Does Cracking Car Windows Help With Heat? Stanford Study Says No

Cracking your car windows does not make a parked car meaningfully cooler. In a 2005 Pediatrics study from Stanford University researchers, windows opened 1.5 inches made no significant difference to how fast or how hot a dark sedan got, and a 2017 test by Germany’s ADAC club found the same with one or two windows open. Cracked windows also do not make a parked car safe for a child or pet: NHTSA says rolling windows down does little to change the interior temperature, and if you see a child alone in a locked hot car, call 911.

Does Cracking the Windows Really Work?

Short answer: no, not in any way that matters. Both controlled tests we read compared cracked windows with closed windows and found essentially no difference. Celsius figures below come with Fahrenheit conversions.

What does help is keeping sunlight off the surfaces inside the car. In ADAC’s 2021 test an interior reflective shade lowered cabin air by 4 degrees C (about 7 degrees F), and ADAC says an interior shade only works when it matches the size of the glass. A windshield shade sized to the glass is the product type that test examined.

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What a 2005 Stanford Study Found About Cracked Windows

The most-cited research is a paper in Pediatrics (July 2005) by Catherine McLaren and James Quinn of Stanford University’s emergency medicine division and meteorologist Jan Null; the full paper is available as a PDF. They parked a dark-blue 2000 Honda Accord with a medium-gray interior and untinted windows in full sun in Fremont, California, on 16 cloud-free days in 2002, with outdoor temperatures from 72 to 96 degrees F, and logged the cabin temperature every 5 minutes for an hour. On 2 of those days they also ran a second hour with the windows cracked 1.5 inches.

The authors concluded that leaving the windows open slightly does not significantly slow the heating or lower the maximum temperature. Limits: one vehicle, only 2 of 16 days with cracked windows, and a thermometer in the shade about 15 inches above the rear seat. They wrote that a child in full sun would face greater and faster heat stress than their readings show.

What ADAC Measured With One or Two Windows Open

ADAC, Germany’s national auto club, parked three identical cars in the sun at about 28 C (82 F) in 2017: one with closed windows, one with a window open about 5 cm (roughly 2 inches), and one with two windows open a narrow gap. In every case the cabin reached about 50 C (122 F) after 30 minutes and about 57 C (135 F) after an hour. ADAC’s conclusion: never leave children or animals alone in the car. Its 2021 article adds that midday sun can bring a parked car close to 60 C (about 140 F) after 90 minutes.

Risks of Cracking Your Windows

Pet and Child Safety

NHTSA says a child’s body temperature rises three to five times faster than an adult’s, heatstroke begins at a core temperature of about 104 F, and death occurs at 107 F or above. It reports 31 child heatstroke deaths in vehicles in 2025 and more than 1,000 over 25 years; see its heatstroke prevention page. The National Weather Service adds that a toddler who climbs into a car can die in 10 minutes, so lock your car even in your own driveway.

NoHeatStroke.org, run by Jan Null of San Jose State University, counts 1,070 child deaths since 1998 as of September 28, 2026, an average of 37 a year. Of 1,041 cases examined for 1998 through 2025, 52.9 percent were a child forgotten by the caregiver, 23.8 percent a child who got in alone and 21.9 percent a child knowingly left by the caregiver.

NHTSA recommends:

• Check the whole vehicle, especially the back seat, before you lock the doors.
• Put a purse or briefcase in the back seat as a reminder, and ask your childcare provider to call if your child does not show up.
• Keep keys and fobs out of children’s reach, and lock your car when you are not using it.

Pets face the same risk: the National Weather Service lists pets alongside children as never safe to leave in a locked car. See our guide on whether leaving a dog in a car is illegal.

Hot Surfaces and Security

ADAC measured steering wheels and dashboards above 70 C (158 F) in an unprotected car and warns of skin burns at those temperatures. The Pediatrics paper also lists burns from buckles among the risks of leaving a child in a car, and cites a survey in which fewer than one third of mothers would leave windows half open because of theft concerns.

Factors That Affect How Well Cracking Windows Works

Window Opening Size

The 2005 study cracked windows 1.5 inches and ADAC opened one window about 5 cm (2 inches). The Pediatrics paper also cites a 1981 study by King and colleagues in which opening the windows 20 cm (about 8 inches) had minimal effect on temperature rise and maximum temperature. No tested gap changed the result.

Outside Temperature and Humidity

In the 2005 study the rate of rise was the same whatever the outdoor temperature from 72 to 96 F, and the final cabin temperature followed the starting outdoor temperature. The sources we read report air temperature, not humidity, so this guide cannot say how humidity changes the result.

Parking Location

ADAC’s 2017 advice is to park in the shade and use shading on glass roofs. The sources give no figure for shade versus sun parking, so none is claimed here.

Car Color and Materials

In ADAC’s 2021 test a black car’s paint reached 65 C (149 F) against 44 C (111 F) for a white car, and the white car’s interior ran 5 degrees C (9 degrees F) cooler. ADAC found heated dark interior surfaces add more heat than paint color; NoHeatStroke.org reports dark dashboards and steering wheels often reach 180 to 200 F when outdoor air is in the 80s. See do black cars really get hotter and do cars run hotter in the summer.

