A parked car can reach 117 degrees on a mild 72-degree day, and the study that measured it found that cracking the windows made no meaningful difference to the final temperature inside at all

Picture this: it’s a nice 72 degrees outside, you park in the sun for a “quick” errand, and you don’t think twice about leaving your kid buckled in the backseat. According to a 2005 study published in Pediatrics, that decision can quickly turn dangerous. Researchers from Stanford University and San Jose State University measured the inside of a parked sedan on 16 different sunny days with outside temperatures between 72 and 96 degrees. Even on the mildest of those days, the inside of the car still hit 117 degrees in an hour.

The heat doesn't care what the thermometer says outside
Now here’s the interesting part. It was almost irrelevant whether it was 72 degrees outside or 96 degrees outside, the study noted. In either case, the car warmed up at about the same rate, picking up about an average of 3.2 degrees every five minutes. The only difference was the final number: on average, cars were about 40 degrees hotter than the starting outside temperature, with individual days ranging from 28 to 49 degrees of increase. And most of that rise happens quickly. The study found that 80 percent of the total temperature increase happened during the first 30 minutes.

Cracking the windows doesn't actually help
Many parents probably leave the windows open by an inch or two, thinking that it will let the heat out. According to the study, it hardly makes a difference. Researchers compared cars with windows cracked open 1.5 inches to cars with windows fully closed, and found the two heated up at a similar pace, between 3.1 and 3.4 degrees every five minutes, and landed at the same final maximum temperature regardless. A cracked window can feel like a safety net, but the data says it is not.

Why does this hit kids so much harder than adults
A hot car is bad for everyone, but it’s especially dangerous for a little kid. In earlier research on toddlers, the researchers had found that their core body temperature rose faster and higher than their own mothers’ in the same hot room, even when the kids were sweating more relative to body weight. Fluid loss also adds up quickly. A 12-month-old can lose about 1 to 2 milliliters of fluid per kilogram of body weight every hour from sweating in 86-degree heat, enough to add up to meaningful dehydration over just a few hours, according to this study.

Doctors say that heat illness exists on a spectrum. The study says it starts with heat stress: just feeling hot and strained. It can advance to heat exhaustion, leading to dehydration, weakness, dizziness, and headache. The core body temperature runs between about 98.6 and 104 degrees. It can turn into heat stroke, a serious medical emergency, with core temperatures over 104 degrees, and confusion, seizures, or worse.

This is not just a 2005 problem; it is still happening
The original research that supports this is now over 20 years old, but the danger has not gone away. The federal safety data collected by the National Highway Traffic Safety Administration indicates that over the past 25 years, more than 1,000 children in the US have died of vehicular heatstroke, and a child’s body temperature rises three to five times faster than an adult’s in the same hot environment. More than half of these deaths are caused by a caregiver forgetting a child is in the back seat.

The ongoing national database, managed by the same meteorologist who co-authored the original 2005 car-heating study, indicates that the US has averaged about 37 of these deaths a year since 1998. The pattern hasn’t really changed: warm sunny day, quick stop, a few forgotten minutes.

What actually helps
You do not have to do anything complicated to protect your child. Make it a habit to look at the back seat every time you park, not just on the days you remember your child is with you. Keep something you can't walk away without, like your phone, wallet, or work badge, in the back seat as a physical reminder. If your child’s daycare offers to call when your child does not show up as expected, sign up for it.

And if you ever see a child alone in a parked car, don’t hesitate to stop and check if everything’s fine. Get the child out of the heat as quickly as possible and call 911 right away. On a day that feels perfectly mild outside, the inside of that car is anything but.