Cars alone in the sun can feel like an oven in little more than an hour for a child left inside

It happened again this week, a 1-year-old girl was accidentally left in a car seat in the back of her parents’ pickup truck in their Nashville, Tennessee, driveway. The young girl died later that day.

It takes about an hour for a child left in a hot car to suffer from heat stroke.

A new study tested different types of cars in the sun and in the shade to see just how quickly temperatures inside the vehicle can reach a level that's lethal for inside occupants. Researchers from Arizona State University (ASU) and the University of California, San Diego School of Medicine did their tests in Tempe, Arizona, using a 60-minute span as their basic measure, looking at what temperatures would do to a typical 2-year-old.

“We are hoping that our study can invoke awareness, send a new message with a human health-centered focus, support technological adoption from car manufacturers and other device manufacturers, and advance new policies that give people legal immunity if they need to save children and pets trapped in hot vehicles," Jennifer Vanos, lead study author and assistant professor of climate and human health at University of California, San Diego, told ABC News. "Those types of actions can decrease risk."

Over the last 20 years, about 750 children in the United States have become heatstroke victims after being left unattended in a car by a parent or a caregiver. Even more, 37 children die each year from pediatric vehicular hyperthermia (PVH) -- a process in which the body warms up to above 104 degrees and cannot cool down. More than half of those deaths are from children under 2 years old being left accidentally -- more than a fourth of the children were left “playing” in the car.

Heatstroke and hyperthermia effects happen along a spectrum. Even below 104 degrees Fahrenheit, heatstroke can lead to brain and organ damage.

This study, published in the science journal "Temperature" shows, on average 80 minutes in a sunny car is enough to kill a child. In a shaded vehicle, it takes a little under two hours for a 2-year-old’s body to reach a core temperature of greater than 104 degrees Fahrenheit.

"Our study not only quantifies temperature differences inside vehicles parked in the shade and the sun, but it also makes clear that even parking a vehicle in the shade can be lethal to a small child, if left long enough in the car," said Nancy Selover, an Arizona State climatologist and research professor in ASU's School of Geographical Sciences and Urban Planning.

Six vehicles -- two silver mid-size sedans, two silver minivans and two silver economy cars -- were moved from the shade to sunlight multiple times, as they measured interior air and surface temperatures. For cars parked in the sun for an hour, the average surface temperature for the dashboards was 157 degrees Fahrenheit, the steering wheels were 127 degrees Fahrenheit and the seats were 123 degrees Fahrenheit.

Vehicles parked in the shade were cooler, with average surface temperatures of 118 degrees Fahrenheit, steering wheels 107 degrees, and seats 106 degrees. The various vehicles warmed up at different rates, with the economy car warming up faster than the mid-size sedan and minivan.

"These tests replicated what might happen during a shopping trip," Selover said. "We wanted to know what the interior of each vehicle would be like after one hour, about the amount of time it would take to get groceries. I knew the temperatures would be hot, but I was surprised by the surface temperatures."

The average estimated core temperature for a 2-year-old after 60 minutes in shaded vehicles was up to 101 degrees Fahrenheit. In sun-exposed vehicles, it was up to 103 degrees Fahrenheit, with higher final core temperatures in sun-exposed vehicles.

Of course, in the real world, different children would reach hyperthermia earlier or later, based on the climate, and a child’s size, clothing, ethnicity, and age.

"I don’t think our study can address the overall ‘risk’ of heatstroke because that depends on human behavior and actions more than anything," Vanos said. "All cars heat up to lethal temperatures across every state, and although it’s the level of heat that in the end causes the death, it’s the act of forgetfulness that is the trigger. Deaths have occurred in not-so-hot states as well. And even though parking in the shade decrease the heart rate of the child’s core temperature, the risk of death is likely the same as if parking in the sun."

Sima Patel, MD, is a third-year physical medicine and rehabilitation resident at the University of Minnesota who works in the ABC News medical unit.