Even on mild weather days, the temperature inside a closed vehicle can reach dangerous levels within an hour, posing major health risks to small children or pets left inside, Consumer Reports testing shows.

Summer has officially ended, but parents and others still must be vigilant about the ongoing danger of hot cars. In some parts of the country, it can be a four-season threat, and there are tragic examples every year.

CR testing showed that even when it was 61° F outside, the temperature inside a closed car reached more than 105° F in just 1 hour, an extremely dangerous and potentially fatal level for a child.

“Temperatures that might seem comfortable for adults can quickly become dangerous for children,” says Orly Avitzur, M.D., medical director for Consumer Reports. “And elderly passengers who can’t care for themselves, or those with cognitive problems, can also be at risk if they’re left in a car on even a mild day.”

The CR test results help dispel the myth that hot car deaths or heat stroke only happen on blisteringly hot days in the dead of summer.

And the idea that your car’s color can significantly mitigate that heat inside the vehicle is also largely a myth, CR found from its testing.

“Children should never be left unattended in a car for even a short period of time. Even when it’s not that hot outside, our test results show how quickly temperatures inside the car escalate, regardless of whether your car is light or dark,” says Jennifer Stockburger, director of operations at CR’s Auto Test Center.

Heat stroke is the leading cause of deaths in vehicles (excluding crashes) for children 14 years old and younger. Since 1998, an average 37 children have died each year in the U.S. of vehicular heat stroke.

So far this year, 39 children have died after they were left inside a hot car, according to the National Highway Traffic Safety Administration (NHTSA), the federal agency that polices auto safety.

The danger from high temperatures is particularly acute for young children because their bodies heat up three to five times faster than adults, according to the American Academy of Pediatrics (AAP).

Young children, especially babies, lack the ability to efficiently regulate their body temperature. Children dehydrate more quickly than adults.

In general, dogs suffer hyperthermia if their body temperature reaches above 103° F, and heat stroke sets in at 106° F, according to PetMD.com. Dogs with short noses and flat faces, or with extra-thick coats, are at increased risk, according to the website, which advises never leaving pets unattended in closed cars.

Some automakers have begun integrating alert technology into vehicles designed to remind parents or guardians that a child or pet might be left behind in a vehicle.

Consumers Union, the policy and mobilization arm of Consumer Reports, supports legislation introduced this year that would require warning systems in new cars.

Our Findings

Consumer Reports conducted several temperature tests inside closed vehicles at CR’s Auto Test Track in Colchester, Conn., to better understand how rapidly cabin temperatures can become unsafe. The experiments were conducted with precision instruments.

On June 28, when it averaged 61° F outside during the first hour of testing, the inside of a parked car reached more than 105° F.

On July 10, when it averaged 78° F outside during a 1-hour test period, the inside of a lighter-colored sedan reached more than 104° F.

On July 10, during the same 1-hour test period, the inside of a dark-colored sedan reached more than 109° F.

CR reported readings at the end of an hour. But interior temperatures continued to rise as more time passed.

NHTSA warns that on a 60° F day—something akin to autumn weather—the temperature inside a vehicle can reach a dangerous 110° F over the course of several hours.

The threshold for heat stroke in children is when the internal body temperature reaches about 104° F. And a child is at serious risk of death if his or her internal body temperature reaches 107° F, according to medical experts.

The risk is year-round in some regions with mild winters, including the South and Southwest. In all seasons, children have died inside hot cars when the average outside temperatures are in the 60s and 70s.

In May of this year, 5-month-old Kyrae Vineyard died from heat stroke after being left in a car for about 4 hours in Caldwell, Idaho, according to news reports. The high temperature there that day was 76° F.

And 9-month-old Bryce Balfour died from heat stroke on March 30, 2007, after he was left inside a vehicle on a 66° F day in Charlottesville, Va. His story was posted on the NHTSA website as a cautionary tale.

Car Color and Temperature

The charts below demonstrate the rise in vehicle cabin temperature when CR tested the two sedans on July 10.

Starting Temperature (° F) 78.0 78.8
Temperature After 1 Hour (° F) 109.4 104.1
Consumer Reports
Chart describing vehicle temperature rise in 1 hour

Why Cars Heat Up

Closed cars can get super hot quickly because sunlight heats up elements inside, such as the dash, upholstery, steering wheel and more, according to noheatstroke.org. Those elements radiate their heat into the air, increasing the ambient temperature inside the car.

