The man, who delivers ice cream using dry ice, had left four coolers of it in the back seat of the car, which displaced the oxygen inside, causing his wife and mother to choke when they left Thursday evening, according to The News Tribune.
"At this point we're just looking at this as a horrific accident," sheriff's spokesman Ed Troyer told the News Tribune. "If you do some research on it, it's out there. It happens."
The man awoke early Friday to go to work and realized his wife had not returned home. He found her and his 77-year-old mother a few blocks away in the vehicle unconscious. He broke a window, pulled out his wife and called 911, according to the News Tribune.
His mother was pronounced dead at the scene. His 51-year-old wife was taken to the hospital in critical condition.
Dry ice, which is carbon dioxide in solid form, transforms to a gas when exposed to open air and can be dangerous in non-ventilated areas causing difficulty breathing, loss of consciousness and death, according to University of Washington's Department of Environmental Health and Safety.
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