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At 160°F (71°C), you can safely fry an egg. That’s the kind of temperature you can find not just on the surface of a frying pan, but also on a car’s dashboard. According to a new study, it takes under an hour for a car left in the summer sun to reach this kind of ungodly temperature. In that same time, an infant accidentally left in the car can easily suffer heat-related injuries or even die.

Just this year, there have been six recorded cases of infants who have died after their parents left them in the car in the United States. On average, 37 children die annually because of such incidents from hyperthermia — a condition due to failed thermoregulation that occurs when a body produces or absorbs more heat than it dissipates (when the body warms to above 104°F and cannot cool down).

More than half of such unfortunate cases are due to one of the parents or caregivers forgetting about the child in the car while shopping for groceries.

Jennifer Vanos, lead study author and assistant professor of climate and human health at UC San Diego, wanted to find out just how long it takes for a car to reach deadly temperatures inside. She and colleagues used three pairs of identical vehicles: silver mid-size sedans, silver economy cars, and silver minivans. Each car was moved from direct sunlight to shade for different periods throughout three hot summer days in Tempe, Arizona. The temperature outside was around 100°F (37°C).

The tests were designed to mimic a shopping trip. After one hour, which is about the time to get groceries, the researchers measured the temperature both inside the vehicle’s surface and ambient air.

For vehicles parked in the sun during the simulated shopping trip, the average cabin temperature hit 116°F (47°C) in one hour. Dashboards averaged 157°F (69°C), steering wheels 127°F (52°C), and seats 123°F (50°C). In the shade, temperatures were significantly lower but still dangerous, with interior temperatures close to 100°F (37°C) after one hour. Dashboards averaged 118°F (47°C), steering wheels 107°F (41°C) and seats 105°F(°C).

A hypothetical 2-year old boy’s body temperature was then modeled using the ambient conditions recorded during the experiments. The team found that the child’s temperature could reach levels that make him susceptible to hyperthermia and heatstroke in about an hour if the car was left in the sun and just under two hours if the car was parked in the shade. Internal injuries due to hyperthermia may surface at temperatures below 104°F (40°C) and heatstroke survivors can incur brain and organ damage.  

Vanos told me that there are a couple of caveats that ought to be considered. She and colleagues had to make many assumptions, “such as what the child was wearing, the size of the child, gender, and the age.” As such, the body temperature values may be under- or over-estimated. However, the researchers ran multiple trials across different cars and solar conditions that all suggested a high risk of organ damage or even death in highly heat sensitive toddlers.

“This inherent inter-person variability makes it difficult to predict an absolute threshold of heat injury or death, and that can make it difficult to provide directed messaging or firm conclusions as we just don’t know, and will never know, such information for toddlers. This is because it’s unethical to complete a research study of that nature (monitoring core temperature in extreme heat conditions) on children, which is why many body temperatures threshold estimates come from adults and animal-based study. But we know that infants, toddlers, and children are more vulnerable to heat for physiological and behavioral reasons,” Vanos told ZME Science.

When I asked her if there’s anything we can do to keep our cars cool during hot summer days, Vanos reiterated that it would still be dangerous no matter what we did.

“In summertime heat in Arizona, it’s impossible to keep a car cool when it’s parked. Of course, as many know, shade helps a lot to keep the car less hot and keep metal and plastic from heating up. Thus, seeking shade for your car can help, and many people already do that. Supporting initiatives to bring more shade into cities over hot open parking lots via trees, solar panels, and shade sails can also help, and the vegetation can help cool the air as well. We want people to remember though that parking in the shade is better but can still result in lethal heat levels after ~2 hours of time being conservative. Given that most deaths are caused by a child being forgotten or locking themselves in a car for extended hours (3-8 hours), shade will not save a life,” Vanos said.

Some of you reading this might find it horrific that some parents are so careless as to trap their children inside a scorching hot vehicle. The truth, the researchers say, is that it could happen to anyone, simply because of the way human memory works or, rather, how it can be hijacked by other activities. Memory failure can easily occur when a routine behavior, such as driving the same route to work, is interrupted by a novel activity, such as an emergency call.

“Memories fail. Likely the biggest mistake someone can make is assuming that this can’t happen to them, and even the most perfect of parents can make mistakes. This narrative has repeated itself time and time again – the forgotten child, which is responsible for 54% of deaths. But there are options. General Motors has started to put sensors in the back seat as an indicator if a child is left, and many new devices or apps are becoming available for reminders.  Many people are unaware of the number of lives needlessly lost per year, and we are hoping that our study can further the conversation, support life-saving initiatives, support technological adoption from car manufacturers, and advance new policies that allow people to save children and pets from hot vehicles if they believe they are in danger without legal consequences,” Vanos concluded.