Why don’t cracked windows help enough? Partially opened windows do allow for some heat to escape, says Jake Fisher, director of auto testing at Consumer Reports, but as long as the heat source (the sun) continues to beat down and heat up the inside car elements, the temperature can stay dangerously high.

Checking Tesla’s Cooling Feature

Vehicle manufacturers have begun integrating technology aimed at preventing child heat stroke.

Tesla introduced its Cabin Overheat Protection feature in their version 8.0 software update for Models S and X. The automaker points out that this feature is not meant to protect children, but it’s the first of its kind to automatically lower cabin temperatures when it gets too hot inside a car that is turned off.

The feature turns on the vehicle’s air-conditioning system when the car is off if the cabin temperature exceeds 105° F.

CR testers evaluated this system in our Model S 75D on June 28, 2017, and compared the interior temperature in it with a similarly colored and shaped control sedan that doesn’t have the cooling feature.

The evaluation was conducted on a partly cloudy day over an 8-hour period. Over that time, the outside air temperature at the test track reached a high of approximately 72° F. Vehicle cabin temperature was measured in the rear seat under an infant-seat carrier canopy installed in the center seat in both vehicles.

The sedan without the software reached a high of 169.3° F in the rear seat, while the Tesla reached a high of 116.8° F in the rear seat.

The Tesla rear-cabin temperature dropped and fluctuated around an average of 111° F. While these temperatures are potentially lethal for children, the test shows that the technology can cool the cabin.

The Tesla owner’s manual is clear about the system’s limits, and intentions: “The climate control system can reduce cabin temperatures in extremely hot ambient conditions for a period of up to twelve hours after you exit the Model S. When enabled, air conditioning turns on when cabin temperatures exceed 105° F (40° C). Warning: Never leave children or pets in the vehicle unattended. Due to automatic shut-off or extreme outside conditions, the inside of the vehicle can become dangerously hot even with Cabin Overheat Protection enabled.”

The manual also notes, “Cabin Overheat Protection does not operate, or stops operating, when the energy remaining in the Battery is 20% or less.”

The system runs off of Tesla’s super-large batteries, and using it would clearly impact driving range.

Preventive Measures

General Motors has created a warning feature designed to prevent children from being left behind. GM’s Rear Seat Reminder notes whether a rear door was opened within 10 minutes of the vehicle being turned on or anytime after the vehicle has been turned on.

GM has implemented the feature in a number of 2017 and 2018 models. The warning holds promise for broader integration into conventional vehicles, which don’t have large batteries to run a cooling feature after the car is turned off.

When asked if GM would introduce a cooling feature like the one in Tesla models, spokesman Nick Richards says that the automaker “continuously vets technologies, but we haven’t found one reliable enough yet to put in our vehicles.”

Nissan has developed a feature similar to GM’s called the Rear Door Alert. Standard on the 2018 Nissan Pathfinder (with plans to roll it out to other models in the future), this system shows a warning in the instrument cluster and honks the horn if it detects that a rear door was opened before a trip and not opened again after the trip was completed. The driver can choose to limit the warning to just the display or to disable it altogether, if it doesn’t suit his or her needs.

CR’s Take

Consumer Reports believes that automakers should create integrated features that remind parents to check for children in the back seat and get the technology into as many models as they can, as soon as they can. Aftermarket products can fall short because they still depend on parents recognizing the risk and taking some sort of action.

Unfortunately, parents rarely expect to forget their children in a hot car, and so fail to act proactively.

To encourage car-based protections, Consumers Union endorsed the HOT CARS (Helping Overcome Trauma for Children Alone in Rear Seats) Act of 2017.

This bill would direct NHTSA to set a rule within two years of passage requiring new passenger cars to come with alert systems that remind drivers to check for rear-seat passengers. CR’s experts believe that these alert systems should incorporate visual and audible components.

Tesla, General Motors, and Nissan have demonstrated that integrated preventative solutions are possible, even if they have not been perfected. CR will continue to evaluate those and other technologies as they emerge, and will advocate for the widespread adoption of simple, reliable, and effective backseat occupant alerts to be integrated in all new vehicles, says David Friedman, Consumers Union’s director of Cars and Product Policy and Analysis.

The AAP also endorses the integrated approach.

“Having something that is in the vehicle that is a default, that you would have to opt out of, is the right way to go about it,” says Elizabeth Murray, D.O. and an AAP spokeswoman. “These are not people making malicious decisions to try to hurt their children. These are terrible accidents that are happening, so if we can make it default to take any human error out of it, then that is the right decision.”