Leaving a child or pet inside a parked car is certainly a no-no on hot days, but new research suggests that parked cars could get dangerously hot even on relatively cool days.

Researchers from Stanford University School of Medicine found that the temperature inside a parked car can quickly spike to life-threatening levels if the sun is out. They hope their findings will put to rest the misconception that a parked car can be a safe place for a child or pet in mild weather.

"There are cases of children dying on days as cool as 70 degrees Fahrenheit," lead researcher Dr. Catherine McLaren said in a news release.

Researchers measured the temperature rise inside a parked car on sunny days with highs ranging from 72 to 96 degrees Fahrenheit. Their results, published in the July issue of the journal Pediatrics, showed that a car's interior can heat up by an average of 40 degrees within an hour, regardless of outside temperature. And 80 percent of the temperature rise occurred within the first half-hour.

"On a cool day, you don't feel hot so you believe it will be OK," said researcher Dr. James Quinn. "But ambient temperature doesn't matter; it's whether it's sunny out."

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Parked Cars Get Hot On Mild Days

Much like the sun can warm a greenhouse in winter, it can also warm a parked car on cool days, the researchers said. In both cases, the sun heats up a mass of air trapped under glass.

And cracking a window or running the air conditioner before parking won't keep the car cool. The researchers said a cracked window had an insignificant effect on the rate of heating and the final temperature after an hour, and the air conditioner trick only delayed the temperature spike by about five minutes.

"If more people knew the danger of leaving their children in the car, they probably wouldn't do it," McLaren said.

She said the solution is simple: Take your child with you when you park the car.

Study Abstract