On days when an oppressive sun strikes streets, temperatures spike and so do fatal car crashes. Over the last decade researchers in countries including the U.S., China, Spain, France and Iran have investigated how temperature affects car crashes and found the same trend: hot weather increases the risk of a lethal wreck. Many researchers say this risk may grow worse as climate change fuels record-breaking heat.
Exposure to extreme heat disrupts the body and brain with severe and sometimes even deadly consequences. Dehydration, drowsiness and heat-related mood disruptions can increase impulsiveness and decrease cognitive performance—serious concerns for anyone behind the wheel. And the way cars trap heat amplifies the risk of a crash.
“A car is a little bit like a greenhouse,” says Ronnen Levinson, lead researcher of the Heat Island Group at Lawrence Berkeley National Laboratory. As a vehicle sits in the sun, light passes through the windows and windshield. Inside, exposed surfaces and the very air itself soak up the heat until the temperature becomes intolerable. In 2011 Levinson and his colleagues found that the temperature inside a black car that sat in the sun for an hour on a pleasant, 77-degree-Fahrenheit morning rose to over 117 degrees F. After four more hours, even with the air conditioning intermittently turned on, the air inside rose to a blistering 148 degrees F while it was 96 degrees F outside.
The human body can’t tolerate such excessive heat for very long, according to Neha Raukar, a practicing emergency medical physician at the Mayo Clinic in Rochester, Minn., who specializes in sports medicine and heat illnesses. “Even above 108 degrees with very low humidity—that will be considered, in sports, the danger zone,” she says. Although each person’s body handles heat differently, Raukar says that 108 degrees F “is a no-play temperature” for athletes, and she recommends resting and drinking water. Sitting in a car with temperatures that can approach or even exceed 150 degrees F can be life-threatening—“no matter what your body chemistry is,” she says.
Car cabin temperatures—which can reach dangerous levels on even a pleasant 77-degree-F day —will be able to hit even worse extremes as heat waves continue to grow more severe. The temperature outside is the first thing that determines just how hot the inside of a car can get, Levinson explains. So when the thermometer climbs well over 100 degrees F outside, a car can quickly become an oven that’s literally hot enough to slow roast a lamb chop.
Once a car gets that hot (in excess of 150 degrees F on the inside), it can be difficult and time-consuming to make it tolerable to sit in. The seats can singe your skin. Rolling the windows down or blasting the air conditioning helps, but this sensible strategy won’t always make a difference fast enough. Levinson found that after running a car’s air conditioning on full blast for 30 minutes, the temperature inside dropped from 148 degrees F to around 94 degrees F—cooler, but hardly comfortable.
And even if you’re not exposed to the heat for long, it can affect your driving ability itself. In a 2014 study, a group of researchers in Italy found that people operating cars that had sat in an unshaded parking lot were likely to have more difficulty keeping their vehicle on a straight path, identifying traffic signals and responding quickly to what was happening on the road.
The negative effects of driving a hot car can occur anywhere, but heat seems to affect traffic risks in some regions less than in others—or, in rare cases, not at all. Health geographer Connor Wu, lead author of a 2018 study that found an increase in fatal car crashes across the continental U.S., published a 2021 follow-up study focused on Alabama, a southern state known for its routinely steamy summers. He determined that the state experienced slightly fewer collisions across all heat wave days, compared with days without extreme heat. Wu suggests that “people living in the South are more well-prepared for the heat,” resulting in lower accident rates. Southerners are more likely than their compatriots in cities such as Seattle or San Francisco to have central air conditioning at home, which can reduce brain-fogging sleep disruptions. Additionally, they’re more acclimated to the heat to start with, Wu says. The 2021 study also took other weather conditions into account, and Wu found that crashes increased during heat waves when there wasn’t precipitation. Fewer people take to the roads on rainy days, and those who do are more likely to drive with caution, he explains. On sunny days, people are more active; traffic volume increases, and the risk of a wreck also goes up with exposure to heat.
Kai Zhang, an associate professor of public health at the University at Albany, also points to larger traffic volume as a factor in the increase in heat-related car crashes over the last three decades. He and his colleagues published a study investigating global trends in heat-related car crashes, and they found a significant increase in fatal wrecks attributable to heat between 1990 and 2019—particularly in countries such as India, Nigeria and Brazil. The climate crisis has hit these countries, and some others in the Global South, far harder than other parts of the world.
Amid the growing concerns, there are proactive strategies drivers and urban planners can take to reduce collision risks during a heat wave. Levinson points out that planting more shade trees to shield cars and parking spaces can help keep vehicles cooler. A 2020 study in Slovenia found that after a few hours, a car left in a parking lot with ample tree cover was cooler by 36 degrees F than one left in an unshaded parking lot on the same day. Levinson also adds that adding reflective shades to the front windshield and rear window is an effective way to limit at least some sunlight entering your car. These simple strategies can make driving at least a little safer and more comfortable.
Beyond heat-proofing one’s car, Raukar says it’s important for drivers to be aware of their individual susceptibility to heat. She points out that certain medications—including some psychiatrics, diuretics, stimulants and antihistamines—can reduce the body’s ability to manage heat. So can several medical conditions, including hyperthyroidism and multiple sclerosis. If you fall into any of these categories, extra precaution is crucial. But everyone can benefit from other safety measures: open windows or doors and turn on the air conditioning for a while before sealing yourself inside a car, and ensure you have water close at hand. If you start to feel uncomfortable mid-journey, even if you’re in a rush, sometimes the safest option is to simply remove yourself from the road. “If you feel not-quite-right,” Raukar says, “pull over and drink cool fluids.”