Never leave children in a car, say researchers in California.

Cars get hot very quickly, and babies and young kids are especially vulnerable to the heat, say Catherine McLaren, MD, and colleagues.

Leaving a window slightly open or counting on milder temperatures aren’t good ideas, they say.

“Prevention of heat illness in children and potentially other injuries is straightforward: Do not leave them in the car,” the researchers write in Pediatrics.

Read Web MD's "Kids and Cars Don't Mix."

Hot Cars Can Be Fatal for Kids

Last year, 35 children died from heat stroke after being left unattended in a vehicle. In 2003, 42 children lost their lives that way.

Those numbers appear in McLaren’s study.

The researchers didn’t just cite statistics. They did an experiment to see how fast cars heat up.

They checked temperatures inside an empty, dark blue sedan. The car was parked in full sun on 16 different days.

Outdoor temperatures ranged from 72-96 degrees Fahrenheit. The temperature inside the car was checked every five minutes for an hour.

Read Web MD's "Traveling With Kids: A Survival Guide."

Sky-High Temperatures in a Short Time

The mercury inside the car soared 40 degrees higher than outside temperatures, on average.

Even on a sunny, 72-degree day, temperatures inside the car could reach 117 degrees Fahrenheit in an hour, say the researchers.

Eighty-percent of that rise happened in the first 30 minutes, they say.

“Vehicles heat up rapidly with the majority of temperature rise occurring within the first 15 to 30 minutes,” they write.”

Open Window Not a Big Help

On two days, the researchers lowered a window 1.5 inches. That didn’t make much difference in the car’s temperature.

“Cracking open windows is not effective in decreasing either the rate of heat rise or the maximum temperature attained,” they say.

The researchers placed the sensor in a shaded part of the car’s interior.

“This suggests that if a child were positioned in full sun, then the heat stress would be greater and more rapid,” they say.

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Other Risks

Leaving children in a car also “puts them at risk for other harm,” say the researchers.

They say those hazards include thermal burns from buckles, abduction, and injury from the child’s putting the car in motion or operating power control features.

“If more people knew the danger of leaving their children in the car, they probably wouldn’t do it,” says McLaren, in a news release.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: McLaren, C. Pediatrics, July 2005; vol 116: pp e109-e112. News release, Stanford University.