Better Ways to Keep Your Car Cool

ADAC’s 2021 test at its Landsberg am Lech technical center put seven identical Dacia Duster SUVs in midday sun for about four hours: four with sun protection, one with tinted side and rear windows, one white and one unmodified reference (ADAC’s press summary, in German).

Surfaces mattered more than air. Every accessory lowered surface temperatures and kept the steering wheel 14 to 26 C (25 to 47 F) cooler, and tint dropped rear-seat surfaces from 57 C (135 F) to 48 C (118 F). The tint row subtracts ADAC’s reported 2 degrees from the 53 C reference, and the Fahrenheit values are conversions.

Use a Windshield Sunshade

An interior shade cut cabin air by 4 C (7 F) in ADAC’s test, less than the exterior film or the full cover, and ADAC says it helps only when it matches the interior size of the glass. ADAC’s 2017 advice is to put a shade behind the windshield to reduce incoming radiation. See do car sun shades really work and the best car sunshades for heat.

Park in the Shade

Shade keeps direct sun off the dark interior surfaces that ADAC found drive much of the cabin heat.

Ventilate Before Driving

ADAC’s 2017 advice for a sun-heated car is to open the doors, windows and sunroof before you set off and close them as you start driving, then switch briefly to recirculation at maximum cooling.

Tint Your Windows

Tint lowered cabin air by only 2 C (4 F) in ADAC’s test, but it kept rear-seat surfaces far cooler, which ADAC says matters especially for children and infants. ADAC adds that film on the rear side and rear windows blocks UV and infrared as long as the driver’s view is unaffected. Compare does window tint keep a car cooler, sunshade vs. tint and legal tint for car windows; UV also passes through untreated glass (do UV rays go through car windows).

Use a Car Cover

The full-window cover ADAC calls a half garage performed best, 10 C (18 F) cooler than no protection, because it shields the whole cabin. ADAC also recommends covering the steering wheel, dashboard and child seats (with a light cloth) against burns from hot surfaces. That is about burns: it does not make it safe to leave a child in the car.

How to Use Your Car’s AC Effectively

Air the car out first, then use the AC. ADAC puts a comfortable cabin temperature at 22 to 25 C (72 to 77 F) and suggests aiming airflow over the shoulders of front occupants instead of straight at the body. Pre-cooling does not protect a parked car, as the 2005 study showed. More: make your car air conditioner colder, does recirculate heat a car faster, sitting in a car with the AC on, and what it means if your car is running hot.

Myths About Cooling Your Car

• Myth: cracked windows make it safe to leave a child or pet. NHTSA and the National Weather Service say otherwise (see above).
• Myth: only very hot days are dangerous. A 72 F day produced a 117 F cabin within an hour in the 2005 study.
• Myth: cooling the car first keeps it cool. The tested car returned to outdoor temperature within 5 minutes of the AC going off.

Conclusion

Cracking your car windows does not cool a parked car in any way the tests could measure, and it does not make the car safe for a child or pet. Block the sun, air the car out before you drive, and never leave a child or pet in a parked car. Related: does cracking car windows help with heat. This guide does not cover how humidity changes cabin temperature, heat damage to window glass, or vehicle-specific ventilation features.

FAQs

Does Cracking Windows Cool the Car Faster with AC?

No test we read measured cracked windows with the AC running. ADAC advises airing out a sun-heated car with the doors, windows and sunroof open before you set off, then closing them once you drive. The 2005 Pediatrics study found that cooling the car with the AC beforehand had a negligible effect on how fast it heated up again.

How Much Should I Crack My Windows?

The studies tested a 1.5 inch gap (Pediatrics, 2005) and about 5 cm, roughly 2 inches (ADAC, 2017). Neither found a meaningful cooling benefit, and no gap size has been shown to make a parked car safe for a child or pet.

Is It Safe to Leave Windows Cracked in a Parking Lot?

The studies measured temperature, not security. The Pediatrics paper notes that in a survey fewer than one third of mothers would leave the windows half open because of theft concerns, and the measured cooling benefit of a gap was insignificant.

Can Cracked Windows Keep Pets Safe in a Hot Car?

No. The National Weather Service says it is never safe to leave a child, disabled person or pet locked in a car, and NHTSA says rolling the windows down does little to change the interior temperature.

Does Car Color Really Affect Interior Heat?

A little. In ADAC’s 2021 test a white car’s interior was 5 degrees C (about 9 degrees F) cooler than a black car’s, and ADAC found that heated dark surfaces inside the car add more heat than the paint color.

How Hot Does a Car Get With the Windows Cracked?

The 2005 Pediatrics study found an average rise of about 41 F in an hour with windows closed or cracked 1.5 inches, 80 percent of it in the first 30 minutes. As arithmetic from that average, an 85 F day would put the cabin near 126 F. ADAC measured about 50 C (122 F) after 30 minutes and 57 C (135 F) after an hour with one or two windows open on a 28 C (82 F) day.