Cars parked in the sun on a hot day can reach dangerously high temperatures in just one hour, a new study has revealed.

One hour is roughly how long it takes for a young child to suffer heat injury, or even die from hyperthermia, which is when the body's temperature goes above 40 degrees Celsius and cannot cool down.

US researchers looked at how different types of cars warm up on hot days depending on whether they are in the sunlight or shade. They assessed how these differences would affect the body of a hypothetical two-year-old left in a car on a hot day.

The study involved six different vehicles - two identical silver city cars, two identical silver mid-size saloon cars and two identical silver minivans.

During three days when temperatures reached above 35 degrees Celsius, the researchers moved the cars from the sunlight to the shade for different periods of time. The cars' interior air temperatures and surface temperatures were measured throughout the day.

"These tests replicated what might happen during a shopping trip. 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," commented one of the study's authors, climatologist, Nancy Selover.

For cars parked in the shade during the simulated shopping trip, interior temperatures reached an average of 38 degrees after one hour. The dash boards averaged 48 degrees Celsius, the steering wheels were 42 degrees and the seats were 41 degrees.

However, when parked in the sun for just one hour, the interior temperature averaged 46 degrees Celsius, the dash boards averaged 69 degrees, the steering wheels were 53 degrees and the seats were 51 degrees.

The cars warmed up at different rates, with the smaller city cars heating up faster than the mid-sized cars and minivans.

"We've all gone back to our cars on hot days and have been barely able to touch the steering wheel. But, imagine what that would be like for a child trapped in a car seat. And once you introduce a person into these hot cars, they are exhaling humidity into the air. When there is more humidity in the air, a person can't cool down by sweating because sweat won't evaporate as quickly," Ms Selover explained.

While scientists cannot predict exactly when a child will suffer heatstroke, most cases occur when a child's core body temperature rises above 40 degrees Celsius for an extended period. In this study, the researchers estimated that their hypothetical two-year-old's body would reach that temperature in one hour if parked in the sun, and just under two hours if parked in the shade.

"We hope these findings can be leveraged for the awareness and prevention of paediatric vehicular heatstroke and the creation and adoption of in-vehicle technology to alert parents of forgotten children," commented the study's lead author, Jennifer Vanos.

Details of these findings are published in the journal, Temperature.