ice that he had packed with the ice cream on the back seat had started to warm up and he had been inhaling carbon dioxide.

In 2016, a delivery driver in Missouri fell into a coma and died after being found unconscious in his parked car, a St. Louis Post-Dispatch columnist wrote. After learning that he’d had five boxes of strawberries chilled by dry ice in the back seat, his sister became convinced that the carbon dioxide had killed him. Doctors told her it was unlikely, but they also couldn’t explain how he ended up in a coma.

The Post-Dispatch also cited another incident in Columbus, Ohio, where a delivery driver passed out on the side of the road after a seal broke on the dry ice in the back of his car. When he failed to show up at his destination, the courier company where he worked used the GPS tracker inside his car to locate him and sent a coworker to check on him. He survived.

Troyer speculated that summer heat may have played a role in Whiting’s death and so may have the fact that her son had recently purchased a new car.

“Somehow or another, the fumes escaped from the coolers, possibly because it was so hot outside and he had a newer car, which probably had better seals and less ventilation,” Troyer said in a video posted on Twitter by Suzanne Phan, a reporter at KOMO News in Seattle. “This all happened due to a bunch of circumstances lining up. Dry ice by itself isn’t going to kill anybody.”