Leaving a bottle of water in the car is a habit shared by most drivers. It seems harmless and convenient, especially during hot weather. However, in reality, even a simple bottle of drinking water can become a fire hazard if placed in the wrong spot under scorching conditions.

During the summer, almost everyone carries a bottle of water to avoid having to walk in the heat to buy a drink. And if you drive a car, there’s a good chance you often leave that bottle somewhere inside the vehicle—usually on the dashboard, in the cup holder near the driver’s seat, or even on the passenger seat for quick access. While this habit feels natural, it may carry unexpected dangers.

A demonstration by Dioni Amuchastegui, an electrical engineer from Idaho (USA), showed that a water bottle left in direct sunlight can concentrate light and generate temperatures exceeding 100°C. When placed on a dark surface—such as a car seat—the focused heat can become strong enough to ignite the material and potentially cause a fire. Amuchastegui himself once nearly experienced such an incident, which prompted him to conduct the experiment and share the warning publicly.

Although several conditions must align for a water bottle to actually start a fire, this does not mean the risk doesn’t exist. If your car is parked outdoors under blazing sunlight, even a harmless-looking bottle can turn your vehicle into a literal “fire car.”

The reason lies in how water bottles are shaped. Most are made from clear plastic and have curved surfaces. When filled with water, the bottle can act like a magnifying lens. In midday heat, the temperature inside a parked car can climb to around 60°C—or even higher. Sunlight passing through the bottle can be focused onto a single point, dramatically raising the temperature and creating a potential ignition source in a matter of minutes.

Many drivers admit that leaving a bottle on the seat is something they’ve done countless times without ever considering the consequences. Besides the fire hazard, some car experts warn of an additional danger: the bottle could roll off the seat and get stuck near the pedals. A bottle wedged under the brake or accelerator during an emergency stop could lead to a disastrous situation, especially if the driver needs to brake suddenly.

There is also a health concern associated with leaving plastic water bottles in a hot car. Research has shown that drinking from plastic bottles that have been exposed to high temperatures for extended periods can pose serious health risks. In some cases, bottles left for months behind the driver’s seat may still be consumed by drivers who simply feel thirsty while getting into the car—unaware that prolonged heat exposure can cause harmful chemicals to leach into the water.

According to one study, when outdoor temperatures range from 80°F (27°C) to 100°F (38°C), the temperature inside a vehicle without air-conditioning can reach between 130°F (54°C) and 172°F (77°C). This level of heat is comparable to the temperature inside a low-setting oven. Another study from the American Academy of Pediatrics found that car interiors can heat up rapidly regardless of whether the windows are closed or slightly open. Even on a mild 22°C (71°F) day, the temperature inside a car can rise by 40°C within an hour.

With these risks in mind—fire hazards, driving dangers, and health threats—it’s best to break the habit of leaving water bottles in your car. If you must store water inside the vehicle, keep the bottle covered, out of direct sunlight, and placed in a secure area where it cannot roll around. Your safety, and that of your vehicle, may depend on it.