Their report, published Tuesday in the journal Pediatrics, showed that a car interior can heat up by an average of 40 degrees Fahrenheit within an hour, regardless of how cool the outside air might be. And 80 percent of the temperature rise occurred in the first half hour.

The researchers hope the findings convince parents and pet owners that parked cars are not safe havens even on mild days.

"There are cases of children (left in parked cars) dying on days as cool as 70 degrees," said Dr. Catherine McLaren, lead author of the study and an instructor in emergency medicine at the Stanford University Medical School. The study measured how a car heated up in the sun on days with temperatures ranging as high as 96 degrees and as low as 72.

'Ambient Temperature Doesn't Matter'

"On a cool day, you don't feel hot, so you believe it will be OK. But ambient temperature doesn't matter -- it's whether it's sunny out," said Dr. James Quinn, a co-author of the study and an associate professor of emergency medicine.

Past research has documented that temperatures spike in cars on very hot days, with the glass and metal acting as a greenhouse.

"Cars get hot, we know this intuitively," said Jan Null, an independent consulting meteorologist who used sensors inside a car to measure heat buildup. "This study tells us that cars get hot very fast."

A study by Massachusetts researchers published earlier this year examined the circumstances of 171 heat-related deaths to young children (5 and younger) in parked cars in the United States between 1995 and 2000, gathered from online news accounts.

They found that deaths occurred in outside temperatures ranging from 63 to 115 degrees, but three-quarters of the incidents were in June, July and August. About the same proportion involved children who were left behind in the vehicle by an adult, either due to forgetfulness or a deliberate decision.

McLaren said the findings should particularly discourage those caregivers who might think it's acceptable to leave a child or pet in the car for only a few minutes.

Cracking a Window

Null said drivers sometimes think that by cracking a window or running the car air conditioner before stopping the car they can make the practice safe.

But he's found that cracking a window has an insignificant effect on either the rate of heating or the temperature inside after an hour, while the air-conditioner trick only delays the temperature spike by about five minutes.

Null still hopes to do more studies on other variables, like the color, shape and interior size of a vehicle, or the effect of tinted windows on how fast an interior heats up. However, the researcher said he's convinced that things heat up rapidly in all cars left in the sun.

"If more people knew the danger of leaving their children in the car, they probably wouldn't do it," McLaren said. She noted that other than studies based on accounts of tragedies, there's almost no information on how many parents leave kids behind in cars either on purpose or by accident.

The Kids And Cars advocacy group, which attempts to track incidents of children put in jeopardy from parked cars in various circumstances, estimates that about 24 percent of the more than 650 children under 15 killed in or around cars between 2000 and 2004 in the United States died from heat-related injuries.

The organization notes that car temperatures can easily reach 120 degrees even when it's only in the 70s outside, and urges parents and caregivers to leave a regular reminder, like a teddy bear or dashboard sign, to remind them there's a young passenger riding in back who needs to come with them when they leave the car.