"It only happens with a good amount of snow and the car is surrounded by a little wall of snow that impedes normal ventilation," said Bruce Anderson, director of the University of Maryland's Poison Control Center.
During the storm of January 1996, two people in Philadelphia and one in New York were killed by the fumes, and 21 in New York were hospitalized. In all cases, the cars were surrounded by snow banks that blocked the escape of toxic fumes.
Carbon monoxide leads all toxins as a cause of sickness and death, according to the Centers for Disease Control and Prevention. Each year, it kills an average of 544 Americans in accidental exposures. An additional 7,000 to 15,000 people are hospitalized annually, and far more are treated in emergency rooms.
Inhaled, carbon monoxide binds to hemoglobin, the blood molecule that normally carries oxygen to tissues. Carbon monoxide is far more efficient than oxygen in binding to the molecule, and the result is that tissues become starved of the very thing they need to survive.
It works insidiously, first by dimming a person's mental faculties and thus the ability to recognize the need to escape to fresh air. If exposure is prolonged, multiple organ failure can set in.
"Since the heart and brain are the most oxygen sensitive tissues, they require a lot of oxygen all the time to keep functioning," said Dr. Kit Lorentz, an emergency physician with St. Joseph Medical Center in Towson.
"All those things add up to organ malfunction. The brain doesn't function as well, headaches cause confusion and lethargy. You have difficulty seeing, speaking, and eventually comes coma and death," he said.
Carbon monoxide is an odorless, colorless gas produced by the incomplete burning of hydrocarbons, fuels including gasoline, kerosene and natural gas. Frequent sources of carbon monoxide poisoning are improperly vented furnaces, gas ovens used to heat rooms and kerosene heaters not vented to the outside.
How fast it can kill depends on many things, including how much fresh air it mingles with inside an enclosed space and a person's size. Children are apt to succumb faster than adults, Lorentz said. Smokers might be more vulnerable because tobacco delivers low-level contamination to the bloodstream.
"If you're breathing pure [carbon monoxide], you could be history in 20 to 30 minutes," Lorentz said.
People who get help fast enough revive when given pure oxygen to breathe. In extreme cases, they are placed inside a hyperbaric chamber, which exposes the cells to concentrated oxygen under pressure.
One such case was that of a 4-year-old Philadelphia girl whose parents let her sit in an idling, heated car while they dug it out of a snow bank in 1996. They assumed she was napping when she fell asleep but panicked when they could not get her to wake up. She had spent half an hour in the car.
They rushed her to an emergency room, where she became alert when treated with pure oxygen. Later, she was transferred to a hyperbaric chamber, which sped her recovery.